hookup a scope to monitor serial i/o
Figure 1. A simple RS232 monitor can be built with some back-to-back LEDs and a couple of resistors. For basic monitoring only TX Data and RX Data need monitoring. Source: Zonker's RS-232 Passive Signal Tracer Page.

The real minimalist circuit is just one pair of back-to-back LEDs to monitor the TX line. In normal operation one LED should stay on when the line is quiet and both should blink (at the baud rate - depending on the data pattern) when the line is active.

Try this and report back.


  [OP's comment:] I built a small one-line (TX) circuit ... on a breadboard. One light dimly flashes during data transmission but the other is always off, so I'm experimenting with different ohmages and baud rates.


Right, now we know that there is activity on the line. Next tackle why the scope isn't working. Hook the scope probe to the LED TX Data line and the ground to ground. What do you see? If you see nothing while the LED blinks then you know you have test equipment trouble - faulty probe, connection or software.