Monitoring ASCII Stings between Two Serial Devices

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Monitoring ASCII Stings between Two Serial Devices

Postby pmonahan » Sun Mar 16, 2014 9:50 am

I Love CoolTerm. Mac and Windows.

I have a home automation controller with a serial port that connects to a third party serial device for lighting control.

I've used CoolTerm to send and receive strings from the automation controller with success. All of the ASCII is correct.

I've also used it to interface to the Lighting Controller to send and receive ASCII strings and that works correctly as well.

The problem is, when connected, the two systems are not playing well and I need observe the communication between both devices to determine where the fault is. I need to observe the response packets that the 3rd party device sends back in response to commands from the automation controller and I also need to observe the command packets sent from the automation controller *when connected* to the 3rd party Lighting Controller.

Hoping one of you serial-killers might have some ideas because I'm certain CoolTerm can't loop through a COM Port to a 2nd port.


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Re: Monitoring ASCII Stings between Two Serial Devices

Postby roger » Mon Mar 24, 2014 1:50 am


You can use 2 terminal windows in CoolTerm in conjunction with a spy cable to monitor traffic between two serial devices. You will need 2 extra serial ports on your computer to do this. Connect the TX signal from the serial connection you want to monitor to the RX input of the first port, and the RX signal from that serial connection to the RX input of the second port. You will be able to see what the first devices sends in the first terminal window, and the responses from the other device in the second window. Here is the connection diagram for such a spy cable:
spy cable.png

Alternatively, if your serial connection is half-duplex, i.e. only one device is talking at once, you can make a different cable and only use one extra port to monitor your traffic in a single CoolTerm window. However, you will not be able to distinguish which of the two devices sent what data. The cable for this setup looks like this:
spy cable half-duplex.png

I hope this helps.

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