[sebhc] H8 optoisolators - aka: disk transfer program

Steve Thatcher melamy at earthlink.net
Fri Jun 18 12:02:49 CDT 2004

Hi Lee, thanks for filling in more technical details as to why I asked someone to point me at the right schematic.

best regards, Steve Thatcher

-----Original Message-----
From: Lee Hart <leeahart at earthlink.net>
Sent: Jun 18, 2004 11:07 AM
To: sebhc at sebhc.org
Subject: Re: [sebhc] H8 optoisolators - aka: disk transfer program

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.

There are circuits that can partially correct this, at least up to 9600
baud. They usually drive the LED very hard (20ma), and use a very small
collector resistor (like 100 ohms). You then need some kind of
amplifiier on the output side to get it to be TTL compatible.
"Never doubt that the work of a small group of thoughtful, committed
citizens can change the world. Indeed, it's the only thing that ever
has!" -- Margaret Mead
Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net

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