Technical Description - Max-i Fieldbus

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Technical Description
Comparison between CAN and Max-i
Max-i is very similar to CAN, which is probably the only other event driven multi-master bus based on bit-wise bus arbitration except for the old, obsolete STL-Net, but as can be seen from the table below, Max-i 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 single chip interface
NoYes
Product certification and registration needed
Most protocols
Not necessary
Use of unshielded, standard installation cables
NoYes
Maximum practical power transfer per segment
384 W at 24 V  1)
≥1500 W at 20 V  1)
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 control
No
255 groups
Data
Multiple master bus with bit wise bus arbitration
YesYes
Publisher/subscriber model
Partly 2)
Yes
Identifier length
11 or 29 bit
12 or 31 bit
Multiple use of same identifier
No 3)
Yes 3)
Number of addressing modes per identifier
14
Local and global data and global poll of local data
NoYes
Possibility for temporary change of values
No
Yes 4)
Maximum number of bytes
8
1028 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
Unshielded cable
No
Yes
No termination resistors = high failure tolerance
No
Yes 6)
No bias distortion at capacitive loads
No 7)Yes 7)
Ideal triggering level not affected by attenuationNo 8)Yes 8)
Line-ringing between devices during arbitration
High, but damped
Low (short pulses)
Sensitivity to voltage drops in negative supply
High 9)
Low 9)
Uncritical timing on all bus length and no setup
NoYes
Timing not affected by galvanic separation
NoYes
Tolerant to contact or conductor failure
No
Up to 2 contacts 10)
Contact fritting
No 11)
0.15 - 0.2 µm 11)
Typical transmitter power / total power loss
0.2 W / 0.4 W
6.8 W / 1-1.6 W 12)
Receiver hysteresis
0.1-0.2 V
±1 V, 3-level
Sensitivity to vibrations and low temperatures
Crystal oscillator
RC oscillator
Safety
Error detection
15-bit BCH 13)
20-bit CRC
Protection against masquerading
No
7-bit Hamming code
Detection of wrong number of telegrams
No
7-bit serial number
Predictable response time
No
Yes, deterministic
Designed for IEC 61508 SIL 3
No
Yes
Speed
4-bit/20-bit polled values/s on 1 km line
612 / 530

4-bit/20-bit polled values/s on 1.5 km line
(1.5 times slower)

480 / 350  14)
4-bit/20-bit event driven values/s on 1 km line
1136 / 880 15)

4-bit/20-bit event driven values/s on 1.5 km line

480 / 350 14)
4-bit/20-bit polled values/s at maximum speed
9800 / 8472
30500 / 17600 14)
4-bit/20-bit event driven values/s at max. speed
18176 / 14080 15)
30500 / 17600 14)
1) 1500 W requires 5 x 4 mm2 flat cables with power supply from both ends. For the 384 W power level, CAN requires a very expensive thick DeviceNet cable (12.2 mm with 15 AWG / 1.65 mm2 conductors for DC).

2) CAN uses the publisher/subscriber 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 multi-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 4, 20 or 36 bits, and it is possible to change a value temporary, which can save a lot of time during commissioning for example in case of sensor errors, to try an alarm limit or to simulate the presence of material.

5) Max-i can send individual 8-bit, 16-bit, 24-bit or 32-bit values to more devices in a common telegram with up to 1028 bytes and in this way ensure 100 % data synchronization and a very high efficiency for example for motion control, positioning systems and for stage light where Max-i with advantage can replace DMX512. Standard CAN (not CAN FD) is only able to transmit 8 bytes in each telegram.

6) In Max-i, the traditional termination resistors have been replaced by voltage clamps in each device. This gives a very high failure tolerance as the bus may be cut in as many parts and each part can still work if it is powered up! The clamps also reduce the ringing between devices during bus arbitration, 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) Bias distortion occurs in case of capacitive loads or lines if the output impedance of the circuit, which drives the line high, is not the same as the circuit, which drives the line low, as it is the case with CAN, where the line is driven high by a driver with a low impedance, but driven low by two 120 ohm termination resistors in parallel. This may cause a kind of signal rectification where it takes much longer time for a signal to go low than to go high, which destroys the signal integrity and makes it very difficult to chose the right triggering level. In Max-i, the driver is fully symetrical so even very high capacitive loads does not affect the communication.

8) Many fieldbus systems, which use termination resistors including CAN, have the problem that the low level is always 0 V, but the high level depends on the signal level and any attenuation. This makes it even more difficult to select the right triggering level. If the level is chosen too high, a high level may not be recognized in case of attenuation, and if the level is too low, the bus will be very sensitive to noise and it may take too long time for the signal to go low in case of bias distortion. In Max-i, the high signal level is always equal to the low signal level just with opposite sign, so the ideal triggering level is always in the middle no matter how much the signal is attenuated. This makes the signal/noise level even better in practice compared to other fieldbus systems.

9) The CAN transceiver is usually connected to the negative supply line so a voltage drop on this line causes bias distortion. Max-i uses the midpoint between the power supply lines as 0-V reference, but must then require that the voltage drop in the two lines are approximately the same.

10) 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.

11) 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 20 V, approximately 0.2 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.

12) When a transmitter is activated, two waves are generated with a typical power of 3.4 W in each direction. Because Max-i does not use any termination resistors, the current in each wave falls to zero after the time it takes for the wave to travel to the end of the line and back again to the transmitter. If for example a device is placed in the middle of the line, the two waves will arrive simultaneously, so the power will fall from a total of 6.8 W to zero after a time corresponding to the propagation delay of the line. If the device is placed at the end of the line, one wave arrives immediately, so the power falls from 3.4 W to zero after a time  corresponding to two times the propagation delay. No matter where a device is located on the line, the energy (power multiplied by time) is the same, and if the line is shorter than the maximum length, the energy is reduced correspondingly. This reduces the emitted noise and enables battery operation and operation in explosive atmosphere. The power loss in the transmitter and the clamps depend on the supply voltage and the sum is maximum at maximum voltage.

13) In CAN, two bit errors may on rare occasion remain undetected when the first generates a bit stuffing condition and the second then removes a stuff condition (or vice versa), shifting the position of the frame bits between the two bit errors. The shifted area may lead to a burst error that is too long for the CRC mechanism.

14) 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.

15) 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 as it is the case for stage light and motion control systems.

Specification
The Max-i specification can be downloaded here: www.max-i.org/specification.pdf

This page is created with WebSite X5 and updated June 11th 2020

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