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Communicating between "boxes" using Modbus. Master is a 4-wire system and Slave is a 2-wire system. What is recommended cabling? What is recommended termination resistor?

Master signals:
TXA, RXA
TXB, RXB
Gnd

Slave signals:
+
-
Gnd

connect A signals and call them +
connect B signals and call them -

connect + to +
connect - to -

One question on the slave device, is this device configurable for RS485 2-wire & 4-wire system? If it does, then I would suggest to implement 4-wire full dulpex system.

As for the communication line termination resistor, usually is 120ohm to match typical communication cable impedance. Each resistor has to be installed only at the end of the Receiver cable pair (Rx+ & Rx-) on Slave device and (Rx+ & Rx-) on Master device.

I'm having a similar problem, except the master device is half-duplex and the slave is full-duplex. I tried this wiring scheme, but when the slave device responds, an echo occurs (the response is sent back to the slave as well as the master.) Any way to prevent this behavior?

Thanks,
DG

Yes, a kind of "pot luck" problem. Some 4-wire produces turn off their receiver during transmit and thus you can just short the pairs togather and make them 2-wire. Others (as you've found) do not and thus see an echo. Common symptom is Masters is it sends only 1 byte of a request, then "shuts up" since it sees itself talk and thinks the RS-485 network is in fault.

When echo causes problem, your only solution will be to use a piece of hardware (a 2-to-4 wire or RS-422 to 485 converter) that prevents an echo of 4-wire data received back on the other pair of the 4-wire. http://www.robustdc.com and others sell such products.

best regards
- LynnL, www.digi.com

I'm having a similar problem, except the master device is half-duplex and the slave is full-duplex. I tried this wiring scheme, but when the slave device responds, an echo occurs (the response is sent back to the slave as well as the master.) Any way to prevent this behavior?

Thanks,
DG