Conneting 1747-NET-AIC DH-485 connector output to same type of HMI

[mahone]

Hello There,

In my control panel, port 1 or RS232 port of a 1747-NET-AIC is connected to a compactlogic l32e and DH-485 or port 3 of the converter is connected to a HMI.

The HMI used Modbus RTU protocol to get the data into the HMI components. One HMI communicates well without any issues. But i must add the second HMI to display the same details.

I connected another pair of wires to the DH-485 port A and B pins of the same 1747-net-aic and terminated to RS232 port of the second HMI but now i only see some values in both HMIs

Can I split the DH-485 signal this way is it correct?

I appreciate your answers.
Thanks

 

[Ken Roach]

The problem is the protocol, not the wiring.

You are using the 1761-NET-AIC as an ordinary RS-485 to RS-232 converter.

It is not using DH485 protocol. That is a specific Allen-Bradley protocol that can run over RS-232 and RS-485 wiring.

Modbus RTU protocol is a "master / slave" protocol. There can be only one Master in the system.

In almost every implementation I have seen, the HMI is the Modbus Master. You cannot have two Masters on a single Modbus RTU network.

 

[mahone]

I loaded sample Modbus Master program from RSLogix5000 and changed the HMI protocol to Modbus RTU server. According to the HMI datasheet this protocol can support multiple slave and one master.

1747-NET-AIC shows right communication transmission. When i change the slave addres inside the logic to what i gave in in HMI they can communicate.

Without a modbus mapping list document or anything i find difficult to decide the Master command details.

One time i establish clear communication between the PLC and the two HMis i could change the Modbus HREG4[0] - Modbus HREG[4] change in the HMI when i change them in the logic.

then after one HMI displayed only 0 in all the tags.

should i continue using Modbus RTU server or DH485

I just have to display same PLC data in two HMI using RS485 wire because on HMI is installed far from the main panel

Thanks