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Reading/writing to serial port w/ VISA in Labview

I'm writing a Labview program to control and to read data from a Varian vacuum pump controller. It is connected to the serial port in my computer, and I have been trying to open a VISA session to communicate with the instrument. So far, however, the computer cannot see the instrument--I get the same error messages reading and writing to the instrument as I do to an empty serial port. Does anyone have any suggestions on what the problem might be? Thank you.
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There are a lot of possiblities. Is the cable correct? You probably need a crossover cable (TX and RX) swapped. You also need to check and make sure the serial ports on both ends are set the same (i.e. baud rate, handshaking, stop bits, etc.). Make sure that the port is enabled. I've seen people try to use a comm port only to find that a modem card installation caused the port to be disabled. You could also try using hyperterminal to talk to the instrument. If you can communicate there, you can eliminate any hardware problem.
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