digital-logicrs232ttl
I'm just looking for a historical answer as to why TTL and RS232 don't transition logically in the same directions. The signals are the same, but basically level-shifted and inverted.
Was there a technological reason behind this, was it a mistake, or was there hardware that inverted that was cheap and easy to come by to drive the outputs for RS232 communication?
Thank you for helping to satisfy a curious question.
Here is how I would suggest going about debugging this problem:-
Check the connections.. if you've mixed up signal ground and chassis ground (shield) you could get funny behavior just like you describe. Pin 7 on the DB-25 is the one you want to connect to DE-9 pin 5. I'll assume you're using a commercial adapter/cable or have one wired like this.
Maybe check the transmit out line to ensure it's idling at -12 ( < -3 anyway).
Make sure the instrument is set to RS-232, and not current loop (Jumper 7,8 and 1,2).
Try the most common serial port setting (9600/8-N-1), 9600 baud, NO parity, 1 stop bit switch 1, 2, 3 OFF switch 4, 5 ON switch 6 ON switch 7, 8 ON
Make SURE you understand which is 'OFF' and which is 'ON'. Don't laugh, I've seen this happen more than once, and some types of DIP switches are kind of ambiguously marked. If in doubt, an ohmmeter across the switch.. in fact you could check all of the switches to ensure they're working.
I like Realterm as a terminal program. It's free, and easy to change the parameters. Try it on something known working first of course, and look for an online tutorial. Really worth it.
If still gibberish after that, get a scope screen cap or photo and post it.
If you have a continuing need for a logic analyzer, this one is very cheap and relatively useful. Not as good as my Tektronix boat anchor, but this one will fit in a coat pocket. But I'd get a scope first if you don't have one. Even the worst scope is better than nothing.
Your question was way to long to read, but it seems you are having some problems with a MAX232-type of converter circuit. I have made what seems to be a very similar circuit using one of the TI variants of the chip, and even sell it as a product. Go to www.embedinc.com/products/rslink2 and you can see all the documentation, from a picture to the board layout to the schematic. Perhaps you can see what you are doing differently.
One thing I noticed while skimming your question is that you are using electrolytic caps. Double check that the polarity of each is correct. Another issue is that some COM ports, particularly USB to RS-232 converters, won't work without RTS/CTS, whether this is enabled in software or not. Note how I have pins 7 and 8 connected to each other on the DB9-F connector.
Best Answer
This is partly speculation, but here goes:
Originally, asynchronous serial interfaces were current loops. In order to make things like detecting wiring faults easier, the "idle" state was defined as current flowing, and the "start" bit was a brief interruption of the current.
The first devices were strictly electromechanical, but as soon as digital electronics became common enough, there was a desire to use the teletypes for I/O. It would make sense, then, that the logic that drives the current loop would be "on", or high for the same condition, because that would be the obvious way to drive, say, an NPN transistor.
Now, switch to voltage signaling. The telephone line sits at a negative voltage with respect to ground, because it was discovered early on that this reduced electrolytic corrosion in the wires caused by moisture.
So, for the same reason, the "idle" state in RS-232 was defined to be the negative voltage, because that is the state that the line is in most of the time.
Therefore, the translation between logic and RS-232 needed to be inverting.