RS232 WITHOUT level converter

[JBM]

Simple question (which I will no doubnt take a lot of flack for)

What RS232 interfacing circuitry can I use to receive data from my PC that does NOT use a voltage level converter like a MAX232?

I have done this many times with the PICAXE system and not had any problems. I can send data to my PC fine without a level converter - I'm just having a few 'issues' transmitting to the PIC.

Any suggestions? Ideas?

[rnielsen]

I've used a transistor to convert the level from +/- 12v to TTL. The transistor(NPN) won't conduct when the RS232 goes negative which gives you your 0v and the transistor can be tied to a 5V source (limiting the current by a resistor) you can get a nice 5v signal out. You'll need to calculate how much your base resistor should be to limit the current at 12v but this is a fairly simple way to do it. This only works with the RXD line.

Ronald

[SherpaDoug]

I use TTL level serial all the time, sometimes for hundreds of meters at 1200 baud in a low noise environment like a shielded cable at the bottom of the sea. If there could be a difference in ground potential or if high voltages are bundled in the same cable I tend to AC couple the serial line. Just put a cap, maybe 1uF in series and a resistor, maybe 100k from the reciever input to whichever rail gives a Stop bit.

If you have one device WITH a level converter talking to another WITHOUT a converter it gets a little harder. On the TTL to RS232 line you should AC couple as above except tie the 100K to a voltage that just barely gives a Stop bit. Probably use a resistor divider instead of a single resistor.
On the RS232 to TTL line use a series resistor and a diode clamp to prevent the RS232 voltages from damaging the TTL input. A 1K resistor and a 5V zener will clamp to +5V and -0.7V. I would add another 1K resistor from the zener to the TTL input so the TTL input diodes can clean up any overshoot.
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[JBM]

After installing a new set of eyes, I found the problem. The RS232 pin was connected to 0V.

*slap*

Thanks for your ideas