RS-232 Inverter

[Kalle Pihlajasaari]

Hi,

> I posted this message last month but never saw it come back.
> This is a rs232 to ttl level converter that should work for the 8192
> baud ecm's.

Seen it 3 times now.

> I used it a few times on non auto related projects. I haven't tried it
> yet
>  on a car. You can delete the inverter if if you want a one way
> interface.

To keep the polarities of the signals the same you would have to 
have the same number of inversions in each path.  The MAX 232 chip will
invert both signals once.  The 74LS07 device is NOT an INVERTER so
you can use it any number of times in this circuit.  It is
just an open collector output BUFFER.

As has been mentioned there may be 0 .. 12V on the signal from the
ECU so one should add a bit of protection to the logic side input
pin of the MAX232  which is intended for 0 .. 5V working.  A
series resistor of about 10kOhm should prevent random latchup and
stuff in this application.  It is likely that the output of the
ECU is current limited in some way so you should not cause serious damage.

Another alternative that MAY work is to use a level converter designed
for use in connecting a CASIO BOSS or Digital camera to a PC.  These
cables are to be had for about US$25 or so.  One guy in the US is selling
them that you can see at the following adress

http://www.ip.co.za/people/kalle/sirplus.htm#larry

I have added the protection resistor to the diagram below at pin 10
of the MAX232.


>               +-------------+  +             
>   +-----------|1          16|---+---o +5V (VCC)
>  +|          +|             |  ===
>  === +5V o-||-|2  MAX232  15|---+---o GND (card & RS-232)
>   |           |             |
>   +-----------|3          14|
>            +  |             |        o +5V                     
>      +---||---|4          13|        |
>      |        |             |        \          
>      +--------|5          12|        / 10K
>           +   |             |        \
>      GND o-||-|6          11|        /
>               |             |        |
>      RxD o----|7  ---<--- 10|--/\/\/-+----------+----o ALDL I/O
>               |             |  10K         |\   |
>      TxD o----|8  --->---  9|--------------| |--+
>               +-------------+             1|/ 2
>                                           74LS07
>     
>        (also connect to 74LS07, pin 7=GND, pin 14=VCC)

Cheers     *<<<-|
-- 
Kalle Pihlajasaari   kalle at ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle