Testing Serial RS232 Ports

Rybett Controls WebDoc 102   (Copyright 2008)   Return to [Index]

Shop computers and controls are often interconnected by their serial RS232 ports. Since lightning and power surges can travel on port cables, RS232 ports can be a maintenance problem. Serial port test procedures for various types of hardware follow.


All Rybett DNC software includes the programs TESTCOM1 and TESTCOM2.

1. Exit to the DOS prompt and change to the directory holding Rybett software. For instance, if you are using Rybett Camstore Menu software, just type "CD \CAM3" followed by <Enter>. Then type "TESTCOM1" or "TESTCOM2".

2. When in the test program, any key pressed is sent out the serial port and any character received is shown on the display. Locate the appropriate port connector on the back of your computer and unplug any cables attached to it. Most PC COM ports use 9 or 25 pin male connectors shaped like lopsided rectangles. Jumper pin 2 to pin 3 on the port connector on the back of the computer. You may use a jumper wire, a paper clip or the blade of a screw driver. Now, if the port is working, any key pressed will be sent and returned by your jumper and will appear on the display screen.

3. If the port works, you may use it to test your long shop cables. Remove the jumper between pin 2 and 3 and replug your cable onto the port connector. Then use the jumper to connect pin 2 to pin 3 at the far end of the cable. This just makes a very long jumper. If the cable is good, the port test program will again display typed keys.

4. The TESTCOM1 and TESTCOM2 programs operate at 9600 baud using even parity. If your interconnected equipment also uses these settings, plugging everything back together and running the test program will allow you to send and receive characters. For instance, Rybett Camstore 2 and Camstore COM BTR units are normally setup for 9600 baud operation and may be tested by placing the mode switch in "REMOTE" and pressing the "RESET" buttons. The letter "S" should be sent and should appear on the PC display if everything is working.

5. Note that all Rybett DNC software uses a common method of setting port parameters. If you are unsure of your settings, just choose "Machine Setup" from the DNC menu and look at the first resulting line. It should read something like "COM2:9600,e,7,1,p". In this example, the port used is COM2 and the baudrate is 9600. If your DNC menu does not display a "Machine Setup" option, it may be present but hidden. Try typing one menu number larger than the last menu item. Typical passwords are "password", "PASS" or "ADAMA" without quotes.

6. The TESTCOM1 and TESTCOM2 programs are just batch files that call the master program RTERM at 9600 baud. If you wish to operate at different port settings, you may call RTERM directly. For instance, if you wish to communicate over port COM1 at 4800 baud, using even parity, 7 data bits and 1 stop bit with software handshakes, at the DOS prompt type "RTERM COM1:4800,e,7,1,s". Or, if you wish to use COM2 at 1200 baud, with no parity, 8 data bits and 2 stop bits using hardware handshakes, you may type "RTERM COM2:1200,n,8,2,h". When using the "h" flag, be sure to satisfy CTS/RTS hardware handshakes.


Up to 16 additional serial ports may be used by Rybett Phantom DNC systems. To test any Phantom DNC port, you must first reboot the computer (or start the Windows Phantom task) with the background TSR communications program disabled. For DOS applications, edit the line in the AUTOEXEC.BAT file that calls "CAMCTSRx" to read "rem CAMCTSRx" which turns the line into a remark. Then reboot the computer. (For Windows applications, edit the batch file that launches Phantom.)

To test ports COM1 or COM2, use the TESTCOM1 or TESTCOM2 program as above. To test ports COM3 thru COM18, use the DIGILOOP program in a similar manner. To properly test DIGILOOP ports, use must jumper pin 2 to pin 3 and you must jumper pins 6, 8 and 20 with another wire.


Rybett Camstore and Numeritronix Exec BTR products share a number of common features. They all may be tested by connecting to a PC computer running the RTERM program set at a matching baudrate as described above. In the case of units with a built-in display, the test is even easier. If a bad port is detected, try replacing the 1488 and 1489 RS232 line driver chips on the UART card. Locate these chips by following the cable from the port connector to the UART card. These IC's are widely available at electronic stores. If this does not fix a bad port, send to Rybett for repair. CAUTION: before servicing any Camstore or Exec, unplug from power.

1. CAMSTORE 1 terminals have built-in CRT displays and may be placed in a TERMINAL emulation mode by pressing <Break> and then <Ctrl> T. (Just hold down the <Ctrl> or <Shift> keys and press the letter T at the same time.) When in Terminal Mode, any key pressed is sent out the serial port and any character received is displayed to the CRT. If you jumper pin 2 to pin 3 and pin 4 to pin 5 on the serial port connector behind the CRT, any key pressed should be displayed.

2. EXEC 1501 and 1502 terminals are simply early versions of the Camstore 1 described above. Use the same procedures to test. Some Execs were configured with RS422 ports which can communicate over greater distances than RS232 ports. When testing RS422 ports, you must additionally jumper pin 15 to 16 and pin 17 to 18. To repair this type of port, try changing the 3486 and 3487 driver chips.

3. CAMSTORE 2 BTR interfaces do not have a CRT display. However, they may be tested using the TESTCOM1/2 or RTERM programs described above. The Camstore 2 consists of a small blue/black Operator Box usually mounted on the outside of an NC control and a larger computer module usually mounted on the inside. The baudrate is controlled by a small DIP switch on the computer module. Switch positions 1,2,3 set the baudrate. Some common settings are:

                   1 OFF and 2,3 ON for 9600 baud
                   2 OFF and 1,3 ON for 4800 baud
                   3 OFF and 1,2 ON for 1200 baud

The DIP switch may have a tab that is slid to an ON position or may have a rocker which is pushed in or out. Sometimes the rocker is labled with an arrow that points to the opposite side of the switch. In such cases, it is very easy to misread the DIP switch setting.

Connect the Camstore 2 to a PC computer running TESTCOMx at 9600 baud or RTERM at a specified baudrate. Place the Operator box mode switch in REMOTE and tap both RESET buttons at the same time. A letter "S" should be sent and should be displayed on the PC screen. Only run this test after you have confirmed a good port on the PC computer.

4. CAMSTORE COM BTR interfaces are very similar to Camstore 2 units but are repackaged into a single box. They may be tested just like a Camstore 2 as described above. The DIP switch is located in the lid of the Camstore COM enclosure which may be accessed by removing the four lower screws and rotating the lid upward. The Camstore COM may also be self-tested without using a PC computer.

a. Unplug the DNC cable at the Remote port on the side of the unit and jumper pin 2 to pin 3.
b. Place the Mode switch in Remote and press both Reset buttons. (This clears the port.)
c. Place the Mode switch in Bypass and hold down both Reset buttons for several seconds.
d. A green light indicates a good port while a flashing red light indicates a bad port. The test continues, so removing the port jumper will cause an error.

5. CAMSTORE 1SX, 2SX and SHOPBOX interfaces are shop hardened PC computers. Their basic serial ports may be tested using the TESTCOMx programs as described above. Note that on some computers with high impedance ports, the stray capacitance between port pins is enough to simulate a jumper even if one is not present. Connecting a 1K ohm resistor to pin 7 ground will load such ports so that they behave properly during testing. PC computer serial ports are best repaired by replacing the serial adapter card. The serial adapter cards are generally sold as combo cards which include other functions. Always use the cables provided with such cards as there are several wiring variations in use. If the serial port is part of the computer motherboard, disable the bad port by accessing the BIOS during boot and add a new serial port adapter card. Use the new cables.