Interfacing with serial port on IRB6600

[richardmcc2]

I've been having trouble interfacing with the serial port on my IRB6600 robot.  I have an ADC (http://www.omega.com/pptst/D1000.html) with serial output that is hooked up to COM1.  The ADC requires a $1RD command to request data from it, so I'm trying to send that string to the serial port, and then read the data value that comes back.   When I try and run this program, it hangs on the line that tries to read the serial port.  Can anyone tell me what's wrong?

Code:

MODULE Controller
VAR iodev com_port1;
VAR string signal;
PROC GetSignal()
OPEN "COM1", com_port1\BIN;
While True Do
WriteStrBin com_port1, "$1RD";
ReadStrBin com_port1, signal;
TPWrite signal;
EndWhile
CLOSE com_port1;
ENDPROC

ENDMODULE