Wouter van Ooijen wrote:
>> But the standard that defines what we call RS232 really came before
>> all but the very first primitive commercial ICs (first revision was complete
>> ca. 1962), so I think you'll have to look back much further in time to
>> see why 1 is defined as < 0 and 0 is defined as > 0.
> 
> I agree. But my question was why the TTL 'version' of RS232 (the 
> interface between a UART and the driver) is inverted wrt. the RS232, 
> which requires the driver (besides shifting the level) to invert. Was 
> the inverting driver first and did the UARTs follow, or was it the other 
> way round?

This may not be the answer but when I was learning about TTL in 1978
I recall the professor saying that it was easier to build an inverter
than a buffer. A buffer required more transistor. With a NAND gate
you could build an AND gate but not the other way around. So as a guess
it probably had to do with the cost of the design. I recall that a AND
gate was more expensive than a NAND gate.

Also a rather obscure note on something not directly related. In the
1980's I worked for a company which produced products for the
newspaper industry. One day the AP asked us to build an interface to
a machine which produced photos used in news paper articles (it stank
like vinegar). It was a machine from the 1940's which had a centronics
like interface with RS232 like logic (+25 = 0, -25 = 1). Not sure
what that really means in this conversation but thought it of interest.

-- 
Linux Home Automation         Neil Cherry       ncherry@linuxha.com
http://www.linuxha.com/                         Main site
http://linuxha.blogspot.com/                    My HA Blog
Author of:    	Linux Smart Homes For Dummies
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