To: "'\[*AH\]'" analogue@hyperreal.org,
From: "Caloroso, Michael E" CalorosoME@corning.com,
cc:
Subject: RE: [AH]MemoryMoogPlus MIDI in fail
Date: Mon, 03 Jan 2000 19:21:42 -0500
> Now my MemoryMoog Plus doesen't respond to MIDI in. (MIDI out,thru are OK)
>
> I found schematic of MIDI/sequencer boad and trace MIDI in signal, and I
> found opt isolater and buffer(7417) is OK and USART(2661) pin 14 output
> signal repond to MIDI in.
>
> I have no more resource. Anybody have any ideas as to fix this?
>
This isn't too hard.
The USART is probably fine if MIDI OUT is working.
You need to trace the RxRDY signal (pin 14 of U6/USART) from the USART to the
NMI input on the Z80 CPU.
Start with U8/74LS02 on the Sequencer/MIDI board. The output (pin 1) is HIGH
only if pins 2 (RxRDY, U6/2661/pin 14) and 3 (U16/74LS378/pin 7) are both LOW.
Next, check that the output U9/74LS04/pin 2 is LOW every time its input
U9/74LS04/pin 1 is HIGH. This output goes directly to the NMI line on the Z80.
NMI means Non Maskable Interrupt, meaning that when this signal goes LOW the
Z80 stops whatever it is doing and runs an interrupt code vector that handles
MIDI input on the Memorymoog, then goes back to where it left off. On the
Z80 itself, you can't mask (disable) this control. But in the Memorymoog,
it *can* be disabled; this is the purpose of U16/74LS378/pin 7, if it's
HIGH then the RxRDY signal from the USART will never make it to the NMI
line of the Z80. My guess is this is probably true whenever you put the
Memorymoog in Autotune or if it's in POST (Power On Self Test).
Next check the digital board. There should be a jumper from S4X/pin28 to U1/Z80/pin
17, which is the NMI line. Make sure that this jumper isn't shorted against
any protruding IC pins, check to see if the insulator of the wire hasn't been
pierced.
If you see NMI working all the way to the Z80, you probably need a new CPU
(extremely rare).
You might also have oxidation on connector contacts, just the act of unplugging
and plugging them back in will scrape off the gunk and things will start
working again. This is a very common problem on polysynths with gobs of
connectors.
MC
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