I've mananged to hookup a 16C450 UART to my C64. I can read/write registers and the chip seems to respond.

My schematic is here:
http://www.54.org/condev/c64uart.png

However, I cannot seem to get it to transmit the THR (Transmit Hold Register).
The default registers are set up for 2400 baud (DLAB MSB=0, LSB=0x30), 5 data bits, 1 stop bit, no parity.

I used an oscilloscope to see if the 1.8432mhz crystal is oscillating (which it was). Now I'm stuck at this point.

Any suggestions?

I am not familiar with the 16C5x0 series of UARTs. Is the chip reporting that it even CAN transmit? If it's configured to recognize hardware handshaking, make sure that the CTS (Clear to Send) signal is asserted.

From my understanding (after reading the data sheet), 8250/16450/16550 UARTs leave CTS/RTS DTS/DTR up to the programmer. Before writing to the THR (transmit hold register), you should check the MSR (modem status register), to see if CTS is in the correct state before writing.

I do believe the 6850 UART (Motorola), does indeed need CTS in the correct state to transmit.

Solved the problem.
The crystal was not oscillating reliably, replaced it with an oscillator and all is well.