I've been planning to use an X-10 CM17A serial firecracker, a TM751 transceiver and a universal module so I can manually reset a remote PC when it freezes. (Fortunately there are two PCs at the remote office so as long as they don't get stuck simultaneously I can reboot the frozen one from the working one.) On the way to the forum (as it were) I discovered that the PC I wanted to use would not fire the transceiver. I got it to work from three other PCs, one running XP and others running Win98. But the PC that had to do the job would not, even if I moved the transceiver right next to it.
I was about to blame the NVidia serial port implementation on my Asus A7N8X-X but then found on CNET downloads a different software interface for the firecracker. It worked... as long as the transceiver was within 8 or 10 feet of the firecracker. At the edge of the range the ON command would work but OFF would not. Guessing there are some differences in the effective power transferred to the firecracker from the port and also the power required to transmit the codes.
I was going to settle for that solution but then ran across FireworX 1.3 from triplewhitefox. It was of interest because unlike the official X10 software (and the alternative that actually worked) FireworX can do scheduled events (which I want for my house) so I installed it and found the effective range was 20+ feet! I know X10 says it should work up to 100 feet but 20 is really exciting compared to my prior experiences.
Still looking into factors that affect serial firecracker range, but I found it interesting that different software would have such an impact, especially when the official X-10 software would not work at all with this PC. For those having joy issues with the firecracker, try different software (and different serial ports!). It does work after all! [Dave]