[time-nuts] GPDSO is working

[Chuck Harris]

It all harkens back to the old teletype machine.  Teletypes used a
current loop interface, and to make sure that it was immediately
obvious if the interface was broken... eg. the lines were down, the
teletype was designed so that when there was current, the printer
was silent, and it was the lack of current that caused the asynchronous
code to progress.  This "negative" logic has persisted in all teletype
machines... and was incorporated by Western Electric into the RS232
interface.

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.

-Chuck Harris

Bob Stewart wrote:
> Hi Brian,
>
> That's just strange.  There are a whole lot of these MAX232 and MAX3232 devices
> being sold.  Hmm, I'm looking at the UT+ User's Guide, and it lists the voltage
> levels as follows.  These would imply that an inverter is necessary, right?  Could
> it be that someone programmed your PIC upside down - i.e. using negative logic?
>
> TTL 0 V to 0.8 V = logic 0 2.4 V to 5.0 V = logic 1 RS-232 (reordered from manual
> to put logic 0 on top) 5 V to 15 V = logic 0 -5 V to -15 V = logic 1
>
> Bob - AE6RV