Max-i Fieldbus

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Welcome to the official homepage for the Max-i fieldbus and Max-i Association
Max-i is a new, extremely powerful and efficient fieldbus, which enables the lowest automation and LED-lighting costs ever seen combined with maximum performance! It may be regarded as a combination between a highly improved CAN bus and a high-power 20-V power supply and may be used for virtually all low to medium speed applications (up to 25,000 telegrams per second on short lines) like:

  • Green smart house systems and building automation.
  • Intelligent LED lighting including demanding stage light, architectural lighting and traffic lights.
  • Industrial automation including motion control and safety applications.
  • Transportation applications such as automotive, special vehicles, railway, ships and aerospace.
  • Internet of Things - IoT.
  • Military applications including equipment, which must survive very strong electromagnetic pulses.

Max-i has a lot of outstanding benefits:

  • It is possible to make a complete bus interface in one integrated circuit (IC), which is small and cheap enough to be built into even the smallest and most price sensitive actuator, sensor or lamp, gets its supply voltage of 20 V (15.4 - 24.6 V) directly from the bus and only needs an absolute minimum amount of external components - often only a few small ceramic capacitors. The very small size is also the key to protection against electromagnetic pulses and weapons.
  • The interface can handle a lot of simple functions entirely in hardware like UART and SPI interface, 4-bit Boolean I/O, A/D and D/A conversion, keypad scanning and very advanced LED driving. This gives a much higher reliability and failure tolerance than any fieldbus, which need a microprocessor on the sensor and actuator side, and it saves an expensive and time consuming conformance test.
  • It is possible to use cheap, standard, unshielded and un-terminated installation cables instead of special communication cables. This saves a lot of money and troubles with shield connection and shield currents and makes it possible to transfer much more power over the bus than any other fieldbus - up to several kW.
  • It uses bit-wise bus arbitration, which has many very important benefits compared to any other access method:
      • It is the only multi drop technology (not point-to-point), which does not need any reconfiguration when devices are added or removed, so this may happen at any time on the fly. In all other systems, a new device needs to be inserted in the communication sequence before any direct communication is possible, and if a device is removed without reconfiguration or fails, time is lost forever.
      • The response time is much faster for the same speed as a device does not need to wait for a poll, a token or a time slot before it is allowed to send a message.
      • The bandwidth utilization is much better since no time is lost on devices, which do not need communication.
      • The failure tolerance is much higher since there is no dedicated master unit and no need for a centralized database. Since Max-i uses voltage clamps in each device instead of resistor termination, the cable may even be cut in two or more parts and each part can still work if it is powered up.
      • It is possible to use many different protocols simultaneously. It is for example possible to run any CAN protocol like DeviceNet or CANOpen together with the Max-i protocol and in this way save extra cables and communication channels.
  • It uses the very efficient publisher-subscriber model, where it is the various values, which have an address/identifier, not the various devices. In this way, the same value may be utilized simultaneously by any number of devices and it is not necessary to add a long list of source and destination addresses when gateways are used.
  • It is extremely efficient. It may not take longer time to poll a value than to send it event driven, it is possible to transmit data to more devices in the same telegram for example to stage lamps and servo axes, and with the short 12-bit identifier, a 4-bit value can be transmitted in only 5 bytes including a 20-bit CRC-check to detect errors and a 7-bit Hamming code on the identifier to protect against masquerading. Even with the long 31-bit (PNS) identifier, a 20-bit process value with exponent, scaled to SI-units only requires 9 bytes. Efficiency is a much better means to get many data through than speed because it does not reduce the signal-to-noise (S/N) ratio. If for example the probability of a bit failure (p) is 10-7, which is usually accepted in a field bus system, just a factor 2 (N) increase in speed will reduce this to approximately p(1/N) = 3.2 x 10-4, which is unacceptable.
  • It is extremely safe. It is the first fieldbus designed directly for highly demanding safety applications according to IEC 61508 SIL 3 (death of 1 - 3 people) without the use of additional protocol layers or safety monitors, and it is fully deterministic. A "babbling idiot protection" prevents high priority devices from taking over the bus and if all devices want to transmit simultaneously, they just get through one by one.
  • It combines the outstanding performance with a remarkable and unusual simplicity that will please everyone from makers to professionals. There are just a few registers to setup and a new numbering system - 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 approximately 220 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.

"Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away."
Antoine de Saint-Exupéry

The name Max-i means Multiple Access Cross (X) coupled interface. It refers to Max-i being a multi-master bus, 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) to prevent mutual coupling to other cables. The name also refers to the Max-i-mum performance, which by far exceeds comparable fieldbus systems on most fields.

Released specifications can be downloaded from the Technical Description page and used free of charge for all non-commercial use. Commercial use requires membership of Max-i Association.

Specification 11.0 has been released.
  • Number of I/O increased to 8 inputs and 8 outputs.
  • Soft start possibility added to boolean outputs.
  • Boost function added for very thin or long transmission lines.
  • Flash function for lamps changed from 4-bit to 20-bit message.
  • Lamp output with square root gamma added for dim-to-warm.
  • Programmable start level and timeout color added to lamps.
  • Inputs for up and down buttons added to lamps.
  • Analog status signals added to monochrome lamps.
  • X16 and X8 UART clock outputs added to MODEM object.
  • 2-bit boolean outputs added to KEYPAD scanner object.
  • 4-bit bolean output possibility added to ADA object.
Max-i Association
C/O Innovatic
Bøgebakken 3, Gjessø
8600 Silkeborg
Phone: (+45) 86 84 72 92

Chairman Carsten Kanstrup
Mobile: (+45) 20 15 51 60
This page is created with WebSite X5 and updated March 29th 2020

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