Possible Issue with reading Virtual Serial Ports

[joraguero]

Hello,

I am experiencing issues when trying to communicate with Modbus RTU clients through virtual serial ports, resulting in communication errors.
I have two scenarios that I can describe. In both cases, I am able to communicate successfully using another Modbus client application.

Case 1
I have a physical device that creates a virtual serial port over a USB connection (for example, COM20 configured as 9600E1).
ModbusTool correctly detects the port, but when it attempts to communicate via Modbus RTU, a Timeout error occurs.

1.1) Additionally, I observed that if I first communicate with/read COM20 using another Modbus application, then close that application and subsequently try again using ModbusTool, I am able to communicate with and read the device successfully.

Case 2
On the computer, I create a Modbus client using a Modbus Slave application. Then, I create a virtual serial port (for example, COM100 using VSPE) and connect the Modbus slave to the virtual port (COM100). I then start ModbusTool, which correctly detects the port. However, when attempting to communicate via Modbus RTU, the following error appears:
Modbus exception with code 0x02.

Best Regards.

[joraguero]

Hello, I'll provide new information.

In case 1 (specifically investigating point 1.1), what's needed is to activate the DTR signal after opening the port.
When running this simple program before ModbusTool, it works perfectly.

import serial, time
port = serial.Serial('COM20', 115200, timeout=1)
port.dtr = True
time.sleep(0.3)
port.close()