from the Modbus department...
2-wire 4-wire dilema for Phoenix Contact Modbus RTU bus terminator
Local and wide area networking in factory automation. topic
Posted by dsperry101 on 4 June, 2013 - 6:54 am
I am hooking an EZtouchplc to a Phoenix Contact Modbus RTU bus terminator. The EZtouchplc is a 4 wire device and the PhoenixContact is a 2 wire device. When I use a 2-wire to 4-wire converter the link works and updates the i/o OK.

I wanted to eliminate the 2 to 4-wire converter since its on a continually moving device. I saw on the modbus site that you can make a 4 wire a 2 wire by tying the +'s together and the -'s. So T+ and R+ are tied together and R- and T- are tied together. I also incorporated 120 ohm resistors to eliminate echo. When I hook up this way the light start blinking like the link is working but the inputs do not update on the other end. Looking at the Modbus PLC block status it shows no errors.
Anyone know what's going on here?

Posted by Lynn August Linse on 4 June, 2013 - 9:40 am
> I saw on the modbus site that you can make
> a 4 wire a 2 wire by tying the +'s together and the -'s.

This is ONLY true in limited situations & if the 4-wire expects half-duplex behavior:

1) The first problem occurs if the 4-wire device keeps it's RX pair on all the time. If so, it will see an echo of its request as the response, which will be invalid and the transaction fails.

2) The second problem occurs if the 4-wire 'Master' device leaves its transmitter drivers powered FULL-TIME, forcing the TX pair to an idle ('1') state. This means any slave trying to reply causes TX driver contention (aka: a fight or battle), the results will be undefined.

So one can only hope to convert a 4-wire to 2-wire this way. It often works for 'slave devices', but rarely 'master devices'.

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