This page is updated May 3rd 2012 and optimized for 1280x1024 pixels.
It is only available in English.

Welcome to the official home page for the Max-i fieldbus
and Max-i Association.

NOTE! Specification v4.0 has been released!

• Addition of flash synchronization.
• Signature on network telegrams changed to enable simple devices to issue such telegrams.
• Enhancements to enable fieldbus connections in for example railway applications.

Overview

Max-i is a new, very cheap, but extremely powerful fieldbus, which enables the lowest, total, automation costs ever seen combined with maximum performance! It may be regarded as a combination between a highly improved CAN bus and a 12-V power supply and may be used for virtually all low to medium speed applications like:

• Industrial, building and home automation.
• Transportation applications such as automotive, off road vehicles, railway, ships and aerospace.
• Supervision on bridges and tunnels.
• Stage light control.
• Intelligent LED lighting.

Max-i has four basic properties:

• It is possible to make a complete bus interface in one integrated circuit (IC), which is small (3-8 pin) and cheap enough to be build into even the smallest and most price sensitive actuator, sensor or lamp, gets its supply voltage of 12 V directly from the bus and doesn't need any other components than a decoupling capacitor over the supply voltage rails. This enables a whole new world of single-chip sensors and it not only saves the traditional distributed coupling boxes, components for transient protection and a lot of cabling, but also the expensive and time consuming conformance tests, which are required by most other fieldbus standards. It also makes it possible to do with only one bus in automotive applications because Max-i is as cheap as LIN, more powerful than CAN, as deterministic as TTCAN (Time Triggered CAN) and for a given amount of money, it offers the same speed and much higher safety than FlexRay and its predecessor Byteflight.

• It is possible to use cheap, standard, unshielded installation cables instead of special communication cables. This not only saves a lot of money, but also makes it possible to transfer much more power over the bus than any other fieldbus - in practice up to approximately 450 W on 6 mm² cables! Because the cable is not shielded, the traditional dilemma with the shield connection is avoided. For the sake of noise immunity, a shield must be connected to ground/chassis in both ends, but it is simultaneously a very bad idea to establish ground loops and potential equalization between different parts of the plant through a thin fieldbus cable - especially in plants, which use a TN-C net with a common neutral and protective earth! Nevertheless, it is common practice with most other fieldbus systems!

• The total reliability of the bus with single chip interfaces is at least as high as the internal parallel or serial bus in a microprocessor plus the necessary I/O cards. This enables the replacement of hundreds of cables and conductors with a few fieldbus cables without any sacrifice in reliability if the application uses a microprocessor.

• It is the first fieldbus designed directly for highly demanding safety applications according to IEC 61508 SIL 3. A lot of fieldbus systems are approved to this level, but they all obtain that by means of an additional safety protocol on top of the fieldbus, separate safety monitors etc. In Max-i, all data processing for simple functions supported by the chip like Boolean I/O, A/D conversion and SPI interface is done entirely in hardware, which cannot "go down" in the same way as software and is much more predictable and failure tolerant. If a transistor fails or there is a logical bug in the design, you will only lose the function(s) that transistor or bug is a part of. In a microprocessor, all data processing is done in the same core (CPU) and it may be possible for one program to overwrite the memory of other programs so it is much more likely that you lose everything.

The name Max-i means Multiple Access Cross-coupled interface. It refers to Max-i being an event driven multiple master bus (no dedicated master and slave units), which for industrial applications, railway applications, stage light control and long distance communication uses a cable with 4 conductors connected as a balanced 4-wire line (X-coupled). For applications in homes, a partly unbalanced, 3-wire cable is used, and for automotive applications it is enough with a 2-wire line. The name also refers to the Max-i-mum performance, which by far exceeds comparable fieldbus systems on most fields.

Simple, but Powerful

Max-i combines the outstanding performance with a beautiful simplicity. There are just a few registers to setup and a new numbering systems - PNS, which allows the various process values to be addressed directly as properties of the equipment to which they belong like for example HX127AT4 for Heat Exchanger 127A Temperature 4. The Max-i specification fills only 124 pages where approximately half is used for background material and annexes. As a comparison, most other fieldbus systems have specifications way over 1000 pages, which requires several months to study and implement. The specification can be downloaded from this page and used free of charge for all non-commercial use. Commercial use requires membership of Max-i Association.

Highly Improved CAN

Max-i is very similar to CAN and is actually able to run any CAN protocol although it has its own, but as can be seen from the table below, it is not just much cheaper, but also much more powerful, reliable and safe.

Comparison between CAN and Max-i		CAN	Max-i
Economy	Possibility for cheap, single chip interface	No	Yes
	Product certification and registration needed	Most protocols	Not necessary
	Use of cheap, unshielded, standard installation cables	No	Yes
	Maximum practical power transfer	384 W at 24 V 1)	≥450 W at 12 V
	Maximum practical number of devices per bus	64-127	≈1000
Environment	Power saving mode / sleep mode (≤0.5 mA)	Only partial network	Not necessary
	Group switch off of Boolean outputs	No	255 groups
Data	Multiple master bus with bit wise bus arbitration	Yes	Yes
	Producer/consumer model	Partly 2)	Yes
	Identifier length	11 or 29 bit	12 or 31 bit
	Multiple use of same identifier	No 3)	Yes 3)
	Local and global data and global poll of local data	No	Yes
	Possibility for temporary change of (erroneous) values	No	Yes 4)
	Maximum number of bytes	8	516 or infinite
	Different data to more devices in same telegram	No	Yes 5)
	Specified layers of OSI 7-layer model	1, 2	1, 2, 3, 4, 6, 7
	Setup attributes per I/O (OSI layer 6)	0	16-1024
Reliability	Uncritical timing on all bus length and nothing to setup	No	Yes
	Timing not affected by galvanic separation/insulation	No	Yes
	No bias distortion at capacitive loads (symmetrical drive)	No	Yes
	No termination resistors = high failure tolerance	No	Yes 6)
	Possibility for closed ring topology	No	Yes 6)
	Unshielded cable = no ground loops and potential equalization	No	Yes
	Tolerant to contact or conductor failure	No	Up to 2 contacts 7)
	Contact fritting	No 7)	≈0.1 µm 7)
	Typical transmitter power / total power loss	0.2 W / 0.4 W	4.6 W / 2.6 W 8)
	Receiver hysteresis	0.1-0.2 V	3.0 V
Safety	Error detection	15-bit BCH (CRC)	22-bit CRC
	Protection against masquerading (wrong identifier)	No	7-bit Hamming code
	Detection of wrong number of telegrams	No	8-bit serial number
	Predictable response time (babbling idiot protection)	No	Yes
	Designed for IEC 61508 SIL 3 without additional layers	No	Yes
Speed	2-bit / 24-bit polled values/s on a 500 m bus	1225 / 1059	1270 / 930 9)
	2-bit / 24-bit event driven values/s on a 500 m bus	2272 / 1760 10)	1270 / 930 9)
	2-bit / 24-bit polled values/s at maximum speed	9800 / 8472	40640 / 29760 9)
	2-bit / 24-bit event driven values/s at maximum speed	18176 / 14080 10)	40640 / 29760 9)

¹) Requires a very expensive thick DeviceNet cable (12.2 mm with 15 AWG / 1.65 mm² conductors for DC).

²) CAN uses the producer/consumer model, but many protocols such as DeviceNet and CANopen need to establish a communication channel between devices before communication can take place and may even divide the network into masters and slaves.

³) Without this feature, it is not possible to make for example two-way/landing switches for LED lighting, and it is not possible to have more control buttons for the same process function or the same function in for example coupled trains, but most (all) CAN protocols such as DeviceNet and CANopen actually has a feature to prevent multiple use of the same identifier!

⁴) Most process values use 2, 18 or 34 bits, and it is possible to change a value temporary, which can save a lot of time during commissioning in case of sensor errors. It also makes it possible to run tests without material simply by simulating the presence of material.

⁵) Max-i can send individual 16- and/or 32-bit values to up to 256 devices in one common telegram and in this way ensure 100% data synchronization and a very high efficiency for example for motion control, positioning systems and for stage light control where Max-i with advantage can replace DMX512.

⁶) In Max-i, the traditional termination resistors have been replaced by multi diode clamps in each device. This gives a very high failure tolerance even without multiple communication lines as the bus may be cut in as many parts as there are power supplies, and each part will still work! A closed ring with a length less than half the maximum length can be cut at any place without any loss of communication. The diode clamps also reduce the power loss in the line termination to approximately the half compared to termination resistors and they utilize the reflections to improve the signal waveform and prevent bias-distortion due to noise rectification.

⁷) In case of a balanced 4-wire line, where the two communication conductors are connected together in all devices, Max-i will usually survive a failure on one of these conductors or connectors. If more power supplies are used, Max-i may also survive a failure on one of the supply lines so that Max-i is able to survive a failure on two neighbor conductors or connectors.

Usually, a fritting voltage of approximately 100 V/µm is required to burn through contact corrosion. Since the supply and communication voltage of Max-i is approximately 12 V, approximately 0.1 µm can be accepted. Below 3-5 V, no fritting can be expected. This makes CAN inexpedient for connections between for example tractors and trailers (trucks) and between train wagons.

⁸) Because Max-i does not use any termination resistors, the transmitter only draws current for the time it takes the pulse to travel to the end of the line and back to the transmitter, so the energy (power multiplied by time) may be quite small. This reduces the emitted noise and enables battery operation and operation in explosive atmosphere - especially if the speed is lower than the maximum speed for the given line length.

⁹) In Max-i, it does not take longer time to poll a value than to transmit it event driven as the first and last part of the telegram are just transmitted by two or more devices.

¹⁰) Because CAN does not have any "babbling idiot" protection, it is not possible to reach this number of telegrams in practice without a completely unpredictable delay of low priority telegrams. Max-i may run even at 100% and is faster than CAN for safety telegrams where CAN needs an extra layer and it is much faster if the possibility for different data to more devices in the same telegram is utilized.

Supplement to Ethernet

Ethernet is growing fast in industrial process control. This puts traditional 5-V based fieldbus systems under pressure. Because they need a power supply to convert the supply voltage to 5 V, a timing crystal and some microprocessor assistance, they are too big and expensive to be included in the smallest and most price sensitive devices like push buttons, lamps, micro switches, motor contactors etc. It is therefore only possible to use these bus systems for distributed I/O and for connection to more complicated devices, but Ethernet can do that too at very much the same price. There may even be Ethernet in the building already, which may save some money for cabling. Ethernet will however, never get out to individual actuators and sensors - even if the price and size is not taken into consideration. Because Ethernet is a point-to-point communications, the signals in a big plant would have to pass so many routers and with that so many cable connectors and so much electronics that the reliability would fall to a totally unacceptable level with maybe up to one failure per week.

In the future, there will therefore probably only be room for two types of bus systems - Ethernet with an added layer to make it deterministic, and an ultra low-cost, but still high performance and deterministic bus system for connection to individual actuators, sensors and lamps. This can very well be Max-i.

No Charger or
Converter Chaos

Today, smartphones, tablets, laptop computers, cameras etc. all require often very clumsy chargers, which already destroys the smartness of the technology, but in a few years when LED lighting is expected to replace traditional incandescent (filament) bulbs, new solutions will be necessary if it should not lead to a total chaos of converter boxes and cable spaghetti combined with clumsy and expensive bulbs with build in converters. Besides, due to the laws of nature, the efficiency of many small converter boxes is way below what can be achieved with fewer devices with higher power - especially if 50 Hz transformers are used, and there will be a potential fire hazard with many converters of often doubtful quality and cooling.

Max-i offers the ideal solution for that. The high power and use of standard installation cables also makes Max-i extremely suited as a supplementary 12 Vdc power supply and control in the houses of the future where it may be used not only for all kinds of battery chargers and intelligent LED lighting, but also for window openers, alarm systems, energy management etc., and as an ultra-low-cost replacement for short distance wireless power transfer with for example up to 1000 times faster communication than Qi (from WPC) and much smaller size. Calculations done by the Danish engineering company Rambøll shows that such a network partly driven by solar cells can save the Danish households in the order of 1 billion Danish kroner per year corresponding to approximately $200,000,000.

Max-i has all the features needed for these kinds of applications. For example, it is very easy to make two-way/landing switches and light dimmers, and all Boolean outputs may be divided in up to 255 groups, which may be switched off in three steps for example during lunch breaks, when you leave a building or in case of failing "green" energy supply or a heavily loaded power net.

Evaluation Board

Until the final IC solution is available, Innovatic can offer an evaluation board - EB1, which simulates an 8-pin IC solution by means of an FPGA and other standard components. This solution may be very interesting for vendors, who want to take the step into the future and have a possibility to influence the standard before the IC is made. It may also be interesting for semiconductor companies, who may be looking for a new product family and/or can see the enormous potential in one fieldbus for virtually all low to medium speed applications from the most price sensitive ones to the most demanding.

Vendors, who want to utilize the outstanding properties of Max-i and embed this preliminary Max-i interface directly into their products, can buy the pre-programmed FPGA from Innovatic (or through Microsemi - previously Actel), which simultaneously grants the vendor a royalty free, non-exclusive right to use the schematic of the evaluation board and the PCB layout. In this way, it is not necessary to wait for the final IC! As the interface does not need any microprocessor assistance, it is not more expensive that most other fieldbus interfaces and may therefore be used for example for distributed I/O, but of course, Max-i will not release its potential of very low cost and very small size before the IC is ready.