home made RS232... is it that simpe?

[ikalogic]

hello, i was just thinking about the r232 protocol - which i never used, but want to-

i want to explore the things, even more i'd like to build the iterface using op-amps to convert the TTL signals to 12/-12 volt signals.

i guess i'll have to boost the the 12 and the -12 volt from the original 5V source and that's the hard part. but supposing i could do this, is the op-amp appraoch correct?

(i allready know there is the MAX232 to handle this!)


thx

 

[Sceadwian]

First thing you're going to want to try is find out if the two devices that are communicating can already do it on their on at 0 and 5 volts levels for logic with zeners or some other form of over voltage protection on the micro controller side, that's often all that's needed. Sending from the micro controller side can get more difficult as a negative voltage may be required.
Generally you only need -3 volts to be in Rs232 voltage specs. And that can be easily done with some very simple digital inverter circuitry and a few capacitors. That's all the Max232 is really, a switch controllable 12 volt charge pump.

 

[Nigel Goodwin]

Opamps would be completely the wrong way to go - as suggested a MAX232 is a ready made single chip solution, and generates the required voltages internally. It's simple to do with a couple of transistors, but to more accurately meet the RS232 spec you really need an external +/-12V supply.

I would suggest you use a MAX232!.

 

[gramo]

The DS275 is really easy to use, and best of all, it only requires a single 5V power supply!

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DB9 Pin 2 - Connect to Pin 7 on DS275
DB9 Pin 3 - Connect to Pin 5 on DS275 (Earth)
DB9 Pin 5 - Connect to Pin 4 on DS275

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The circled pins arnt required if you use VB and disable the handshaking on the MSCOMM control