QJ71MB91 Fail to read holding register

[kivafishy]

Hi Folks,

CPU: Q03UDVCPU, Modbus Module: QJ71MB91

Anyone knows what have I done wrong with my setup? Parameters, wiring connection or I need to write special instructions?

Theoretically, it shouldn't be so difficult to get the readings. Now, I can't monitor any values from data register D0 ~ D199.

[kivafishy]

I'm using 2 wire connection for RS485.

[Gambit]

You got an exception message so the slave is responding. You should check which one it is.

I see you are reading 120 holding registers. I suggest you reduce the amount first to 1 or 2 until it is working or at least to 50 like you did in your test tool.

[kivafishy]

Thanks Mr.Gambit, I had reduced the number of access points to 10 and I was able to read the data. 

I had done a few tests. It seems the Mitsubishi PLC only can see the data when access points were fixed at 30 or below. If I set more than that, the PLC will show 7360H error and no readings.

I'm not sure is it slave problem or master configuration problem.

[Gambit]

It's not the master it can handle more but the slave is the issue in this case as it is replying an exception error.

If you look in the slave manual than probably Holding register 30 does not exist.

[pturmel]

The ability to read up to 125 holding registers in a single command is the maximum the Modbus protocol allows.  Devices are not required to support that many in one command.  Try using multiple commands, each reading 30 of the desired registers.

[kivafishy]

Problem has been solved. Modsim simulator has to set correctly before master plc modbus module interface with the simulator.

Thanks all.