- 1). Connect one side of an alligator clip to serial cable pin 4. There are nine pins on a serial cable, and each pin has a separate function. The pins that are used to power devices are pin 4, known as DTR, or data terminal ready, and pin 7, known as RTS, or request to send. These are the first pins you should check.
- 2). Connect the second alligator clip to pin 5, the ground pin on the serial cable. Connect the alligator clips to the oscilloscope. Connect the pin 4 wire to the positive input on the oscilloscope, and the pin 5 wire to the negative terminal.
- 3). Switch on the oscilloscope. The serial cable voltages vary between +12 V and -12 V. Turn the Volts/division knob on the oscilloscope to the 10 V/division setting. Use the vertical position knob to move the flat line to the center of the screen, and place it directly over one of the horizontal marker lines.
- 4). Connect the free side of the serial cable to COM port 1 on your computer. Switch on the computer and launch the ComTest program. Check the DTR option in the ComTest window. The DTR indicator in the window should turn red. At this point, the DTR signal line is outputting a digital high of 12 V. Check the oscilloscope trace. The horizontal line on the screen should have shifted up by 1.2 units, indicating a positive voltage reading of 12 V. Uncheck the DTR option in the ComTest window. The oscilloscope signal should show a -12 V corresponding to a digital "0." This indicates that the DTR pin is working. Connect pin 7, the RTS pin, to the oscilloscope and test it in the same way.