On Wed, 17 Dec 2003 20:23:49 -0300, Dennis Crawley wrote:

>Based on a lot of readings, I did this interface. After many people
>successfully used it, it fails in my client computer, not in only one
>computer but in two! =8O
>

You're not sending a valid RS-232 level to the PC. By spec, the
minimum legal swing is -3 volts to +3 volts. (IIRC the maximum is -12
to +12 but don't quote me on that; I'm not certain of it.) Some PC
serial ports will operate with a 0 to 5 volt swing but many won't.

Is there some specific reason that you're using this circuit rather
than a more conventional circuit based on an interface chip like
Maxim's MAX232 chip?

>It works fine form PC to pic but I had to decrement the resistor value
>in the PC receiver part.
>

A lower than expected positive voltage on TxD will give you less
current through the opto and keep it from ever turning on. The 4N26
doesn't have all that great a current transfer ratio to begin with.

>Any hint on how can take current dynamically without burn out the PC port?
>Changing the opto perhaps?
>

A higher CTR device would probably improve reliability on the PC to
PIC path but you really need to go below ground on the PIC to PC side.
The MAX232 solution would eliminate both problems and reduce your
parts count as well

.
Regards, Bob

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