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Thursday, January 18, 2018
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Allen Bradley PanelView Modbus
An HMI act as Modbus master, fail to get response message from slave, despite the slave is sending response messages

I'm trying to build a project on PanelView C600 Touch Screen HMI. communicating it using Modbus with a pc via serial link. the pc act as a Modbus slave. after configuring both the slave simulating program and the HMI communication settings, I add a pushbutton command to the screen and linked it to a certain signal. when I run the project and press that command, the value of the register changed on the pc, but I got an error on the HMI saying "Write Error for Alias l63, Controller PLC-1, Address is 00001, Communication Flag is 0". looking at the messages, it seems like the pc receiving the message and send its response to HMI; same thing happened with reading and writing all other data types.

Hope any one can help solving this issue.

The slave in the PC may be responding BEFORE the PLC Host releases the RS485 lines.

Thanks for reply, I've already tried to add delay time to slave response, that doesn't solve it.

>The slave in the PC may be responding BEFORE the PLC Host
>releases the RS485 lines.

Does the RS485 driver on the slave have an "Automatic Control Circuit" such as the B&B 485SD9R? Because typically the PC RTS Control is not precise enough for required Key Up Key Down timing.

Thanks for reply, I'm using RS232, not 485.


Then you may have RS232 cabling issues such as no ground continuity.

>Thanks for reply, I'm using RS232, not 485.