AVR-Chat

Bruce Parham wrote:

> Zack Widup wrote:
>
> > On Tue, 13 Dec 2005, sheena licarte wrote:
> >>Mr. David and Mr. Zack thanks for the advice. i'll  try to test things
> >>one at a time. by the way is it possible to connect  avr directly to 
> the
> >>rs232 level converter such as max 232 and then to  pc?
> > Yes, the MAX232 series all have TTL inputs/outputs for the device and
> > RS-232 level inputs/outputs for the line.  They are very easy to use.
> Just a little heads up about comm ports on PC's. The RS-232 spec defines
> valid levels for the data line to be > +3V and < -3v. So you would think
> a MAX232 type device running on 3.3V and outputting a +/-6V signal 
> would work
> just fine with a PC. Not so... The PC folks (maybe IBM 20 years ago?) 
> decided
> to attenuate the inputs by a factor of 2, setting the switching thresholds
> at +/- 6v.
>
> This bit me in the rear a couple years back when I fired up a new 
> prototype
> board and connected it to my desktop Dell box. Comm was dead but the 
> scope said
> it was working... A bit of fiddling with a function generator revealed 
> that all
> the desktop and laptop Dells and Sony boxes needed about +/- 6.2V to 
> switch!
>
  Still better than what IBM offered on the PC for serial originally.  
Drove us nuts in a vertical software environment.  I was writing code to 
query and download data from electronic theodolites, and massage the 
data into a standard format for input to a CoGO (Coordinate Geometry) 
package our company sold.  Damned thing would NOT work with IBM PCs 
(whihc, at the time, was a minor inconvenience, because the vast 
majority of our customers were still using CP/M systems.  Biut the 
writing was already on the wall...if you expected to remain successfu;l 
on the market, you *HAD* to be PC compatible).

  it turned out that IBM, in yet anohter oif their infamous and 
braindead noves to "save costs" implemented their serial ports with +5V 
and 0V.  SOME RS-232C transceivers could talk to them, many could not.  
We finally had to locate an addon serial card for the PC, and include it 
with the software for PC purchasers.  What a pain in the arse.

   IBM, like  Mickey$lu$h, only believes in "standards" that THEY create.

tp,