How to Test a Null Modem Cable

Verify null modem full handshake cable connections

Null modem cables are used quite often in telecom, networking and other electronic devices that feature serial communication ports. Connecting two devices with a null modem cable when required is usually very reliable. However, if the two devices fail to communicate, confirming continuity between each set of connections within the cable is a logical troubleshooting step.

Things You'll Need

• Multimeter with continuity "beep" feature
• Null modem full handshake cable with DB9 connectors at each end

Instructions

1. Set the configuration dial on the multimeter to the "sound wave" icon. Touch the two probes together and verify that the multimeter makes a beeping sound whenever the probes touch.

2. Connect and hold one multimeter probe on pin 1 at one end of the cable. Connect and hold the other multimeter probe on pin 1 on the other end of the cable. The multimeter will beep if there is continuity. If the multimeter does not beep then the connection for that set of pins must be repaired.

3. Repeat the previous step for each set of connector pins according to the list below:

   Test pin 1 to pin 1

   Test pin 2 to pin 3

   Test pin 3 to pin 2

   Test pin 4 to pin 6

   Test pin 5 to pin 5

   Test pin 6 to pin 4

   Test pin 7 to pin 8

   If the multimeter does not beep when the probes are connected to a set of pins, then the connection between that set of pins must be repaired.

References

• Ladyada:Multimeter Tutorial -- Continuity
• Zytrax:Tech Info - RS232 Cables and Wiring