Three "obvious"'s when nothing is obvious.
When no context is given, what we assume is that the problem is
Or perhaps your simply trying to overanalyze the issue? It's not
complicated. You can easily gather basic information from the original post
to remove most doubt. First, it's for digital logic by the pic. Second, the
PC900 works in the same circuit. Third, it's most likely not some complex
circuit(just by shear probability). This is enough information, if you have
seen this behavior before, to know what it is caused by. If you haven't seen
it before then obviously you will want more information because you'll not
have a clue what is causing it.
In any case I'd imagine that if I showed the circuit(which I won't because
it is too obvious) you still wouldn't know what is causing it and would then
want to know what kinda scope I'm using, what resistor types, tell me to
remove the capacitors(which there are none but that won't stop you), A photo
of the actual circuit since surely I must have wired up something wrong,
If I give you too much information it will simply give you more
opportunities to be wrong. The key here is that the PC900 works fine in the
exact same circuit. This alone is enough to deduce the behavior(since the
6N138 is similar specced and generally used as a replacement).
The point is that if you didn't get an idea of the problem in the first post
your not going to get it without knowing way more information than I'm
willing to sit down and type in.
Don't be ashamed of not knowing though. It's not that big a deal. I'm sure
you could figure it out if you had the problem but I'm not willing to be
asked what color socks I was wearing when I took the pic. AGAIN, either you
have seen such behavior before and have come clue what is causing it(one may
think initially it is capacitance but then it should most likely cause the
same problem with the PC900) or you don't have a clue.
If you want to know what the circuit probably looks like then do a google
image search for "Opto circuit" and pick one at random(at random) and you'll
probably have a 19/20 chance of getting the circuit I'm using.