AVR-Chat

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.
> 
> Zack

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!

Just by luck I found an old HP desktop that switched at +/- 5.5V. That one is still
connected to my prototype.

Bruce