[1000mp] RASCAL and 1000MP

[Tom Rauch]

> I had a similar problem with a homebrew PSK interface using serial
> port PTT keying using one of the many circuits with an optisolator. 
> It would only pull the PTT line down from 13.2V to about 2.5V - not
> enough for PTT "closure."  

I'm puzzled why we so often use opto-isolators in systems that do not 
require ground isolation. I see that quite often these days.

On the other hand I see many interfaces that do not isolate the audio 
grounds from chassis, something that is required!

Opto-isolators have two problems in applications requiring a line to 
go almost to zero volts. They have a generally small current transfer 
ratio, and never quite saturate the output device. You are almost 
always left with a large fraction of a volt or higher when trying to 
pull a line low, and have to drive the input with considerable 
current to accomplish that.

On the other hand a transistor circuit could have a current gain of 
thousands, and pull a low current line (like the PTT line) virtually 
to zero volts. It would be virtually invisible to the computer, and 
actually offer more of what you need.
   73, Tom W8JI
W8JI@contesting.com