We plan to realize a sensor cable several 1000m long which has ideally every 1-2m a magnetic field sensor on a small 10x30mm platine with microprocessor/FPGA for DSP and I2C on chip communication. Later this will be integrated into an ASIC.
The sensors slaves are thought to do alarm handling/asynchronous messaging to the master/repeater, if a distinct magnetic field value change is too high. Several limitations exist I'm aware of to yield a nearly real-time and cheap and low power consuming cable. It is important that all sensors in the cable check with at least 10Hz the field, the power consumption of the sensors will be in the 50 microA range. All sensors should be powered by the fieldbus cable.
I studied different fieldbus systems
Best for our purpose seem to be CANopen, MODBUS or Profibus (but no sensor voltage supply for this). For a sensor every 1-2 meter with maximum node of 250 we would need every 250-500m a fieldbus repeater (which is not very cheap).
Is there a technical/fieldbus alternative for our purpose if high data rate and is not the most important criterion but keeping the cable cheap?
Would it make sense to create our own fieldbus protocol which maybe can handle 1000 nodes on 1000-2000m without a hardware repeater or even develop a own repeater for our start-up company on our own? We have a engineering team which will do this if it is possible and feasible (price and expense) and funding.
Best Answer
Your main problem will be data traffic. If you have 250 nodes that should report in with 10Hz, then that's 2500 messages per second. Your main focus needs to be to keep the data traffic down. Maybe you can create sub-systems, maybe you can send an average at lower frequencies etc.
At a glance, it would seem that CAN is the ideal solution for this. It is rugged, it is real-time, you can get fairly long distances. If including supply, you can do with minimum 4 wires. That is, depending on how much current the nodes draw - sharing supply ground with signal ground is a bit of a dirty solution.
However, since your timing requirements are so high, you'll be looking at the higher baudrates. And you want large distances. Which in turn means that dedicated signal ground and cable shield might be needed. So your specification is contradicting itself: you can't get "fast", "good", and "cheap" at the same time.
In addition, there's not really enough info here to draw any conclusions. I suspect that your specification needs to be revised before you run off to pick a fieldbus.
As for the higher layer fieldbus protocols: unless you need to be compatible with some industry standard computer/PLC, there's no apparent need for CANopen or Profibus etc. These add lots of complexity you can do without, and most are restricted to 127 nodes (for good reasons). If this is a closed system, then a custom CAN protocol might be the ideal solution.
Profibus and MODBUS are also quite old technologies, I wouldn't recommend using them for new design.