MIDI trouble

[Ingo Debus]

Hi all,

I recently built a simple box with a 8032 uP, a MIDI in and a few LEDs
in it. It just switches LEDs on/off when some Note-Ons/-Offs are
received. Normally it works fine (thanks for all the hints how to handle
Real-Time messages correctly, BTW).
But when hooked it to a friend's Mac laptop with a MotU Fast Lane MIDI
interface (without power supply) it doesn't work. It looks like several
MIDI bytes are skipped, since LEDs sometimes go on and not off again.
This MIDI interface works fine with other MIDI gear. With two other MIDI
interfaces the laptop and the box work  well.
Since my friend's MIDI interface is powered from the modem/printer port
I suspected a level problem. But with a MIDI patchbay between box and
MIDI interface things didn't get any better (perhaps even worse). OTOH,
MIDI data coming out of that patchbay were fine for other MIDI gear.
My box has just the usual MIDI in, with CNY17 photocoupler, 220 ohms series
resistor at the input and 2.4 kiloohms pullup at the output.
Could that be a timing issue? Curiously I opened the MIDI interface and
couldn't find anything that looked like a quartz or quartz oscillator.
Usually a Mac MIDI interface has to supply a 1 MHz clock to the Mac
(unless it's a multiport interface with its own uP), hasn't it? My box
should do the timing right, there's a 12 MHz quartz connected to the 8032.
(Maybe it's time to get a storage scope now....)

Anyone have any clues or similar experiences?

(OT, but while we're at it, does anybody know a free 8032
cross-assembler for the Mac?)

Ingo