How to Troubleshoot Serial Communications

So something happened, and now your RotorCard or Rotor-EZ isn't responding to computer control?  This article will help you troubleshoot the problem.

The first step is to take the cover off the rotator control box, and inspect the RotorCard or Rotor-EZ, which will be called "the board" for the rest of this article.  Does the control box smell like burnt electronics?  Are there any burnt components on the board?  If everything looks okay, check to see if the board is getting power.  If you have a Rotor-EZ, see if you can make the rotator turn by using the front panel controls.  If you have a RotorCard, you should be able to measure +5 VDC compared to the control box ground at hole 8 on the left side of the RotorCard.

So the board looks okay, and it gets power.  The next thing to do is to start testing the serial communications chain.  Start by testing the serial port with a different device, if you have one.  It is common to have the serial port be the problem.

If the serial port works, then the next step is to try a loopback test.  Unplug the control box from mains power; the loopback test does not require that the board have power.  Carefully remove the MAX232 integrated circuit from its socket; the MAX232 is sensitive to electrostatic discharge, so take the appropriate precautions.  Bridge pins 13 and 14 of the MAX232 socket with a component lead or other piece of thin solid wire.

Next plug the serial cable into the serial port on your computer, and run HyperTerminal or another equivalent serial communications terminal program.  If the test is successful then you should be able to type text and have the text echo to the screen, just as if you were typing text into a word processing program.  If the test fails, then no text should appear on the screen as you type.

The loopback test is very simple, really.  All you are doing is connecting the RS-232 port TX pin to the RX pin.  The signal path includes the serial cable, wire traces on the board, the socket, and your temporary bridge.  If the test is successful then you know that the serial port is good, and that signals are getting as far as the MAX232 socket.  If the test fails then there is a problem with the serial port, the socket, or somewhere in between.

If the loopback test failed, you can use a multimeter to test for continuity between the serial cable connector and the MAX232 socket.  Typical multimeter probes are too fat to make a good connection with the holes in the RS-232 cable connector; I attach the probes to clipped component leads, and poke the leads into the holes in the RS-232 cable connector and the socket.  There should be continuity from pin 2 of the RS-232 plug to socket pin 14, from RS-232 pin 3 to socket pin 13, and from RS-232 pin 5 to ground on the board.  (On the RotorCard the ground is connected to hole 5 on the left edge of the board.)

Good luck with your troubleshooting!