Section 4: Testing RS232 Serial Using A Screwdriver Or Paper Clip

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.

Screwdriver
Use a screwdriver to loop pins for testing.

When would I need to use this?

When you're setting up any kind of communication equipment, you'll probably have to troubleshoot data ports at some point. In our work with remote monitoring and control systems, DPS Tech Support reps frequently help our clients use loopback testing to troubleshoot RTUs that aren't reporting alarms correctly. Shorting pins can confirm that your device and its data port are working properly. With that established, you can move on to testing cables, protocols, and other equipment in your system.


How to perform a simple loopback test:

  1. 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).
  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.
PaperClip
A paper clip or screwdriver can be a great troubleshooting tool.

When should I use a screwdriver vs. a paper clip

A screwdriver is the right tool when you have an open connector with pins on 2 sides. A slot/flat screwdriver head will usually be the right width to bridge pins. You can even achieve a slight diagonal if you need connect 2 pins that are not directly across from one another. The classic example of this kind of port in remote monitoring is the amphenol connector. It has 50 pins, and each discrete alarm (contact closure) input is a pair of contact pins across from one another. It's very simple to use a screwdriver to short a pair of pins together to test your RTU's inputs during diagnostics.

A paper clip takes more time than a screwdriver, but it has much more versatility because it can be bent. You can use a screwdriver to short 2 female pin sockets on a DB9 serial port. You can connect virtually any 2 pins on a 50-pin amphenol connector. You'll simply need to shape the paper clip into an appropriate shape and insulate it from your hands with a napkin or some other insulator. You'll probably want to have at least a few extra paper clips for this kind of work, because metal fatigue will cause breakages after multiple bends in the same place.

Warning: Exercise caution when working with electricity

These tips are intended to be used only on data ports, where the electrical flow is minimal. You should always use industry-standard safety procedures when working with any electrical equipment. This is never more important than when you're operating on power-input circuits. Discussion of electrical safety techniques is beyond the scope of this article, but understand that there are risks whenever you work with electricity.

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