Dear all,
I have a test application that interfaces with a multiple modules using the RS-485 transceiver. I'm looking specifically putting the transceiver as the driver. That being said, I was hoping to drive multipoints but only one module is powered on at any one time.
The issue I have is the terminating resistors for the differential lines are actually located in the modules themselves and not on the test board so I cannot mount them at each end of the transmission lines. One "fixed" module is always connected, producing a impedance of 100 ohm on the line. However, during a different test, another module can also be installed which also has terminating resistors inside. Would this mean that the RS-485 transceiver I have would see an impedance of 50 ohms total considering that would be installed in parallel with the "fixed" module? If this is the case, I am specifically looking at the SN65MLVD206 which supposedly can support multipoint buses with loads as 30 ohm, would I be fine to keep the "fixed" module connected?
If not, are there perhaps any suggestions to how I could go about making this application work? Perhaps, switching the "fixed" module lines in with actually relays. I appreciate any insight that can be given for multipoint use of the RS-485. Thank you very much in advance