RS232 Serial Port Sniffing / Snooping

Have you ever needed to snoop a RS232 serial port to observe the messages passing back and forth? It’s easier than you thought. All you need is application called HyperTerminal and the simple cable below. The cable can be assembled in minutes and requires a couple of diodes which you can probably buy for pennies. HyperTerminal is provided free with the Windows Operating System.
Here is what you need to do

1. Assemble the cable.
You need a 3 conductor cable and a 3 9-pin d-type connectors. We suggest you put a male on one end and a female on the other end and do not cross pins 2 and 3 over. That way you have a cable segment that will extend any cable without reuiring sex changes or worrying about 2-3 cross over. Put a female connector on the snoop end so it connects to your PC port without a sex change.
The Diode part number is 1N4148. The black band on a diode is the cathode end and corresponds to the vertical bar in the schematic of a diode.
A diagram is provided below the text of this article.

2. Connect
Connect the devices with your cable and connect the snoop to your computers serial port.

3. Run HyperTeminal
START, Programs, Accessories, (Communications), HyperTerminal
Set the port and configure it. In this example we use COM2 to snoop on a 9600,Even,8,1 conversation.

4. Capture Messages
In HyperTerminal, select the ‘Transfer’ menu.
Select Capture Text (even if the protocol you are snooping on is binary)
Specify a file name.
When you have captured enough, stop the capture and you can then use the file to view the conversations.

5. Limitations
No timing will be captured. All the messages sent and received by the devices you are snooping on will be ‘received’ only in HyperTerminal. This you will have to deduce the message ends/starts.
No timing information is available
If the messages are binary you will need to view the binary file in binary mode. You cant do this with notepad or WordPad. We use the Visual Slick Edit editor – Use Ctrl-Shift-H to change the view to binary.