Traffic Lights and Car Parks - Max-i Fieldbus

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Traffic Lights and Car Parks
Traditional Traffic Light
Still today, traffic light are often made the way industrial process control was approximately 40 years ago - by means of point-to point connections from the traffic controller to each lamp. This is easy to overlook, but there are a lot of disadvantages:

  • It is a very expensive and big solution, which makes it difficult to earn money.
      • It is necessary with thick multicore cables, which are also very time consuming to mount. Everytime a T-connection to a signal head is needed on a cable with N conductors, there are 2N connections to make. The number of conductors may be reduced if more lamps in the same direction are connected to the same conductor, but then it is not possible to monitor each lamp individually.
      • It is necessary with group cards for signal heads and special interface cards for digital signals such as pedestrian bottons, loop sensors, countdown displays and air-quality detectors. This usually makes it necessary with a 19" rack.
      • It is necessary with a very long clamp row / terminal block.
  • Due to mutual coupling between conductors, it may be very difficult to monitor low-wattage lamps in an AC system.
  • The mutual coupling also makes it impossible to make lamp dimming by means of leading or trailing edge, pulse width or pulse code modulation. The only economical possible dimming method is to change the voltage level globally and this usually limits the dimming to only two visible steps with very limited dimming range.
  • If the communication between the CPU and the interface cards is done on a parallel bus and/or by means of logic level signals, it may be sensitive to electromagnetic disturbances.
  • There are a lot of connection points, which lowers the reliability - especially for low-voltage signals below 5 V. It is often said that the reliability of an electronic control system is almost inversely proportional to the number of connectors so even though a point-to-point system is simple, the reliability may not be as high as it first appears! In a typical controller there are:
      • A multipin connector for low-voltage signals between the CPU board and a backplane.
      • A similar multipin connector, which leads the low-voltage signals to signal group and interface cards and from there back to the backplane for new multipin connectors, which connect the backplane to the clamp row through multicore cables.
      • Clamps and terminal blocks from the controller to the lamps and sensors. High quality screwless clamps with stainless steel springs and tinned copper conductors have however a much higher reliability than connectors, which must have a much lower contact force and usually use copper or even brass springs, which cannot guarantee a long lasting connection.
  • There is usually a fairly limited number of signal groups and with a given rack size it may be impossible to expand.
  • It can only be tested from the controller.

Due to these disadvantages, virtually all industrial process control systems today and a few traffic light systems use fieldbus systems - often with RS-485 communication. This replaces the internal (parallel) bus in traditional systems and saves all the interface cards and a lot of cabling with belonging distribution boards and documentation, but are not without disadvantages too:

  • It is usually necessary with a CPU in all actuators, sensors, lamps and distributed I/O boxes. For many reasons this reduces the reliability to less than what can be obtained with a point-to-point solution:
      • Even a very small ARM M0 CPU uses almost 100,000 transistors and a failure on a single one may cause a total loss of function.
      • A transient or a program bug may cause the program to go down.
      • The program is usually stored as tiny little charges in a flash memory and if just a single bit out of millions changes over time, which especially may happen at high temperatures, the entire device may fail.
      • Many CPU use a crystal ocillator.
  • The signal level and energy in each communication pulse is usually very low, which may cause problems with noise and connectors.
  • If the fieldbus does not use bit-wise bus arbitration, the efficiency may be very low, which may make it necessary to use a high communication speed, but for each time the speed is doubled, the pulse energy and with that the signal/noise ratio is reduced to the half. Efficiency is much better than speed.
  • If 0 bits and 1 bits are not 100 % symmetrical, which is the case for example for CAN, heavy bias distortion, which may destroy the communication, will occur if many devices are connected to the same line and it therefore becomes capacitive.

The Max-i solution
Max-i is the only fieldbus solution without all these disadvantages and with a reliability, which may even exceed a point-to-point system and simultaneously at a much lower cost:

    • Max-i is designed for maximum safety according to IEC 61508 SIL 3 (death of 1-3 persons) even without additional layers, and it has a green safety output with own decoding circuit to prevent green light in case of a failure on for example the lighting controller.
    • All necessary circuitry for traffic lights is implemented entirely in hardware, which cannot go down and is very failure tolerant, and it uses a dual-phase clock and an internal RC-oscillator (no crystal) for absolute maximum reliability. All critical programming is done in only 10, 32-bit EEPROM registers with parity check, which will even be trippled with majority voting in the final IC so that an error is detected and may be corrected before it has any effect on the function.
    • Everything is done to make the communication as reliable and safe as possible:
        • The combined communication and power supply cable is a very rugged, unshielded, balanced 4-wire line, which may even be failure tolerant! If the supply voltage is applied from both ends of a loop as shown in the drawing on top of the sub page "Green Smart House Solution", full functionality is maintained even in case of af failure on two neighbor conductors, and if a Max-i interface is used in both ends, the functionality may even be maintained if the line is cut in two pieces. This is not possible with fieldbus systems, which depends on termination resistors, like CAN and RS-485 based systems.
        • The signal level is approximately 15 V and the pulse power up to 9 W, which is way above any other fieldbus system and ensures an excellent signal/noise ratio, yet it is still a "green" solution. Because Max-i has no termination resistors, the transmitters only draw currents for the time it takes the signal to travel to the two ends of the line and back again, so the energy consumption may be very low depending on the relationsship between speed and line length.
        • It has a 20-bit CRC check to detect communication errors and reduce the test time for IEC 61508 SIL 3 to only two days. With only 15 bits, CAN needs a test time of two months or 32 times more test devices, which increases the error probability 32 times during the test time.
        • Max-i has a 7-bit Hamming code on the identifier to protect against masquerading and a "babbling idiot protection", which prevents that a high priority device can take over the bus so that it can be 100 % deterministic.
        • It has two separate, programmable watchdog timers for implicit messages and group messages, which may be used to turn a signal group off in case of lost or faulty communication.
    • The entire traffic controller may consist of a single CPU board with one or two Max-i interfaces and a power supply. This makes it possible to save the backplane, all interface cards and the long clamp row, which obviously reduces the price and size to a small fraction, and there is never any risk of running out of I/O.
    • It is very easy to connect one or more laptop PC's or tablets to the bus for programming and debugging so there is no need for an operation panel too. This saves even more money.
    • Each signal group including a countdown display can be driven from a single, small IC, which also makes it possible to generate error messages for each lamp and readback the actual status of each lamp without any problems with low-wattage lamps even when dimmed to very low levels. This may be used to increase the safety even more, and an integrated hour counter may be used for preventive maintenance.
    • The publisher-subscriber mode makes it possible to update all lamps i one direction simultaneously and in this way save a lot of telegrams.
    • It is possible to control any number of signal groups by means of a common telegram.
    • The 4-bit Boolean values fit perfectly with the usual number of lamps in a lantern in one direction (red, amber, green and any green arrow) and it is possible to include for example a 7-segment display pattern in the same message, which can be transfered to a time countdown display by means of a simple SPI interface.
    • The excellent dimming possibilities of Max-i with group control and programmable minimum level makes it very easy to optimize the light intensity in each direction according to the ambient light and solar radiation in that direction. Because of the logaritmic characteristic of the eye, this can save a lot of power, make batteries last much longer and prevent dazzling of the drivers at night. It may even be vital for saving lifes in case of loss of mains voltage, where traditional traffic ligths without battery backup just turn off so that nobody in a big city can get through including rescue vehicles, police and busses.
    • The smoothing filter of the lighting controller, which may be set on a per-telegram basis, makes it posible to turn each lamp on and off in a very pleasant way.
    • It is possible with automatic and synchronized flash with two flash frequencies and programmable phase for each lamp and frequency, and the gamma correction ensures that even with a fairly long smoothing filter, the visual duty cycle of flashing lamps is not affected.

    Car Parks
    Car parks with occupied bay sensors and red/green free indicators is a big challenge for traditional fieldbus systems, if a common overview is wanted. Because there may be hundreds of devices on one line, it will unavoidable get a very low characteristic impedance, which may even be capacitive, which the transmitter of most other fieldbus systems are not able to handle, and if the signal is not 100 % symmetrical, heavy bias distortion may destroy the communication. This is however not a big problem for Max-i, which has symmetrical communication, a typical current limit of 1.2 A for the transmitters, no termination resistors to draw current and a very high efficiency, which makes it possible to reduce the speed and use reflective wave switching without any noticeable delay.

    This page is created with WebSite X5 and updated April 18th 2024

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