All,

It seems to me that if a logic 1 in RS232 is -3 to -25 volts, and a TTL 1 is
between 2.4 and 5 volts, there would have to be an inversion there to make
things work out correctly.   Therefore, the RS232 driver inverts the state
of the incoming TTL signal, and drives it to the negative rail for a mark.
Likewise, a TTL logic low would cause the RS232 driver to drive the output
to the positive rail for a space.

Jim

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
William "Chops" Westfield
Sent: Saturday, November 14, 2009 1:28 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: why do RS232 drivers invert?


On Nov 14, 2009, at 6:01 AM, Wouter van Ooijen wrote:

> 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?

IIRC, rs232 is carefully defined in terms of "mark" and "space" signal  
states corresponding to 1 and 0 (except that I'm not sure that the 1- 
 >mark mapping was in the original spec), so matching up with the TTL  
states for 1 and 0 might have been a logical process.  On the other  
hand, I think rs232 drivers pre-dated the LSI logic of the first  
UARTs, so maybe not.  Remember that before TTL and its associated  
"near the power rail" logic states became the "clear winner", logic  
states were not always so obvious (ECL with it's -5.2V supply comes to  
mind.)

BillW

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