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This page is updated February 12th 2010. It is only available in English. If you want to go back to pages in Danish, please use the "Return" button of your browser.

Max-i is a brand-new and revolutionary fieldbus system for almost all kinds of industrial use, building automation etc. - including Ex applications and safety systems. Max-i is as cheap and simple as the simplest fieldbus systems like ASI (Actuator Sensor Interface), LIN (Local Interconnect Network), DALI (Digital Addressable Lighting Interface) and Dupline, but at the same time much faster and safer and in many ways, it outperforms even the most advanced fieldbus systems for low to medium speed operation (up to approximately 10,000 short telegrams per second).

Everything is very simple and straightforward. There are no complicated object models or connection sets, and the total specification fills only approximately 100 pages where approximately 1/2 is used for background material and annexes. As a comparison, the most popular fieldbus systems today have specifications of over 1000 pages! In fact, Max-i is so simple that the specification will be published free of charge for all non-commercial use. There is anyway nothing to hide for anybody, who has a storage oscilloscope. Because the entier interface can be made in one integrated circuit (IC) there is no need for product certification and registration or expensive and time consuming conformance tests - a very big advantage compared to other fieldbus standards. You simply buy a chip, put it into your product and are ready to sell with no additional costs!

Max-i is quite similar to CAN (Controller Area Network) and may with advantage replace it for all applications. Max-i is a little faster than CAN for polling values (1150 24-bit, fixed point values per second on a 500m bus versus 919). On the other hand, CAN is faster than Max-i for event driven transmissions (1838 values versus 1150), but this speed advantage has been paid with a lot of disadvantages:

  • CAN has a very critical timing for long cables (described later). This is the main reason for the speed advantage (for event driven transmissions)! It may be fast, but not reliable! CAN is simply not designed for propagation delay.
  • CAN generates heavy bias distortion in case of capacitive loading (described later).
  • CAN is much more expensive than Max-i, because a complete interface cannot be made in one single integrated circuit (IC) as it is the case with Max-i. For example, CAN requires a crystal-controlled oscillator and requires a power supply to convert the supply voltage to 5 V. Max-i uses an internal RC oscillator and uses the supply voltage (10-15 V) directly. The use of an RC oscillator lowers the speed due to the necessary tolerance (±9%), but it is the key to a single chip solution.
  • CAN is quite fragile due to the necessary crystal oscillator and therefore cannot be used in areas with vibrations.
  • CAN is very sensitive to contact contaminations due to the low transmitter voltage of only approximately 3.5 V.
  • CAN has a signal/noise ratio, which is approximately 25 times less than Max-i for a given line length.
  • CAN depends on termination resistors, so all the transmitter power is lost, and an error tolerant network is very difficult to make. Max-i does not use any termination resistors!
  • CAN is only able to transfer a very limited amount of power (if any) over the bus and can therefore not drive actuators directly. Max-i uses standard installation cables and any number of power supplies and can therefore supply an almost unlimited amount of power - in practice up to approximately 500 W. Max-i may be regarded as a 12 V supply with communication.
  • CAN is not deterministic and does not have a predictable response time.
  • CAN is not able to transfer data to more devices simultaneously in the same telegram and can therefore not guarantee 100% synchronization in case of for example positioning systems and stage light control. Max-i is able to send data to up to 256 devices simultaneously in one telegram.
  • CAN is only able to transfer 8 data bytes and is therefore hopeless for reading log files, download of new program versions, data transfer to many devices simultaneously etc. Max-i can handle an infinite number of bytes.
  • CAN is not able to switch groups of devices off by means of a common telegram. This makes it difficult to use for lighting control and for energy management where it may be desirable to switch for example electrical heating and wash machines off in periods with low "green" supply or very high power consumption.
  • CAN is not able to force an input to a wanted state or value in case of sensor errors. This increases the commission time.
  • CAN does not have a standardized way of programming calibration and configuration parameters for the various values. This makes it necessary with an added software layer, which has lead to numerous incompatible bus systems like CANOpen, DeviceNet, SDS, CANKingdom etc. With Max-i, it is possible with up to 1024 configuration, calibration, set-up parameters and status messages for each value.
  • CAN has a very complicated, but inefficient acceptance filter, which does not allow a SCADA system to select individual signals and pick out samples from a fast data stream.
  • CAN is not designed for demanding safety applications.
    • It only has a 15-bit CRC check. Max-i uses 22 bits plus a 7-bit Hamming check on the identifier.
    • It has no protection against masquerading, that is, a situation where one unit starts to behave like another unit due to an error.
    • It does not have a telegram serial number to detect loss of telegrams or telegrams inserted by hackers.
    Max-i is designed for extremely high safety in accordance with AK6 of DIN V 19250, category 4 of EN 954-1 and SIL 3 (Safety Integrity Level 3) of IEC 61508.

Max-i is faster than CAN in case of safety systems because Max-i does not require an added safety protocol layer! This may also be the case if a protocol, which uses the data field, is put on top of CAN like e.g. a DeviceNet protocol, and Max-i is much faster than CAN if it is used to transfer data to many devices simultaneously in one telegram. On a 1 km bus, Max-i is approximately half as fast as DMX512, but DMX512 is not a two-way multimaster bus and has no safety or error detection at all and is therefore not allowed for controlling pyrotechnics and laser lighting where audience or performer safety is at risk.

Using fieldbus systems for automation tasks gives a lot of important advantages compared to the usual point-to-point coupled systems:

  • It saves a lot of cables and cabling and thereby a lot of money.

  • Any errors are immediately detected. A traditional 4-20 mA signal may give wrong readings for years if water penetrates the cable or a junction box, and a traditional switch signal may not work on demand if it e.g. is shorted to ground.

  • It is possible with a far better diagnostics with warning and error telegrams.

  • It is fast and easy to add new actuators and sensors and calibrate them.

  • Because there is a standard communication protocol, it is easy to connect even very complex equipment from different vendors. Some investigations has pointed out that this reason is even more important than saving money!

  • If the fieldbus is connected to a wireless hand terminal, it is possible to use cheap sensors without local display. You simply type in the number of the sensor and can then read the value in the display. In this way, it is also easy to read hard-to-reach sensors and e.g. compare measuring values from parallel lines.

The disadvantages of using fieldbus systems has previously been:

  1. Reduced speed.
  2. Reduced reliability.
  3. Bad failure tolerance.
  4. Increased sensibility to noise due to very low signal energy and big signal bandwidth.
  5. Switch problems due to low voltages. Traditional mechanical switches need 48 V for a reliable operation in industrial environments, which means that a -24 V load voltage is needed in case of standard 24 V systems! Almost no control systems including fieldbus systems have that except for the Innovatic LC3000 module.

Although no fieldbus can completely match a hard-wired solution when it comes to these points, Max-i tries to cope with all these problems:

  1. It is fast enough for most applications.
  2. It is as reliable as most PLC I/O channels.
  3. It still works even if the bus is cut into more pieces.
  4. The signaling level is close to 12 V, which is much higher than any other fieldbus system and gives a better signal/noise ratio than any hard-wired 4-20 mA or 0-10 V signaling.
  5. The single chip interface makes it possible to build the interface into the sensors and in this way use magnetic activated switches or other switches without low-voltage problems.

The first Max-i evaluation boards, which simulate an 8-pin IC solution by means of standard components, are tested and works fine. It is expected that the evaluation boards and the preliminary specification will be released in the first quarter of 2010 for everybody, who wish to evaluate Max-i and have a possibility to influence the standard before the IC is made. Even though there are only 8 connections to the electronics (2 power rails, communication line, 2 inputs and 3 outputs), the evaluation board has a 26-position clamp row, which makes it very easy to make experiments. The big 4 mm2 clamps can handle bus currents up to 20 A and you can mount all kinds of NPN transistors or N-channel MOSFET directly in the clamp row and in this way drive loads up to 10 A (120W) without any other components except for a decoupling capacitor to limit the voltage fluctuations if heavy loads are driven directly from the bus. You can also connect the I/O's directly to LEDs, opto couplers and solid-state relays and even to most RS-232 ports. The evaluation board also has a 10-pin programming connector for a cheap Actel FlashPro3 programmer, which makes it easy to download new firmware versions as they become available.




The name Max-i means Multiple Access Cross-coupled Interface, but the name also refer to the fact that Max-i in many ways has maximum properties, which by far exceeds all other comparable bus systems.


Max-i-mum Economy

Max-i enables the lowest total automation cost ever seen! This is one of the most important properties since many customers primary focus on price. Therefore, Max-i has been designed to reduce the price of all parts of the automation process - not just the price of the bus interface.

  • With Max-i, it is possible with a complete bus interface in one single simple and cheap IC (Integrated Circuit), which gets its supply voltage directly from the bus and uses an internal RC oscillator (with ±9% accuracy). There is no need for crystals, resonators or any other external components except for a small decoupling capacitor - not even components for overload or transient protection as the connection between the IC and the actuator or sensor is not taken out of the unit. The one-component solution also saves the usual costs for an electronic production with printed circuit board, component mounting, soldering, testing etc. With Max-i, a complete bus interface may be cheaper than a normal general purpose I/O channel! The single chip interface makes it for the first time both technical and economical possible with a direct bus interface to all devices - even the simplest price sensitive ones like motor starters, valves, limit switches, push-buttons, lamps and sensors.

  • With Max-i, it is possible to control an entire process plant by means of one or more cheap Soft-PLC/SCADA (Software based Programmable Logic Controller / Supervisory, Control and Data Acquisition) systems, a bus power supply and a bus cable. This enables very big savings in the form of discrete PLC's, distributed I/O systems, mounting boxes, clamp rows, adapters, cabling etc.

  • Max-i saves a lot of money for cabling and mounting compared to other fieldbus systems.

    • Max-i may use cheap unshielded standard "green" installation cables with PE based or PE like conductor insulation (Er approximately 2.6). There is no need for expensive and troublesome shielded cables or for twisted pair cables! For use in cars and aeroplanes where there is a common chassis, it is also possible to use Max-i with either a coaxial cable for high-speed operation or a simple two-wire interface for low-speed operation. It is even possible to use a one-wire interface if the supply voltage is available in another cable.

    • In many cases it is only necessary with one cable, as it is possible to connect over 1,000 units (depends only on the semiconductor technology) and the cable may be up to 8 km long or 16 km in a closed ring - without repeaters!

    • It is not necessary with an extra cable for power supply as it is possible to transfer an almost unlimited amount of power over the bus (depends only on the conductor cross section). There is therefore no need for a galvanic separation (between bus and power supply), which is a condition for making a complete bus interface in one IC.

    • Because of an outstanding noise immunity there is no restrictions on where and how the bus cable may be drawn. In this way, it is possible to use existing cable ducts. Other fieldbus systems usually requires that the bus cable shall be drawn a distance from power cables, cables from frequency converters etc. or shall even cross such cables in a right angle. This may make it necessary to mount new cable ducts.

    • If an electronic motor starter and a safety fuse is integrated with the motor, even bigger savings are possible as the power cable may then also be drawn from motor to motor. This is the future way of doing automation! Note that because of the outstanding noise immunity of Max-i, the two cables (fieldbus and power) may be drawn in parallel in the same cable duct. This is not allowed with any other fieldbus system! Max-i is the only fieldbus, which is designed for practical industrial applications!

    • It is allowed to use drop cables (T-connections) up to 1/20 of the maximum cable length (50 m at 83.33 kBaud).

  • Due to a very simple possibility for making a self-healing ring (described below) it may be possible to make failure tolerant networks much cheaper than with other bus systems, which may require two cables and double interface to obtain the same wanted safety.


Max-i-mum
Power Transfer

Max-i uses a nominal bus voltage of 12-14 Vdc (minimum 10 V, maximum 15 V). This voltage is low enough for modern semiconductor technology, but high enough to enable a fairly big supply power, and it fits very well with all automotive and Ex applications. Besides, it is very safe as an arc cannot burn at 12 V since the arc voltage is approximately 11.5 V (this is the reason why the Ex standard stops at 12.1 V). At higher voltages, it may be very difficult to break a DC supply in case of a short circuit.

The industrial standard today is 24 V, but it is based on the requirements for mechanical contacts and electromagnetical relays (motor starters). In the future, it is expected that mechanical sensors will be replaced with much more reliable integrated magnetic sensors like hall elements build into the fieldbus interface, and the old starter contactors will be replaced with electronic soft starters or frequency converters where 12 Vdc is the ideal supply voltage for the low-voltage part.

Because Max-i may use standard installation cables, it is practical possible with an output power up to approximately 500 W. This makes Max-i to a true actuator bus with power enough for driving several motor starters, pneumatic and hydraulic valves, lamps etc. Most other fieldbus systems, which supplies the units through the bus cable, are really only sensor busses due to there very limited current capability of typical below 2 A. Such a low current may make it hard to make a direct bus interface to the various units because it may limit the number of units on each bus to a level, which is not appropriate. For example, a typical big feed mill may consist of 400 automatically controlled units. The most appropriate for such a plant is to use one bus for each part of the plant like e.g. one bus for the intake, one bus for the production line, one bus for pillet pressing and one bus for the final conveying, so approximately 100 actuators per bus are needed in this case plus even more sensors. This high number of devices may alone create problems for many other fieldbus systems, but the necessary current per bus may be over 20 A, which no other fieldbus than Max-i is able to handle! With most other fieldbus systems, it may be necessary with 20 or more bus lines for such a plant. This is very inexpedient, but with e.g. heating and power plants with several thousand signals, the necessary number of busses may be so high that the use of direct bus interface is not realistic!

The typical current requirement at 12 Vdc for a modern low energy actuator is:

Three phase 400 V contactor
Motor size Current
0-5 kW 300 mA
5-10 kW 500 mA
10-20 kW 1 A
20-40 kW 1.5 A
Pneumatic valve
30-150 mA

However, the power consumption of older (existing) actuators may be much higher.

Max-i may be regarded as as 12-14 V supply with communication. This makes Max-i the ideal choise for low voltage supply in low energy houses of the future. Today, most houses only have a 230 Vac supply, but many equipments could benefit from a 12-14 V supply instead of having each a clumsy 230 Vac transformer with often very low efficiency, for example equipment like:

  • Intelligent lighting control.
  • Window openers.
  • Intelligent sun shielding.
  • Electronic lock systems - even in hotels with hundreds of doors.
  • Coupled door bells with door indication.
  • Power supply for all kinds of telecommunication products including IP telephones, wireless phones and xDSL modems.
  • All kind of alarms including burglary alarms and baby alarms and coupled smoke detectors with battery backup, which is a demand in all new buildings in Denmark.
  • All kind of battery chargers for mobile phones, cameras, MP3 players, toys etc.
  • Power supply and battery chargers for laptop computers.
  • LED lighting.
  • Small power-saving bulbs.
  • Small halogene lamps like desk lamps, which today have a fairly big stand-by consumption because the switch is mounted after the transformer.
  • Scanners.
  • Aquarium pumps, water art, small fountains etc.
  • All kind of toys including LEGO, doll's houses, electrical trains and race tracks etc. - even with communication. In this way, 230 Vac outlets may be switched off in kids rooms to prevent shock hazard.
  • Weather stations.
  • Heat recovery equipment with low energy fans partly driven by solar cells.

In the green house of the future, 230 Vac outlets are mounted in all outher walls and in the ceiling, and Max-i outlets with intelligent lighting control are used in all inner walls.

Calculations done by the Danish engineering company Rambøll indicates that a 12 V network partly driven by solar cells can save Danish households in the order of 1 billion Danish kroner per year corresponding to approximately $200,000,000.

Max-i is also very useable as a power supply network for mobile phones and laptop computers in for example trains and busses. The communication makes it possible to transfer low to medium speed information like information about route, next stations, expected arrival etc. to over 1000 units on each line.


Max-i-mum
Number of Units

The maximum number of units depends only on the used semiconductor technology (leakage currents and capacitance)! This makes it possible with several thousand units on one bus. Together with the very high amount of power transfer, this makes it possible to keep the number of bus lines at a realistic and appropriate low level.

With Max-i, it is usually not necessary with receptacle busses, hubs or routers.

By means of a brand-new equipment identification system - PNS (Plant Numbering System - see Plant Numbering) it is very easy to manage the high number of units and signals. Unlike all other fieldbus systems (except perhaps for CAN-B), Max-i has been designed to use addressing directly by means of machine numbers instead of the usual special bus address (typically 0-63, 0-127 or 0-255) or a long unique serial number (usually 48-64 bit). For example, HX361T2 means Heat Exchanger 361 Temperature 2. The machine number is known from the drawing and is therefore easy to type in by means of a hand terminal, and if this is done during the mounting of the device, there is no risk for any confusions. With direct addressing on machine numbers, there is no need for the usual huge cross-reference tables, and it saves the trouble with managing bus addresses and recording serial numbers during commissioning and service (device replacement). It is also a great advantage in the daily work where it makes it possible to remote control the equipment by means of a simple Web- or SMS-interface to a mobile phone. If you stand in the middle of nowhere and receives an error message from the plant, it is much easier if the error identifies itself as HX361A3 (alarm 3) than for example point 127! The machine numbers use 31 bits, but for very time critical data, PNS also defines a 12-bit quick number format, which is compatible with the long machine numbers and may be used on the same bus. The 31/12 bit addresses is compatible with the CAN A and B 11/29 bit addresses so that existing CAN identifiers and numbering systems may be used on Max-i. In fact, it is possible to run for example a DeviceNet or CANOpen protocol on the quick-numbers simultaneously with PNS (machine numbering) on the same bus - although there is no reason to do so. It will only add a complicated software overhead, without gaining anything compared to what Max-i offers as standard.


Max-i-mum Efficiency

High efficiency is the keyword for a good fieldbus system. Just increasing the speed for getting more data through, as it is common practice with many of today's bus systems, leads to a critical bus timing and a bad signal to noise ratio, because the energy in each communication pulse is reduced. Remember that energy is voltage multiplied by current multiplied by time. Each time the baudrate is doubled, the energy in each pulse is reduced to the half, but the bigger necessary bandwidth means that the noise energy is approximately doubled so the signal to noise ratio is reduced approximately 4 times! Therefore, Max-i has been designed for efficiency rather than speed! What really counts is the number of telegrams per second. A high communication speed is just a means - not the aim!

  • Max-i is a multimaster bus based on the CSMA-CD+CR principle (Carrier Sense Multiple Access with Collision Detection and Collision Resolution). This gives a much better utilization of the bandwidth than any TT (Time Triggered), TDMA (Time Division Multiple Access), Token-passing, Token-Slot or master/slave networks. There are no dedicated master or slave units, no channel generators etc. and only one type of device! Any number of bus nodes can request the bus simultaneously and any node may initiate a transmission. This enables very fast response times even at a low baud rate and with many units. The bus arbitration is non-destructive so that no bandwidth is lost in case of a collision.

    The multimaster concept also makes it possible to connect e.g. a separate web interface, a debugger or a bus tester. This is extremely practical during commissioning or in those situations where a vendor wants access to a unit for e.g. remote service, but the plant has no common web-interface.

  • Unlike e.g. TT, TDMA, Token-passing and Summation-frame networks it is not necessary to reconfigure the bus when units are added or removed or if a unit fails. The only exception is when Max-i is used to transfer data to many devices simultaneously in the same telegram.

  • Max-i uses synchronous communication so that no time is wasted on start- and stop bits.

  • Max-i has a unique polling method where it does not take more time to poll a value than to send it event driven! This increases the efficiency a lot compared to most other bus systems - especially with many safety values or analog measurement values, which are typically polled.

  • Max-i uses the very efficient producer/consumer model known from CAN. With this model, it is each value, which has an address - not the bus node! This address is called the identifier. Most telegrams like input data from the process may be regarded as broadcast information no matter if they are initiated by an event or polled! Many units may therefore receive the same message and benefit from data requested by other units. This is very useful in e.g. redundant SCADA systems where the method efficiently reduces the necessary number of telegrams and guarantees data consistency - even without a common database - so that all units show exactly the same values. It is also very efficient for synchronizing e.g. clock, program execution, set points etc. in more units, and it is very efficient for information systems where the same information shall be shown on many displays. As every single value - even Boolean values - has its own identifier, there is no need for AND and OR functions etc.

    The producer/consumer model has the further advantage that there is no need for address stacking and address conversion in case of gateways between different bus systems. This reduces the length of the telegrams and makes the gateway function simple and fully transparent, so that a gateway may be added or removed at any time without any changes in the bus nodes.

  • The Max-i telegrams are very compact with a very little overhead, and it uses separate error and warning telegrams so that it is not necessary to include diagnostic information in all telegrams. With the short identifier, it is only necessary with 7 bytes to transmit a fixed point 24-bit process value in SI-units, and Boolean 2-bit values may be transmitted in only 5 bytes.

  • Max-i is able to group devices together in up to 255 different groups, which may be switched off and back on by means of a common telegram. This is very useful in lighting control where it is very desirable to be able to switch groups of lamps and devices with potential fire hazard off when you leave the house or go to sleep. It is also very useful for energy management where it makes it very easy to switch for eksample electrical heating and wash machines off in periods with low "green" supply or a very high power consumption.

  • To make it possible to transfer as many telegrams per second as the network length allows, Max-i has 8 standard speeds. The table below shows the relationship between the bus length, the minimum cable cross section, the number of telegrams per second with 12-bit and 31-bit identifiers, the Max-i baud-rate and the communication speed when a Max-i controller is connected to a serial port e.g. by means of an RS-232 or RS-485 interface.

    Network Length
    km    		 Min. Cross
    Section     mm2    		 12-bit
    identifier    		 31-bit
    identifier    		 Baud rate
    kBaud
    Linear
    topology    		 Closed
    ring    		 Distri-
    buted    		 Point-
    Point    		 2-bit
    data    		 18-bit
    data    		 2-bit
    data    		 18-bit
    data    		 Max-i UART
    8 16 6 1.5 90 70 55 50 10.42 19.2 (14.4)
    4 8 2.5-4 0.75 180 140 110 100 20.83 38.4 (28.8)
    2 4 1.5 0.38 350 280 220 200 41.67 57.6
    1 2 0.75 0.19 700 570 450 400 83.33 115.2
    0.5 1 0.38 0.19 1400 1150 900 800 166.7 230.4
    0.25 0.5 0.19 0.19 2800 2300 1800 1600 333.3 460.8
    0.12 0.24 0.19 0.19 5600 4600 3600 3200 666.7 921.6
    0.05 0.1 0.19 0.19 11200 9200 7200 6400 1333.3 1843.2

    As 14.4 and 28.8 kBaud are not standard UART speeds in the PC world, a higher speed than necessary is used in these cases.

    The minimum cable cross section in square mm implies that the supply voltage is kept within the nominal range by means of a sufficient number of power supplies and/or capacitors. The minimum cross section is 0,19 mm2 (24 AWG) corresponding to a conductor diameter of 0.5 mm. Note that for point-to-point communication where there are no units in the middle of the cable, it is possible to use a four times thinner cable.

    The 18-bit telegrams are used for transmission of process values in (modified) SI-units (meter, kg, °C, Bar, Volt, Ampere etc.) with an accuracy of up to 17 bits plus sign. In practice, this is all process values except perhaps for weigher signals. Only 18 bits are needed, because the (fixed) scale factor (exponent) (see PNS data type FIX) is embedded in the Max-i fieldbus protocol. This improves the efficiency and saves two bytes compared to the IEEE 754 single-precision floating-point format.

    The number of telegrams per second for a given network length is quite outstanding compared to many other fieldbus systems, and in many cases it is more than most SCADA systems are able to handle - especially with more bus lines! A typical Windows or Linux based SCADA system can only handle approximately 300 telegrams per second! Therefore, Max-i has two group of signals - high speed local signals and low to medium speed signals intended for SCADA systems. Unlike CAN, it is possible for a SCADA system to pick out samples from a high speed signal without being overloaded.

    Even though Max-i is a multimaster bus based on a simple RC oscillator and has a 22-bit CRC (Cyclic Redundancy Check) and separate error, warning and configuration telegrams, it is approximately as efficient as ASI, which is a crystal controlled singlemaster bus with only a simple one-bit parity check and no possibility for error, warning and configuration telegrams. On a 100 m bus, ASI is able to address 31 units with 4-bit data (124 bits) every 5 mS. If a corresponding 6-bit identifier (64 units with 2-bit data = 128 bit) and the same cable length were used on Max-i, it would be able to transfer the same number of bits every 6.4 mS. This is 27% slower than ASI, but because Max-i is an event driven multimaster bus, it would be much faster in practice with a response time approximately 25 times less than ASI (0.1 mS compared to an average of 2.5 mS)!



Max-i-mum Bus Length

The maximum bus length without repeaters (8 km) is very close to the maximum length of other copper based fieldbus system with repeaters. For example, the absolute maximum length of a copper based Profibus with repeaters is 10 km and the maximum length of Interbus is 13 km, but Interbus uses all units as repeaters.


Max-i-mum
Industrial
Applicability

Max-i is the only fieldbus, which is 100% compatible with all kind of industrial environments!

  • It is allowed to draw the bus cable in parallel with any other cable so that it is not necessary with new cable ducts.

  • For industrial applications, Max-i uses a fully floating transmission line, that is, no part of the bus has a direct ground (earth) connection. This is very important in industrial environment where the bus cable is probably unable to handle a short circuit current. However, it is also very important even during normal operation! Many process plants use power networks with a directly grounded neutral (TN-C system), where a common neutral and safety conductor (PEN) is used instead of a separate protective grounding. If there is any difference in the loading of the 3 phase's in such a system the PEN conductor will draw current. During worst case conditions, where the currents are not sinusoidal, they may add together so that the PEN conductor theoretically seen may draw up to 3 times the current in a phase conductor - 2 times is quite common in practice. The current in the PEN conductor will create an AC voltage between different chassis parts and a current may then flow between these if they are connected. Depending on the impedance level, the current levels may be quite high. More than 20 Aac has e.g. been measured in a water pipe! It is obvious that it is not a good idea to connect e.g. a cable screen between different chassis parts of such a process plant and in this way establish a low impedance current way, which may disturb the communication and may heat or even melt the cable. Nevertheless, this is common practice with almost all shielded fieldbus systems!

    Note, that with a fully floating bus nothing is gained by using a shielded cable! This saves indeed a lot of money and problems!

    To avoid electrostatic build-up, to avoid breakdown of the galvanic separations in case of a powerful transient and to be able to detect a failure current, Max-i is connected to ground through a network consisting of resistors and heavy duty spark gabs (gas discharge tubes). The voltage on the network is supervised and an alarm is activated if the leakage current exceeds a given limit. Due to the floating cable, most transients will not cause a high current in the cable and will therefore not stress the protection components.

  • Max-i is also directly usable in hazardous areas (Ex area) from class IIC (the strictest class) without any other changes than a special intrinsically safe 12 V Ex power supply and perhaps a reduced network length (due to capacitance). All other data remain unchanged so that intrinsically safe units are also efficient for non-hazardous applications - unlike e.g. IEC 61158-2 based systems! Because there is no minimum current consumption it is possible with several hundred units in EX class IIC environment, and the response time may be as short as 100 µS (at 50 m) making Max-i to the fastest self-powered Ex bus.


Max-i-mum
Signal/Noise Ratio

Max-i has an unsurpassed noise reduction and a signal/noise (S/N) ratio, which e.g. is approximately 150 times better (at 83.3 kBaud) than IEC 61158-2 based systems and approximately 25 times better than CAN (at the same line length). There are more reasons for this:

  • Max-i uses transmitter pulses with a very high energy of approximately 12.5 µJ at 166.7 kBaud on a 500 m bus. On the same bus length, CAN has a pulse energy of 0.5 µJ at 125 kbit/s.

    In spite of the big transmitter power Max-i has a low radiated emission (EME) because of a very efficient bit coding and a special spread-spectrum technique, which spread out the radiated energy over a wide frequency range. The highest fundamental frequency is only 41.7 kHz at 83.3 kBaud so that the EME drops with 6dB/octave from this frequency.

  • The receiver has a 4.5V hysteresis (at 12 V) and an EMI (Electro Magnetic Interference) filter with approximately 1/2 pulse width like a UART.

  • Max-i does not use any termination resistors! This creates many reflections, but due to a unique technology, Max-i utilizes the reflections to improve the S/N ratio.

  • Unlike e.g. CAN based systems, the data coding is fully symmetrical and has no bias-distortion and no need for equalizers or preemphasis.

    Bias distortion occur if the charge and discharge energy of the line is not the same, and the transmission line behaves like a low-pass filter due to e.g. capacitive loading, drop cables or different cable types. The drive impedance of a dominant CAN-bit is only a few ohm, but the impedance of a recessive bit is equal to the line impedance, that is, 60 ohm for high-speed CAN and as high as 4.4 kohm for low-speed CAN. This create a difference in the rise and fall time, and the line will have a tendency of being charged against the dominant state. This tendency is further strengthened by the NRZ (Not Return to Zero) coding of CAN, because NRZ contains DC. A row of dominant 0-bits may charge the line so much that it is impossible to detect a following recessive 1-bit.

  • Unlike e.g. CAN based systems, the bus timing is uncritical even at maximum bus length and maximum clock inaccuracy! The average margin is 1/2 pulse width like a UART.

    The CAN-bit is divided into a short synchronization segment, a propagation segment and two equal phase segments. The sample point is located between the two phase segments. Because the length of the propagation segment shall be equal to two times the maximum delay from one unit to another, the length of the phase segments must be reduced when the line length is increased. This makes CAN very critical at long transmission lines. At e.g. 125 kbit/s and a 500 m cable, the bit length is 8 µs, but the propagation segment must be at least 5 µs with PE insulated cables, so there is only 1.5 µs left for each phase segment. This makes the timing approximately 3 times more critical than with short cables! With PVC insulated cables, the propagation segment shall be at least 7.2 µs, which makes a reasonable timing impossible. Note that many CAN cables are PVC insulated! In case of propagation delay and more units, which tries to access the bus, the width of the dominant bit may be increased with up to two times the propagation delay and the width of the recessive bit reduced correspondingly. If there is also some bias distortion, the signal integrity may be completely destroyed as the recessive bit may be almost wiped out as shown below. CAN is really not designed for propagation delay!



Max-i-mum Reliability

For several reasons Max-i is one of the most reliable fieldbus systems.

  • The unsurpassed S/N ratio makes Max-i to a zero-error system, that is, a system, which under normal circumstances should have an error rate of zero. Other fieldbus systems typically have average failure rates of approximately 10-7, however, there is a tremendous difference between zero errors and just a single error in time. If one error can occur so can 2, 3, 10 or even hundreds or thousands. The error probability (P) depends heavily on the S/N ratio. It is typically of order e-(S/N), where the noise power N is proportional to the baud-rate. This is the reason why Max-i focuses on efficiency rather than speed! If the S/N ratio is changed a factor k the new error probability may be calculated as:

    Pnew = e-k(S/N) = [e-(S/N)]k = Poldk

    If e.g. the error probability is 0.01 and the S/N ratio is increased 3 times (k=3) then the new error probability will be approximately 0.013 = 0.000001. In the same way, the difference between 1 and 3000 errors in a system with an average error rate of 10-7 is only a factor 2 increase in noise level or a factor 1.4 increase in transmission speed!

  • The very short telegrams increase the reliability because it is more likely that an error strikes a long telegram than a short one.

  • The simple single chip interface to the various devices enables a very high reliability because no crystals, optocouplers or other low reliability components are needed. If the IC is mounted in a socket, no soldering process is needed and there may therefore be no solder joints to fail - a very common error source, and the components are not thermally stressed during the mounting.

  • With the high number of units on one bus and the absence of termination resistors, it is not necessary with routers etc. This also increases the reliability. Other bus systems like Ethernet or systems based on light-guides uses point-to-point connections from each device to a router so that the telegrams may have to pass several routers and with that a lot of electronics on its way.

  • The high transmitter power keeps the communication well free of the low-signal range below approximately 5 V where a reliable function of connectors etc. cannot be expected. This is a big problem with the majority of all other fieldbus systems like e.g. Ethernet, all CAN and RS-485 based systems and systems based on LVDS (Low Voltage Differential Signaling).

With Max-i, it is even possible to obtain a higher reliability than a normal PLC (Programmable Logic Controller)! From a reliability point of view, it does not matter if the bus, which connects the I/O's (Input/Output) to the CPU (Central Processing Unit), is a short fast parallel bus internal in a PLC or a long relatively slow serial fieldbus. A typical PLC with a 16-bit internal bus and 16 I/O channels per card uses statistically approximately 1-2 bus transceivers per channel plus one optocoupler. Max-i uses of course only one bus transceiver per channel and saves the optocoupler. In this way, it is possible to obtain at least the same per-channel reliability as with a PLC. Almost no other fieldbus system offers this degree of reliability.


Max-i-mum
Fault Tolerance

The absence of termination resistors makes it possible to make a fault tolerant self-healing ring. This may be done in two ways:

  • Any network in closed ring topology with a total length less than the maximum length in linear bus topology - e.g. 1 km at 83.33 kBaud - will automatically be self-healing. In case of a line break, it will just be converted to a network in linear bus topology. The disadvantage of this method is that no alarm is generated in case of an error.

    This simple type may also be used to enable rework on the network during normal use without interruption of the communication.

  • A more advanced self-healing ring with alarm can be made with a network in linear bus topology, which is bend together so that the SCADA system is connected to both ends by means of two separate interface channels. During normal circumstances the two channels will receive the same telegrams, but if the bus is broken, they will only receive telegrams from there individual half's. Hereby the error it is immediately detected and the SCADA system starts working as a repeater, which retransmits the telegrams to the opposite half. By that means, not even a single telegram is lost and the error position may be found quickly and automatically by polling the units. This solution is of course only possible if the SCADA system can follow the transmission speed.

In both cases, a very high function safety (not just failure safety) is obtained, which by far exceeds all bus systems, which depends on termination resistors, like e.g. all CAN, RS-485 or LVDS based systems.

With the producer/consumer model and the direct bus interface to the various devices, it is very easy to make redundant systems because any number of SCADA systems may be connected to the same bus and may utilize the same telegrams and I/O units. A further advantage is that such a system does not depend on common bus couplers as with distributed I/O systems or depend on a central database.

It is possible to change the input values temporary e.g. to compensate for sensor errors or to simulate the presence of material. This is extremely useful during commissioning and can save a lot of time and money.

With the long 31-bit PNS identifier the probability that e.g. an error on a drawing or a typing error creates a new legal identifier is extremely small as less than 1ppm (parts per million) of the possible identifiers are utilized! With traditional bus systems (or quick numbers), the probability may be close to 100%!


 Max-i-mum Safety 

A very efficient priority system with 4000 levels (12 bits) ensures that the most important telegrams comes first, and a unique "babbling-idiot" protection ensures that no unit or units are able to saturate the bus. All telegrams always come through. Max-i is therefore fully deterministic unlike e.g. CAN and Ethernet! The maximum response time is as short and predictable as e.g. TT, TDMA, Token-passing and master/slave networks, but the average response time may be several hundred times faster depending on the number of units! The "babbling-idiot" protection has the further advantage that it is usually not necessary to worry about priority levels. Unlike CAN, the various identifiers may therefore be chosen freely, which e.g. is a great advantage if PNS or another numbering system is used.

To ensure that no unit or SCADA system is using/showing obsolete information, Max-i has a standardized time-out system, which makes the received values invalid after a given amount of time. This also makes it unnecessary to poll all units to check if they are alive. A hardware watch-dog timer on each channel may e.g. be used for setting the output low if the output has not been updated or there has not been detected any error free bus communication within a given amount of time.

For safety systems in accordance with AK6 of DIN V 19250, category 4 of EN 954-1 and SIL 3 (Safety Integrity Level 3) of IEC 61508, the total system error probability shall be less than 10-7 per hour (and cannot be claimed to be better than 10-8). To guarantee this, Max-i uses a very special, but very simple 22-bit CRC check invented by Innovatic and developed in collaboration with the Technical University of Denmark - DTU. This alone reduces the test time to less than 11 hours. If no errors are detected during this time, the demand is fulfilled with the required statistically confidence level of 99%. However, Max-i also has a 7-bit Hamming check on the identifier, which further improves the probability for error detection. Because a safety system trips no matter which error occurs, this means that if the equipment runs without very frequent error trips, the demand is automatically fulfilled. If the long 31-bit identifier is used, it is possible to transmit 1 data byte and therefore all Boolean values with hamming distance 8, that is, all errors up to 7 bits are detected with 100% probability. The hamming distance is 6 up to 25 data bytes and 4 up to 262143 data bytes. If the short identifier is used, it is of course possible to transmit two more bytes with each hamming distance. With minor limitations, it is even possible to transmit an infinite long telegram with hamming distance 4, 6 or 8! Unlike many other industrial fieldbus systems, it is therefore easy to download programs and big data files.

For these kind of safety systems it is required with two-channel redundant and diverse hardware and data processing. Due to the direct bus interface, the multimaster concept, the producer/consumer model, the fast polling system and the standardized time-out system it is possible to obtain this without a special protocol or special hardware - except for one or more standard safety/emergency-stop relays.

To fulfill the requirement for redundancy, a safety switch or device must be coupled to the Max-i controller by means of a two-channel interface. Max-i always uses 2-bit Boolean values so the Max-i interface circuit always has two input channels. These channels may be used either for Boolean values or for input channels from a (synchronous) voltage to frequency converter for analog values. Each of the two channels has a fully redundant signal processing. Because of the hardwired connection and the direct bus interface it is not necessary with a further check on the data. With traditional safety systems it may be necessary with test pulses etc. on all inputs to ensure that they are functional, but this is not necessary with a direct bus interface, because the connections between the safety devices and the bus interface are not taken out of the unit so that short circuits etc. are not possible.

The receiver may be made redundant and diverse simply by using two PLC's from two different vendors programmed by two different programmers. In this way, it is very unlikely that the two channels have the same hardware and/or software bugs. Because of the producer/consumer model and the fast polling system, which ensures that all data or the lack of a poll answer are received exactly simultaneously in the two PLC's, it is very easy to combine the outputs from the two PLC in one or more standard safety/emergency-stop relays with positive-guided contacts. It is also very easy to supervise the two PLC's and programs and generate an alarm if only one PLC sets an output low.

It may be possible for telegrams to:

  • get lost
  • appear repeatedly
  • be inserted additionally
  • appear in the wrong order
  • appear as another telegram (masquerading)
  • be delayed
  • contain destroyed data

The 22-bit CRC detect destroyed data as well as communication errors, and Max-i uses a transmitter signature with a 7-bit Hamming check on all telegrams to protect against masquerading (point 5) and further increase the probability for error detection beyond what is possible with the CRC check alone. Max-i also uses a telegram serial number on all safety-related telegrams. This protects against the first four points and further improves the safety against masquerading. In this way, it becomes possible to combine safety devices and normal devices on the same bus!

The response time to an event will usually be less than 500 µS with a 125 m bus, however, the safety depends on the standard time-out system. If a PLC has not received data from a safety device for a given amount of time, it must shut down the system. This mechanism also protects against delayed telegrams. To increase the efficiency, Max-i has a heartbeat timer, which ensures that all values are transmitted at regular time intervals from 0.1 second to 10 minutes. It is therefore not necessary to poll the values to find out if a device is alive.

Because Max-i is a pure producer/consumer system, it has no acknowledge telegrams. It has no meaning to acknowledge a telegram, which can be received and utilized by more units. If the issued telegram is not received, but transmitted correctly, the transmitter anyway cannot do anything, even if there were an acknowledge telegram - except for logging the loss of telegram for later use when the communication channel (the bus) becomes functional again. However, due to the telegram serial number the loss of telegram will be detected the next time the data are polled or received.

The telegram serial number also protects against hacking as this is detected when the original device transmits its data (heartbeat) with an "old" serial number. The only way to fool the system is to transmit so many hacking telegrams that the counter wraps around, but due to the babling idiot protection this is a very uncertain method, which is also quite easy to detect. Hacking protection is essential for burglary alarm systems.


Max-i-mum
Compatibility

Max-i and PNS has been designed to enable an unambiguous and loss less conversion between XML (Extensible Markup Language) and the compact fieldbus protocol. Even the shortest Max-i telegrams contain enough information - like e.g. the data type - for a complete conversion to XML. If e.g. a button with the PNS number AV8SFB12EH101 is pressed this may cause the following XML telegram: 

<siteData>
   <siteArea name="Avedoere 8">
      <ProductionLine name="Solid fuel boiler 12">
         <equipment name="Emergency stop 101">
            <function name="Value 1"
                      value="01B"
                      datatype="BOOLEAN"/>
         </equipment>
      </productionLine>
   </siteArea>
</siteData>

If the bus length is 1 km, Max-i is able to transfer approximately 450 of these telegrams per second - more than enough for almost any SCADA system and database!

There is no doubt that XML is going to be the future standard for automated data exchange. Many office programs like StarOffice and OpenOffice store their data in XML and even Microsoft is now turning over to XML in their new .NET technology and in the pro version of Office 2003. The benefit of XML is that it is a very simple text based language, which may be used on many different platforms and generated very easily. Unlike traditional method based systems, XML is self-describing with a name and description for each value and meta data telling how the telegram shall be interpreted. It is therefore not necessary to have an exact number of arguments and to supply them in a precise order, so the various applications may be much looser coupled than with e.g. DCOM (Distributed Component Object Model). Because each value has a name, XML fits perfectly together with fieldbus systems using the producer/consumer model like Max-i and CAN. Both XML and Max-i has a variable data length, but without a data length information.


Max-i Organization

Max-i is a completely open standard, which may be used without royalty by any end user, and by any manufacturer or vendor who is a member of Max-i Organization. For more information on Max-i Organization (or Max-i), please contact Innovatic.