Do you have a breakout box? This is a box with LEDs that show the status of the RS232 lines, like DTR, DSR, RTS, CTS, Rx and Tx. This device is very helpful in troubleshooting RS232 problems. I suggest you get one.
Plug one side only into the breakout box, start with the computer You should see the following LEDs light up green:
DSR & CTS & CD. If DTR & RTS light up, the equipment is a terminal device. If DSR & CTS light up, the equipment is a computer device.
So plug in one side and determine if it is a terminal or computer device. Then remove that connection and plug in the other end. If both devices are terminal devices or both are computer devices, you will need a null modem cable to connect the two together. This could be why your setup is not working. Normally, the computer is a computer device and the pressure controller is a terminal device.
If this is not the problem, then look at the LEDs with both ends connected. The following LEDs should be lit green: DSR, DTR, CD, RTS, CTS. When you try to send data, the Tx and Rx LEDs will blink. If your baud rate is too fast, you might not see the blinking, instead the LED may be on solid. If you see the Tx blinking, or on solid, and the Rx does not come on, then your pressure controller is not responding. If the Tx LED does not come on, then your computer is not transmitting the command.
Without a breakout box, this will be hard to troubleshoot. get one. They are available from electronics stores like Black Box, Newark, Digi-Key, B & B electronics. These places also sell null-modem cables.
If the proper LEDs do not light up, you may have a broken wire.
