Midi-CV part 2

[Isac Georg Jensen]

First, I want to thank you all for the suggestions that you have given,
regarding my midi-cv converter.

Here's a short account of the last few days:

Instead of trying to receive directly from the keyboard, I let the
68HC705C8 send AA(hex) at 31.25kbps ( I also tried sending 55(hex)).
This worked just fine, when monitored on the scope. Hmmm ?!! This
suggest that it's got nothing to do with the u-processor's serial
interface. 

I rewrote the interrupt handler routine, so that it's very fast now. All
that it does now is discarding the Running Status bytes, and placing the
rest in a buffer. Next, I set a breakpoint in the handler routine, where
it deals with the buffer placing. It only trigger whenever I press a
key, so this leads me to belive that it receives something. The problem
is that the received bytes are totally random ! 

Next, I checked the optocoupler circuit. It's a pretty standard circuit
( 220 ohm resistor, diode on the input side. Collecter thru 4.7k
resistor to +5V, and emitter to ground. Output is collector pin). When
applying a 31.250 kHz square signal to the input, I observe a distorted
signal at the output. It now looks more like a Sawtooth signal, than a
Square. I've measured the rise and fall times to about 150 usec.
Acoording to the spec's the rise/fall times should be around 2 usec.
I've tried to replace the CNY-17 (optocoupler) with another, but I get
the same results. Can anyone tell me if the circuit is right, or if I'm
doing anything wrong ?