Modscan64 gives Timeout Errors for all responses

Question

I am using Modscan64 to poll a service using TCP. I always receive this message:

** MODBUS Message TIME-OUT **

I connect to loopback 127.0.0.1.
I have tried with function 3 (holding register) and function 4 (input register), using length 2 and address 0001.
My service (coded in Python using twisted for TCP) receives the request, properly interprets the bytes, processes it and sends a response.
The response I send is received by MODSCAN as verified in the "Show Traffic" mode. The bytes in the response match what my service sent.

Is timeout really the issue?

How do I change the timeout? I see no settings for TCP Timeout. (googling explains how to set it for RTU only.)

If it is not really a timout, then what?

Here is a typical request / response:

01 00 00 00 00 06 01 04 00 00 00 02 04 04 42 ca 00 00

Request (12 bytes):

First two bytes 01 00 are a counter that increments for each request.
First seven bytes are the TCP header: 01 00 00 00 00 06 01
The next five bytes are the MODBUS request: 04 00 00 00 02
In the request, 04 is function code for read input register
In the request, the next two bytes 00 00 are the address to read
In the request, the final two bytes are 00 02 for the quantity of registers to read

Response (6 bytes):

The MODBUS response is the last six bytes: 04 04 42 ca 00 00
The response echoes the function 04 to mean not an error
The response then has 04 for number of bytes to follow
The response then has 42 ca 00 00 which is single precision for 101.0 (the simulated temperature value being tested)

Does MODSCAN expect a TCP envelope in the response?

Do I need to do something to indicate byte order?

Answers (1)