# MAX232 testing

Discussion in 'Electronic Basics' started by FredBear, Jan 29, 2004.

1. ### FredBearGuest

I stuffed up while building a cable for my casio calculator.
The circuit http://hw.cz/english/constrc/casio/casio_en.html uses a
MAX232.

The prob is I initially soldered up the 78L05 around the wrong way. & when I
tested I couldn't get comms
so I went back & rechecked etc...

So now I have the 7805 around the right way and I get 4.58v measured on pin
16 pin 15 of the MAX232
But still no comms.
My breakout box shows pin 20 going high & it has 12V.

Since this is such a basic circuit, I'm starting to think that I've 'blown'
the Max232....

Any ideas / help on how I ca test the MAX & or the circuit would be
appreciated.

FWIW I'm using 1uF Caps for C1 & C2 & C3 and 2.2uF for C4 & C5 (these are

BTW I'm fairly new @ electronics etc, so please don't make any assumptions

As an aside, the following page has a similar circuit except for the pinouts
on the RS232 & an extra diode, as far as I can see...
is this a 'better' design?
http://www.gbnetwork.co.uk/circuits/boss/index.html

TIA For any input / help

2. ### Tim DicusGuest

Hi FredBear,

Check each "TTL in - RS232 out" converter.

Put 0 volts into pins 10 and 11, then check pins 7 and 14. Should have about 10 volts output on each.

Put 5 volts into pins 10 and 11, then check pins 7 and 14 again. Should have about -10 volts output on each.

If that succeeded, then check each "RS232 in - TTL out" converter. Connect pin 7 to pin8. Connect pin 13 to pin 14. This will be a
"loopback test".

Put 0 volts into pins 10 and 11, then check pins 9 and 12. Should have about .6 volts output.

Put 5 volts into pins 10 and 11, then check pins 9 and 12 again. Should have about 4.5 volts output.

By this point, you should be able to tell if all is well, or where the damage is.

Tim