Thanks for the feedback everyone. I apologize for not making myself clearer.
I don't know the why behind it, but whoever designed the system I'm working
with said I have to set the TX bit high. I asked the same question about
voltages and inverted signal, etc. at the RS-232 end, but the engineer told
me all that has to be done is to hold the PC port bit high (I guess on the
UART before the MAX232 or whatever it is). So unfortunately DTR & RTS bits
won't help, but thanks for the suggestion.

If there is no direct memory mapped address to the TX bit, perhaps I have to
send a continual stream of bytes=0FFh? Any ideas?

Craig

>>Does anyone know off-hand the memory address and value for setting the RS232
>>TX line high on a PC? I need to trigger a relay connected to this line.

>First of all, what do you mean HIGH on TX line on RS232 port of PC ?
>The TX and RX lines (in RS232) are not on TTL level.
>The 'logic 1' on TX (idle state) means this line is about -12V respect
>to ground, the 'logic 0' on TX (mark state) means this line is about +12V
>respect to ground.
>Apart from this, you can't change the state of TX in this way (to control
>relay).
>Instead of TX, use the RTS or DTR signal.
>These signals are TTLs, and it is easy to use as general purpose bits.
>To change these signals, set or clear the appropriate bit:
>(DTR)
>[baseaddr+4].0 (ie. base address of serial port, eg. com1=0x3f8,
>so the appropriate bit is the lsb of 0x3fc)
>(RTS)
>[baseaddr+4].1 (ie. base address of serial port, eg. com1=0x3f8,
>so the appropriate bit is the second bit of 0x3fc)