To:  dhenry@sprynet.com
Subject: Programmer, adding schottky diodes?
Sender: frieder.ferlemann@...

Hi Dan,

I just looked at your project page at 8052.com and
fear that the level conversion circuit might
violate the device's input ratings:




    PC                    AT89S'
    DTR>----/\/\/\----+--->MOSI
             10K      |
                   |__|__
       Clip DTR       ^  | 5.1V
       to 0-5V       /_\   Zener
                      |
                      |
                     GND



I'd suggest to add a Schottky Diode in parallel to
the Zener Diode in order to better limit the negative
voltage?
Otherwise the internal diodes of the AT89Sxx might
be used to clip the voltage (not really dramatic
but not nice either:)

Greetings,
Frieder