As you seem to be aware, Modbus RTU and Modbus ASCII are 2 completely different protocols.

With Modbus (either variant) it is notoriously difficult to find the cause of a slave to respond.

This is because the slave device will not inform that master of any issues with the request, and the master cannot determine if the slave sent the response or not.

There are many scenarios in which the slave device would not return a response / the response fails to get back to the master. The request may not have been successfully sent, or it was corrupted en-route. The slave could return a response, but it failed to get back to the master, or the response was corrupted.

Things to check are: I notice that you say the slave device can support both RTU and ASCII variants. Is the slave device definitely configured for Modbus ASCII? Does it use a different port for ASCII and RTU and you are connected to the incorrect port? Are the serial settings correct and the same for both master and slave? Some devices may require different settings for ASCII and RTU (e.g. number of stop bits). Does the slave device have any debug / logging facility? This would maybe give us a clue if the slave is happy with the request.

Do you have a way of capturing the data "off the wire" using a serial port monitor? That would determine if the request is correctly formed and can be received correctly by the slave.

Hello,

to my opinion your problem is with no understand basic principle of Modbus ASCII functionality, so I think your problem is here:

the communication parameters of controler are:
Baudrate:9600, 8-N-1
Port: 1
Address: 40131
Length: 1

From my knowledge the difference with MB ASCII is that only 7 data bits are expecetd here. I recommend better check some resources about difference between MB RTU and MB ASCII in this case to be completelly familiar with that.