and issues could be due to either hardware or software.

 

what data rate are you aiming for

 

The 120 ohm termination resistors are the bias resistors for this particular chip. Other chips (such as the SN65HVD1176) may require them, but not the one the TS is using. The TS is using the MAX485ESA+T, check its datasheet.

The 120 Ohm resistors don't bias anything. They are simply the line termination resistors. Careful reading of the datasheet shows that (like almost any other RS485 recever) the reciever output voltage is guaranteed to be H at a receiver differential input voltage > 0.2 V and guaranteed to be L for < -0.2 V. Between those values the output voltage is not guaranteed. In contrast to other manufacturers, MAXIM (Analog Devices) doesn't specify internal weak pullups or -downs when the driver is disabled. Usually these are in the 50 kOhm range. Applying Ohm's law with 120 Ohm termination leads to a differential voltage of 6 mV, well in the undefined area. Any noise can cause undefined toggling of the Rx output.

As long as one and only one transmitter is driving the line, everything is ok, but when all drivers on the line are disabled, the Rx output signals are undefined. A proper DC biasing guarantees a logic H at all Rx outputs in idle state which usually doesn't cause any problems with UARTS and so on.

And, as stated by Drjohsmith and me, we are missing oscilloscope screenshots by the TS at the driver and reciever end of the line during communication and while enabling the driver to perform a detailed analysis of the problem.

 

this is usefull info on biasing,
bottom line, ensure you use pull up and down,
https://www.ti.com/lit/ta/ssztak0/s...39272&ref_url=https%3A%2F%2Fduckduckgo.com%2F
other links in that article