Prosoft MVI56E-MCM Modules : RS-485 duplex 2/3/4 wires

[AREDA]

Hi All,

I just wanted to confirm something regarding this module. What I understood from the manual, is that if RS-485 mode is selected (jumper settings to match), then it expects a 2 wire link wired to RJ-45 Pin 1 (Data+ aka TXDRXD+) and Pin 8 (Data- aka TXDRXD-), and optionally GND wire to Pin 5 [Not the same role as the screen wire]. (DB-9 pins if the supplied short adapter cable to be used)

If a device is providing 4 wires for this interface, then MCM is not ready for such connection under RS-485, so it either has to
1- change to RS-422
2- change the device to half duplex mode (possibly jumpering TX+ with RX+ and TX- with RX-).
3- Use a gateway in the middle, so the gateway provide only the + & - wires to MCM side.

Is my understanding correct? In other words, does MVI56E-MCM allow RS-485 full duplex without conversion in the middle?

Thanks

 

[danw]

A quick read of the manual confirms your interpretation.


The fact that the module does not support full duplex RS-485 is because it is specifically designed for Modbus. Modbus is a poll/response protocol where the master waits for the slave's response before it polls another device.


There's no real need for separate Transmit and receive buses because the master will not transmit until it has either received a response or until it times out because of lack of a response. The Modbus protocol is not intended for full duplex (talk and listen at the same time) operation.



>does MVI56E-MCM allow RS-485 full duplex without conversion in the middle?


I don't think the module operates in full duplex mode at all. The jumpering converts it to half duplex 2-wire. There's nothing you can do 'in the middle' that will make it full duplex.


Is there a full duplex protocol (other than Modbus) running on RS-485 that you're trying to implement?

 

[AREDA]

Thanks a lot for your response, danw.

Yes it's only Modbus that we're interested in. I shouldn't have used the term "full duplex" I guess I wanted to say 4-wire devices in this context.

So, for whatever funny reason, some devices may offer 4 terminals for RS-485 Modbus interface, and reading the note in the MCM user manual, it appears to suggest that for such devices, the expectation is to have a "Gateway" in the middle. I found this one on the web for example https://www.cyber-sciences.com/wp-content/uploads/2019/01/TN-106_RS-485_2-wire_4-wire_systems.pdf. [It mentioned RS-485 half duplex in one section]

I just want to confirm, that for the MCM module, with RS-485 selected, we only need pin 1 & 8 (5 optionally) on the RJ45 port; and we're not supposed to short pin 1 with 2 and pin 8 with 6.

We'll be conducting a test soon, but wanted to get the draft design right.