AVR-Chat

Thomas Keller wrote:
> 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 back 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.

It is no different now than then. Back then, RS232 was run around and
between buildings for terminals and mainframes. If you still want to
run those distances, you'll want +/-15V.

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

LVDS only works over a few metres length without repeaters and
equalizers. RS232 can do a few km.

There is much less ground noise in the ~400MHz LVDS frequency range and the
short cable lengths it was designed for, and it is differential mode as well.
LVDS is double-terminated 50ohm transmission line, so there is significant
signalling current that is hard for stray electric fields to affect much.
Plus the sync preamble allows the serdes PLL to lock, and a slowish PLL loop
also averages noise jitter. RS232 is a much higher impedance system ~5kohms.

http://www.edn.com/article/CA6288038.html
http://focus.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=SLLA120