I was building that interface and it didn't work for me
(it might work elsewhere but sure didn't like to talk to
ECUs we had to work with).
We had the same problem with all commercial products we could find
(NetWay, B&B electronics and Scantool - which actually uses
ELM chip which is really PIC). we started this ca two years ago.
I am sure all these guys have worked out solutions by now but
at that time we ware quite desperate to get working interface so
simple schematics like in previous post seamed attractive enough
to try them out. Unfortunatelly first two didn't work for us and
we all had sinking feeling and then I came accross circuit that was
using MAX232 and 7407.
Quick comparison with the block diagram of the ISO9141-2 chip
confirmed same inner working.
I've built the first unit and after it worked, I asked our technician
to make more. (so far we did ca 30 of them).
For your circuit (it uses PIC) I would recommend simple interface
like in the K-Line of following drawing. I'm no expert in Russian
language but you should be able to copy two transistor circuit
up to schmidt trigger inverters and substitute those components
with parts you keep in your drawer.
http://www.radioland.net.ua/images/Avto/auto_comp1.gif