Modbus Function 8

[MichaelAur]

Hi all,

We are looking at introducing a new basic RTU into our system, and I noticed in the documentation that it refers to a Function 8 message to allow rebooting the controller. I’ll admit I’m not very familiar with this function, but from looking at the standard, it appears to be a diagnostics function and may only be supported over serial (and we will be connecting by TCP!).

I am following up with the supplier about this as well, but I thought I’d check here: if the supplier has set up Function 8 over TCP as well, is there any way we can send Function 8 messages from Geo SCADA, or even a way to formulate a custom Modbus message to replicate this command?

The image below shows the information I currently have available. Based on this, I would guess the message would look something like:

[TCP header] 00 01 00 00 00 06 01 08 00 01 FF 00

Thanks for any help you can provide, and sorry if this shows I’m missing something basic about Modbus.

[AndrewScott · Accepted solution]

Modbus function code 08 (diagnostics) isn't supported by any of the Geo SCADA Modbus drivers.

The support functions code are listed in the help: Modbus Function Codes Supported by the Geo SCADA Expert Advanced Modbus Driver 

[geoffpatton]

Options I can think of all of with may not be worth the implementation cost for something that in my experience is almost never used. My experience being with DNP3 which support reboot commands.

Maybe something using MQTT and Node-Red if any of the Node-Red Modbus drivers support function 8.

Writing a custom driver.

Writing an external program that Geo SCADA can interact with.

If the RTU has programmable logic, you may be able to program an output address to trigger a reboot.

If the RTU has digital output contacts and one that will be available you could put an interposing relay on the supply power and use the output to operate the relay.