Hello,

I am attempting to install a system of 6 controllers, each of which send and receive data from two sensors, on an RS485 loop. I am using a MODBUS address for each of the controllers. When connecting the 2-wire RS485 in one direction (A to B), I can only read half of the sensors. When the 2-wire is reversed (B to A), I can read the other half of the sensors. Is there any known cause of this problem?

Thank you,
RC

 

I assume that one group of sensors is manufacturer/model X and that the other group of sensors is manufacturer/model Y.

There is no 'standard' for labeling the data lines. Every manufacturer picks their own labels.

I've had an RS-485 Modbus product from one division within a company label the opposite of another division.

Wire the network so that all the (+)'s are on one line, all the (-)'s on the other, even if you must wire A to B.

This happens all the time when mixing brands on the same multidrop network.

 

Check which ones work and reverse the lines on the others.
The A and B are pretty much arbitrary.

Regards
cww

 

It may also be a biasing issue or termination resistors. If you measure the voltage across the D+ and D- terminals, you should see a static voltage when no communication is happening. Check for significant voltage drops. Ideally, you should see about 5 VDC. When you start to communicate, you will see a fluctuation in voltage. If the range drops too low, you can add a repeater to strengthen the signal or a hub to just separate the 2 sets of devices.

http://www.icpdas-usa.com/i_7513.html