Craig Webb wrote:
>At 14:14 3/10/98 -0500, you wrote:
>>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.
>>
>>Thanks.
>>
>>C. Webb

The Csaba replied with:
>Hi,
>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)

RTS or DTR are NOT TTL level signals.  You will have to pass them through a
level translator (such as a 1489 or the RX half of a MAX232) to get TTL/CMOS
level outputs.  From here, the signals can be used to control a relay driver
(such as a ULN2003A).

myke

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