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I am trying to configure a GSM modem on a Windows XP box. It communicates over the COM ports. The device shows no change in status whether it is plugged in to the computer or not (though I am not certain that it should). Attempting communications over the COM port provides the exact same result whether it is plugged in or not.

I have tried an RS232 port, a USB-RS232 adapter, and an RS232 port on a different machine. All of them with the same result.

The COM ports exist with no visible problems on device manager (though no attached devices come up, again don't know if they should). When trying to communicate through powershell it throws no errors trying to open or write to each port, using readexisting returns nothing, and readline appears to hang powershell (even ctrl+c won't return it to prompt). Attempting to connect via Putty appears to hang Putty (presumably it starts by performing a readline.

Connecting via HyperTerminal and setting "Echo Typed Characters locally" only displays one letter per letter entered, while claims it should show two. The second machine I tried was a server2003 machine, therefore could not perform the HyperTerminal test on it.

I do not have the resources while at work to create a loopback RS232 cable, but may try that on a third machine when I get home.

What am I doing wrong?

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With COM ports you have to set, in the PC application, the speed, data-bits, parity and stop bits to match those expected by the device connected to the port.


One way to test a serial port is to use a Port And Peripheral Electronic Reactive Communications Link Interface Probe - otherwise known as a paperclip, bend it so it can be inserted in positions 2 and 3 of a female serial port connector, this provides a loopback circuit that should echo back any characters sent to the com port (it relies on the serial drivers not being fussy about other data lines like DSR, RTS etc). You can probably still get RS232 test widgets.

Hayes "AT"

The classic method of testing a serial modem is to issue Hayes commands such as

  • ATE1
  • ATI0
  • AT

Press enter to send a CR character after each command, the modem should then respond with "OK" or some information.

Legacy support

So far as I know, not all GSM modems provide a Hayes-compatible interface. Some do, even over a virtual COM port created over a pure USB connection.

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