Hello Slobodan,

In answers to your suggestions.

1) Newer MAX232 are "sattisfied" with only 100nF (C9-C12)
Ans. The printed circuit board (PCB) decides which MAX232 IC to use. The PCB is being designed to accommodate any version of this chip.

2) For display contrast (pin 3) you should apply negative voltage (-3 to -5V). And you have -9V.
Ans. This change has been incorporated and the schematic has been updated. Thanks.

3) Backlight for the display requires much higher current than you can get through 1k. This resistor should be about ten times lower.
Ans. This change has been incorporated and the schematic has been updated. Thanks.

4) I suppose the voltage divider R13/R12 and C6 (?) should give 2 levels, depending on P3.7. Do you really get it ?
Ans. This design is based upon an application from Atmel which can be seen at this address. http://www.atmel.com/dyn/produ...ily_id=604

Look over that to see how R13/R12 and C6 work.

5) By opening U12A and U12B at the same time, you shorten them ? Should not it be an inverter to one of them, so only one is open at a time ?
Ans. U12A and U12B is a 4066 analog switch. Only one is on at any given time. Depending upon the level of P3.5 will determine which one of the two is on.

Thanks you for your comments.

Regards,

Charles Bannister