[sebhc] H8 optoisolators - aka: disk transfer program

[Dwight K. Elvey]

>From: "Lee Hart" <leeahart at earthlink.net>
>
>Steve Thatcher wrote:
>> I would suspect that it is not an optoisolator problem, but rather the 
components around it. If someone can direct me to the correct schematic, I could 
probably recommend value changes for the other components that would allow it to 
work at 9600 baud reliably.
>
>Most inexpensive optocouplers are indeed quite slow. This is caused by
>the phototransistor. It needs a large base-collector junction to catch
>lots of light; this gives it a lot of capacitance. It also needs high
>gain, to get the largest possible signal out. The capacitance is
>multiplied by the gain (the "Miller effect") to make turn-off time
>particularly slow.

---snip---

Hi
 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
guess.
 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.
Dwight


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