I've been having trouble getting a rs485 communications bus working with some external devices. I have an Arduino hooked up to a MAX487ECSA+ which goes through some cabling to my devices. Using an oscilloscope, I can see the signal (with sufficient amplitude) reaches my devices but I get no response. The baud rate used is 19200.
I bought this breakout board from sparkfun to see if I could get some communication working. I hooked it up to my arduino and I get a response from my device.
The MAX487ECSA+ is a quarter unit load and the IC used by sparkfun is a one unit load. I'm unsure of the unit load of my devices. Could a difference in unit loads between the 2 ICs be the reason one circuit works and the other doesn't? If this is a reason, could you please explain why I can't use devices with different unit loads on the same bus?
-A and B are correct, I have tried both ways