RS-232 interface and voltage stabilization

Yvan · Post by **Yvan** » Fri Oct 24, 2008 7:57 pm

I need RS232 interface for my second engine modification - LPG Lambda Control System (LCS). Since my knowledge of electronics is very limited, I wonder if someone here might help.

Googling for rs232 interface produce a lot of results, all very similar, and also quite different.

The basics is the same, MAX232, but capacitors seems to vary from 0.1uF ceramic (as used in MJLJ V3), to 1uF electrolite, or 10 uF electrolite ones.

How critical is this? Should I use ceramic or electrolyte?

I also need 5V for MAX232. I have 7805, and again designs, and values vary. Do I need all the components shown below in MJLJ V3:

http://www.autosportlabs.net/asl_dist/m ... v3_sch.png

or do diodes and zener diode serve some other role apart from voltage stabilization (lot of solutions do not have diodes)?

Post by **brentp** » Sun Oct 26, 2008 5:13 pm

Yvan,

Review the datasheet for the proper charge pump capacitors on the MAX232. The MAX232 chip required 1uF capacitors, whereas the MAX232A is designed to work with less expensive 0.1uF capacitors.

For the power supply section you can get away with fewer parts- a reverse protection diode and filter capacitors would be sufficient for many application.

Yvan · Post by **Yvan** » Sun Oct 26, 2008 9:05 pm

Thank you Brent, I have MAX232N, and 78L05. I just finished PCB design in CadSoft Eagle. Hope it's OK

Yvan · Post by **Yvan** » Thu Nov 06, 2008 6:14 am

I have another question. I noticed (now that I got my PCB finished)

at the few web sites that pins 15 and 16 of MAX232 are connected to the ground and +5V. I did not do that, and neather have you for the V3 version here:

http://www.autosportlabs.net/asl_dist/m ... v3_sch.png

So, is it needed or not?