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Section 4: How to Test RS-232 Connections with a Screwdriver or Paper Clip

1. Use a screwdriver to loop pins for testing.

   Remove the cable from the near-end port of the problem connection. Take a paper clip or screwdriver and loop the port's TX pin (typically Pin 2 on RS-232 ports) to its RX pin (typically Pin 3 on RS-232 ports).

A paper clip can be a serious - and handy - troubleshooting tool.

What's going on: Creating a loop with a paper clip may seem silly, but it's actually a great troubleshooting tool. By looping the transmit and receive pins, you can test whether the ports and connectors transmit and receive valid information. This is called a loopback test.

2. If the device will support it, make a Telnet or proxy connection to the near-end device.
3. Perform a loopback test: Type any character. You should see the character echoed on the screen. Now remove the paper clip from the port and type a character. Without the loopback, you should not see the character echoed.
4. If the result of the loopback test is exactly as described in Step 3, the port is physically OK. If the result of the loopback test is different, the port may be defective.
5. The next step is to test the problem port's cable.
6. Unplug the cable at the far end and loop the TX pin to the RX pin.
7. Make a Telnet or proxy connection to the near-end device and perform a loopback test.
8. If the cable fails the loopback test, you might need to replace the cable.
9. But before your replace the cable, double-check your pinouts. The near-end and far-end ports may have identical pinouts, so using a straight-through cable will result in one TX pin being connected to another TX pin. You may need a null-modem adapter, which reverses the TX and RX signal.
10. If the cable is OK, the problem might be on the far-end port. To make sure, loop the far-end port's TX pin to its RX pin, make a Telnet connection to the far-end device and perform a loopback test there.

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