[sebhc] H8 optoisolators - aka: disk transfer program

Steve Thatcher melamy at earthlink.net
Fri Jun 18 19:14:15 CDT 2004

specsmanship is certainly a problem. Different manfuacturers will spec the same equivalent device different ways. I checked Vishay for the 4N26 and they at least kept the same circuit for their specs. When it comes to an optoisolator, the turn on time is always quicker than the turn off time. If you have a base connection, you may be able to speed up the turn off time with a resistor to ground.

Glad to hear that swapping parts got the serial to work okay.

By the way, did Heath use 1488 and 1489 chips for the RS232 along with the optoisolators?

best regards, Steve

 One of the biggest problem with them is that the spec sheets
alway spec them under the best conditions. Turn off time with
a low value load resistor and turn on time with a high value
load resistor. Real life is someplace in the middle but where
or what that'll be with any specific load resistor is anyones
 The darlington ones do allow access to the base lead but
there is no spec on the relative gains of the two transistors
so you can't depend on using that lead for anything practical.
 Anyway, swapping the two on my board brought the recieve
side up to a respectable level. The Transmit side is a little
less sensitive to these issues. I'm not sure why Heathkit
chose to use these on the RS232 path, rather than a simple
transistor or two, as a level shifter.

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