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0.2V the receivers output switches to a 1 and when the voltage goes below -0.2V the receivers output switches to a 0. (or vice versa if there is inversion on the receiver) Since the last bit from a UART will be the stop bit (1), then the transmitter is turned off (the differential voltage goes to 0V, but not having gone less than -0.2V), this should leave the receiver with a 1 being output to the receiving UART. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. The sensitivity treshold and hysteresis in the receiver certainly play a role. 125 mV. The small voltage gradient is intended to improve noise immunity of the idle bus, as 200 mV differential is the sensitivity treshold of the bus receivers (upper toggling point of their built-in hysteresis). The balanced receiver sensitivity treshold is high enough that in many scenarios, ingress EMI is not an issue (some point out that RS485 is more immune than CAN).

So we unplugged the service port, and the bus got clear of that EMI. On the scope I could see that the pulse shapes indeed got cleaner, but at the same time the pulse amplitude dropped quite a lot, and the transceivers probably didn't like that side-effect. So, after some cabling exercises in the lab, I decided to pay a visit to the customer, to see with my own scope exactly what was going on. So there I was, with the 120Ω terminators back in place, and the bus didn't quite work, despite the nice-looking scope traces. Voila, THE BUS STARTED TO WORK! First of all I verified, that the RS485 transceiver is essentially alive, that both TX and RX work (using another converter). The probes communicate using the standard Modbus RTU protocol. When using RS485 communication, try to use short cable lengths to minimize noise interference, and ground the shield of the isolation network together with the main communication line. The morale for me is that, for short sections of cabling, even if you don't have a 120Ω cable, you should still use 120Ω terminators, because it's more important to please the active transceivers, than to correctly terminate the transmission line.

At short distances, RS485 standard the TML's wobbling does less harm than a DC impairment. But it still didn't work. The bus still didn't work. One important aspect though: the shielding was discontinuous (not interconnected at the ends of cable sections) - the customer said he tried connecting the grounds and it didn't work. In the aforementioned customer trouble case, there were actually several problems combined. The customer had my culprit PC connected to just two devices (RS485 slaves): an I/O module (not our hardware) and a meter device - for a total of three nodes, from three different manufacturers. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. It doesn't even have to be a master node - the biasing node can just as well be a protocol slave, or the biasing resistors can be added someplace halfway between the nodes on a bare transmission line. Which essentially meant twisting all the shields together and connecting them to the master PC and the I/O module's power GND (same as its RS485 ref.GND).

This means that in a master/slave configuration, a master can talk to multiple slaves, all of the slaves can talk back to the master, and every device on the network can hear every other device. The baud rate essentially means DC to the cabling. This in turn means that the drivers' heat dissipation increases significantly (more current and a greater voltage drop across the output transistors), which is certainly a safety risk. My explanation is that the RS485 bus drivers' output impedance is optimized (matched) for the typical 120Ω transmission line (the driver's output impedance should actually be half that value, because the driver feeds two sections of the TML in parallel), and if a lower impedance transmission line is attached to the driver, the drivers' output sags accordingly. If they don't communicate, reducing the bit rate may make the two devices function together. The usual transfer rates are just so low, that bit length is several times (orders of magnitude, rather) greater than the transmission-line round-trip.