[time-nuts] GPDSO is working

[Chuck Harris]

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.
>