AVR-Chat

Russell Shaw wrote:

> Peter Jakacki wrote:
> > Ok, this is good, it's getting meaty and I like some of your figures.
> > This only takes into account a discrete BC548 and not the typical input
> > transistor of the receiver which also happens to be buffered by two
> > other transistors in typical inverter follower fashion.

[excess text removed for brevity]

> > Sometimes it's not about margins but about knowing what is redundant 
> and
> > clearing away the clutter. There are a lot of things about 232 
> including
> > the standard itself that are redundant and I get very tired of people
> > quoting the decades old rules that applied back then without taking
> > current usage into account. Sub 120K low-speed short-range coms to a PC
> > needs high plus and minus volts??? ...come on, nobody but nobody 
> designs
> > like this except to keep the voices happy.
> Unless the RS232 line is driven with an open collector/drain output stage
> which guarantees close to 0V, i'd not trust even the fastest most modern
> RS232 receiver to be reliable. The other problem is that the fastest most
> modern RS232 receiver is extra sensitive to line noise when you have a
> low switching margin.


   And, looking at the time when EIA RS-232 was originated, and 
realizing the state of electronic conponents bavk then, it is the noise 
margins that explain the high voltage requirements of the standard.  +/- 
15 VDC wasn't all that radical, considering that noise immunity sucked, 
and noise impulses were often (as measured in practice, not 
theoretically) measured in the 5-10 Volt range.  Given modern components 
and circuit layout techniques, of course noise margins are smaller. 

  One wonders what the engineers who develoiped the RS-232 
recommendation (it is a 'recommendation, *NOT* a "standard, after 
all...RS standing for "Recommended Standard") would have though of the 
LVD transmissioon schemes so common today.  +/- .7 volts!!!   Your 
signal would disappear into the noise!   *grin*

Tom