Probably because you have a serial interrupt that clears TI but also are checking for TI with your JNB TI,$ instruction. The interrupt clears it before the JNB instruction has an opportunity to process it.

Try one of the following:

1. Disable the serial interrupt. See if your transmitting works then. It should.

2. Don't clear TI within the interrupt. Clear it in the instruction after the JNB loop.

3. Define a flag bit. When TI is set in your serial interrupt, set the flag bit and clear TI. Then, your JNB instruction should check the flag bit (and clear it) instead of TI.

Good luck,
Craig Steiner