>> Similarly, I would say that the idle condition being high on the logic
>> side followed the conventions of the time, since that was a lower-power
>> state for most of the earlier logic families.
>>
>> Therefore, the 1488/1489 were designed to interface between these two
>> worlds.
>
> That would be my guess too, but I hope for someone to have solid facts.
> Maybe the piclist audience is too young...

It may also come from mechanical teleprinters which idle in the mark
condition (from Morse's original recording telegraph where loop current
caused the tape to be marked). Teleprinters had a loop current of 60mA
(series pulling magnets) or 20mA (series holding magnets) between
characters. This was probably carried over to UARTs where loop current was
represented by a high. The RS232 drivers and receivers PROBABLY invert
because it takes fewer transistors to make an inverter compared to a
non-inverting buffer.

Harold


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