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MODBUS RTU RS232 shows connection exists without cable being connected


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Hello everyone,

I have a Unistream Built-in 7" PLC+HMI. I am trying to communicate with third party devices which only have an RS232 serial port.

I have one of the devices directly connected with RS232 to the UAC-CX-01RS2 module on the PLC. I then configure the UAC-CX-01RS2_0 port to Modbus Panel with the correct settings, i.e., Baud rate, data bit, parity and stop bit. In Unilogics I'm unable to find the flow control which it uses, does anyone know what this is?

I then assign the PLC  as a remote slave (master) on the UAC-CX-01RS2_0 and try to read a certain register from which I'm not sure whether its active (still waiting on this information). However, the status value of the remote slave shows 3 if there is a connection, but the modbus commands give an error. And it reads 2 if there is no connection at all.

When in online mode, the remote slave status value shows 3, implying there is a connection. However, if I pull the RS232 cable from the port, it still shows 3. Although a connection then is impossible without the wire, of course.

Can anyone help me figure out what is wrong with my serial connection? Or give me advice on how to properly setup MODBUS TRU RS232 connections in unilogics because I can't seem to find matching information with my setup.

Thanks in regards!


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Thanks for the reply Joe T.

Can it not pose a problem that Unitronics doesn't use flow control?

My retry is 1000ms and my response timeout is set to 1500 ms.

The settings I am able to set on my device are:
Baud rate: 9600
Data bit: 8
Parity: None
Handshake: None
In the manual it says the Stop bit is default set to 1 and cannot be changed.

The program is currently only used to setup communication with the third party device (which is a CVC2000 vacuubrand vacuum controller), I uploaded it onto this post.


MODBUS Communication setup.ulpr

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