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RS485 signalling characteristics - collisions
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Simon:
I see that some micro controllers have an automatic bus collision detection which usually fires an interrupt for RS485 modes. So it has me wondering how the actual driving of an RS485 output works as two devices trying to put opposite states on the bus seem to be OK. CAN bus works in such a way, is RS485 the same? Is it a pull up/down resistor with opposing transistors so that the "dominant" state will prevail?

I cannot actually find any information on the bus driving itself unlike with CAN. All the specs say is that you have +/- 0.2-1.5V output and that's that.
ogden:
Modern rs485 drivers have push-pull mosfet output. One datasheet that I know is SN65HVD76, look for "9.4.1 Equivalent Circuits". For MCU that does not have rs485 line driver built-in only way to detect bus contention - compare transmitted bits with received. Hope this helps.
Simon:
Yes the micro controllers just read back in what is transmitted on the bus and if it does not match throws and error and interrupt. But this means that it is deemed acceptable to put both a 0 and a 1 on the same bus, how is that possible if each node is a push pull driver? that would result in a direct short circuit.
Alti:
Proper transceivers are short-circuit protected and as long as power dissipation is not exceeded and you won't start violating maximal ratings(-7V/+12V), she'll be fine.


--- Quote from: SN65176B ---(..)The driver is designed for up to 60 mA of sink or source current. The driver features positive and negative current limiting and thermal shutdown for protection from line-fault conditions.
--- End quote ---
ogden:
Yes, contention is effectively "bus shorted" state. Most drivers have bus short-circuit protection, including one I mention. Usually it is implemented as thermal shutdown. In short: avoid rs485 collisions by design, pay close attention to device ID assignments so they do not match - using warnings and doublechecks for users. Further reading: TI have good rs485 driver datasheets and good appnote as well: "AN-1057 Ten Ways to Bulletproof RS-485 Interfaces".

[edit] Good idea to test supply voltage ripple during bus-short early in the prototyping. Most likely you don't want whole device/circuit to brownout during rs485 short/contention.
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