Hi Craig,

>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?

Sending a continous stream of 0x0FF will just send a +12V "blip" on the line.

If you want to output a continuous +12V from the RS-232 port, set bit 6 of
the "Line Control Register" (Base Register + 3), which is the "Set Break"
bit.  By setting this bit, you will, for all intents and purposes disable
the serial port's transmitter and just send a "Space" (or +12V).

I *think* this is what the engineer is trying to tell you, but if this
doesn't work, maybe he should go back to RS-232 school.

myke

>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)
>
>

"If people don't know what you're doing, they don't know what you're doing
wrong." - Sir Humphrey Appleby K.C.B