If a TTL signal does "just work" with your RS232 receiver, you have
a faulty receiver. The receiver is supposed to have a dead zone from
+3V to -3V. If you can get the receiver to function with 0V to +3V,
it has substandard noise immunity. And then there is the little matter
of what will happen to the TTL input being connected to a real RS232
driver when It sees a potential +12V to -12V input.
I know that TTL level signals used to work with the original IBM-PC
comports, but they were using home made receivers and drivers that
did not meet the RS-232 spec.
The biggest problem with the MAX232 is its receiver was designed not
to the RS232 spec, but rather to work in the same way as the IBM-PC
comports. It's RS side threshold is at 1.3V, and it has only 0.5V
of hysteresis.
It's not a bug, it's a feature?
-Chuck Harris
Chris Albertson wrote:
Whoever programmed the PIC in question inverted the signal for some
reason. There should be no need for an inverter to use the MAX232
devices, they knew what they were doing when they designed them.
If you have an inverted TTL serial signal then you can connect it straight
to n RS-232 port and there is a very good change it will "just work". They
call it "TTL level RS-232". BUt if you want to reliable drive a long
cable it is best to level convert to true RS-232. But today most "re-232"
is actually using zero and five volts.
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