Modbus TCP Device ID

[fischerp]

Hello,
I am trying to connect a Modbus RTU device that is connected to a TCP device as a sub-device.

Using the usual Modbus test tools, I can find the values by specifying the ID used in the internal communication between the two (TCP and RTU) devices as the device ID.

In Rapid Scada, I have now created a new device with the IP address of the “main device” and the numerical address corresponding to the device ID.

As I understand it, this should be the correct procedure? I get values, but not the expected ones.
What am I doing wrong?

• This topic was modified 8 months ago by fischerp.

[Mikhail]

Hello,
Your description is correct.
I suggest to compare Modbus packets sent by Rapid SCADA and the other software.

[manjey73]

Addressing – start with 0 or 1
It is possible to specify byte permutation for 4 and 8 byte data

[JW]

The Device ID (Slave ID) looks correct, since you can connect and get data.

The issue with incorrect data you encountered could be the holding register address instead of the Device ID.

On your 1st screenshot, the address test software is 0 based, and use 40000 data block as holdings register.

While Rapid SCADA administrator is 1 based, and use 03h function code as holding register.

40 holding registers from 40080 on your test app equals 40 registers from 81 of 03h function on scada settings ui.

[Author]

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