Years ago, I worked at Intel, when they made systems.
While I was there, I learned some about specmanship.
We had problems getting serial current loops to work at
9600 and some times even 4800.
We traced the problem to the optical isolators.
The biggest problem was the darlington ones.
Two factors were involved.
First, the turn off spec was done with the higher
end of current flow, while the turn on spec was
done by the lower current flow. Both of the timings
where tested under their optimal condition for
shortest time.
They usually have the lead for the intermediate emitter
base connection.
A resistor can be added there to optimize the circuit
for best turn on and turn off at the desired current
drive.
This only worked for a short time.
It seems, there is no spec for the gain of each separate
transistor. Form one manufacture to another there
is a large difference. In some,the photo transistor
has more or less gain and the difference is made up
by the output transistor.
The fix should have been to not use darlington optical
isolators but it was decided to only use one manufacture.
The conclusion I'm coming to is that if one needs
higher speeds, don't design using the darlington
outputs.
Dwight