PC serial port questions

My old computer had a serial port that I could read 9600bps data exported from a 16F628 operating at 5V without a MAX232 chip. I installed a card with a parallel and serial port on my new computer to be able to interface with some legacy hardware, and when I try to read data from the same PIC, I don't get anything...

So my question is, was my old port operating with TTL as opposed to RS232? And is the new serial port I installed in my new computer operating with RS232? For reference, my old computer was XP, and my new computer has Vista.

Also, the CD with drivers has a bunch of subfolders and options... can I configure the serial port to be TTL as opposed to RS232?

Thanks in advance for any insight!
Dave

RS-232 voltage levels are +/- 3V to +/- 12V. This means that the receiver (your PC) - by specification - doesn't have to read the 0 to +4 V output of the PIC correctly.

The PIC will probably see the output of the PC, since most of them output +/- 10V or so. This also brings up a point - make certain that the input pin of your PIC has resistor in series!

And no, you can't change the driver to output TTL levels.

So how come my older computer could read serial data from a PIC operating at 5V? Was it not "RS232"? I had ground between them connected, so the data line wasn't "floating".

Also, hypothetically speaking, couldn't I convert TTL to an "approximate" RS232 with an op-amp using 6+V with a 3V reference as ground? Or even a transistor tied to a 6+V supply with a 3V ref tied to the RS232 ground?

Using MAX232 (level converter) you will be able to Connect TTL to RS232 Without any problem.

Cheers

Al.

Quote:

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Originally Posted by achilles03

So how come my older computer could read serial data from a PIC operating at 5V? Was it not "RS232"? I had ground between them connected, so the data line wasn't "floating".

Also, hypothetically speaking, couldn't I convert TTL to an "approximate" RS232 with an op-amp using 6+V with a 3V reference as ground? Or even a transistor tied to a 6+V supply with a 3V ref tied to the RS232 ground?

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A lot of things work outside of their specifications - they just aren't GUARANTEED to do so. A PIC pin is rated to sink 25mA. Most will do 80mA,
but that isn't something that I would expect from every device, under all conditions.

Some RS-232 receivers REQUIRE that the signal go negative, some don't. The spec says they must. Your ideas *may* work, but a properly configured MAX232 *will* work.

I suggest you read the Wikipedia explanation of RS-232. You obviously have a computer and an internet connection.