The circuit below is fine if the other end is a 'real' RS232 driver.
If both of them are like this, then nothing will happen since both
ends will wait indefinitely for the other to put a -ve signal on its
Tx.  You go first... no, you go first... no, ...

This is a definite inclusion for the Devil's handbook of dodgy
circuits (available from Hades Publications, cost: 1 engineer's soul).

Regards,
SJH
Canberra, Australia

> From: Gena <gena@EUNET.YU>
> I found this years ago and it works great, tested on several machines
> and long cable length (>>30m ).
>
>
>                          + 5V    +5V
>                          |       \
>                          \       / 2K2
>                          E\ |    \
>                BC557       \|    |
>                            /|----+--/\/\/\/\--- TTL In
>                          C  |         10K
> DB9 (2) ----------------+
>                         |
>                         \
>                         /  2K2
>                         \              + 5V
>                         /              |
>                   +-----+              \
>                   |     |              / 10k
>                   |   -----            \
>                 \--/  ----- 10n        /
>        1N4148    \/     |              |-----------TTL Out
>                 -----  _|_GND       | /C
>                   |          10k    |/
> DB9 (3) ----------+---------/\/\/---|\   BC547
>                                     | \E
>                                        |
>                                       _|_
>                                       GND
> Best regards,
> Gena
>