At 05:55 AM 11/14/2009, you wrote:
>In a lesson I want to explain asynch serial data format and RS232 signal
>levels.
>
>AFAIK the RS232 signal levels predate the 5V TTL levels (correct)?
>
>The TTL-RS232 level converters/shifters are all inverting. Does anyone
>know why? Because inverting drivers are slightly easier to build? Or
>were the asynch-on-TTL levels chosen first, and as a consequence the
>drivers had to be inverters?

The first popular RS-232 receiver was developed by a friend (Jim Thompson,
then working for Motorola). Here is a modern datasheet from Jim's website:

http://www.analog-innovations.com/SED/MC1488-DataSheet.pdf

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 suspect a thorough search will take you back to the time of Mssr. Baudot
himself (19th century) when teleprinters were purely electromechanical
devices and operated on a current loop principle with electromagnets, cams,
and whirring motors.

Here is a manual for a relatively modern (1931) teleprinter:
http://www.bitsavers.org/pdf/teletype/126_Model14_Descr_Dec31.pdf

Probably not much different in principle from the ASR33 and TWX/Telex
machines that some of us old f*rts can remember using.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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