Sequential PROPHET-600 - 7-2 OSCILLATOR B

Shorts may occur within a device or between printed traces. But since the computer is
known to have been running, the problem is more likely to be device failure than a
problem with the traces. Nevertheless it is probably best to begin troubleshooting the
computer with a close visual inspection, especially around sockets. (Magnifying glasses
are often helpful.) If you see no evidence of mechanical problems such as broken
traces, broken sockets, conductive particles, botched soldering or careless repair
attempts, try removing or swapping socketed devices such as the CPU, EPROM, and
RAM. Even if the computer isn't running (due to one of these devices being missing), a
change in static voltage indications could be a good clue about which is the bad part.
If you have checked all socketed components (perhaps even a few unsocketed ones),
then there may be no recourse but to cut printed circuit traces. The customary
technique is to make the first cut at the electrical center of a the bus line, to isolate
the problem to one half or the other. When the cut has yielded information on the
direction of the malfunction, it should immediately be repaired—to prevent unrelated
malfunctions. Then halve the suspect trace again, and so on, until the bad IC (or
socket, or shorted trace) is isolated.
2-6 INTERRUPTS
With the basic architecture of the microcomputer now introduced, we can look at the
input and output processes in more detail.
To accomplish real-time tasks such as calculating envelopes and responding to MIDI
inputs, the Prophet-600 microcomputer is interrupt-driven at a constant rate. The
Z-80 has two interrupt inputs: -INT (Maskable Interrupt) and -NMI (Non-Maskable
Interrupt). The first is constantly clocked, the second is used only for MIDI (see below).
Timer U320-10 which forms the Interrupt Clock was briefly mentioned above. This
third of the triple-timer device is programmed to divide-by-10,000, yielding the 200
Hz (5 ms) -INT signal. Because this interrupt is "maskable," there are some rare
occasions during which -INT is ignored. But normally each interrupt pulse forces the
CPU to:
Calculate the current values and effects of the six separate envelopes.
Calculate the LFO.
Calculate the effect of GLIDE.
Refresh the LEDs.
Alternately read the PITCH or MOD wheel (because these can be expected to
be moving constantly or quickly).
Read one other control knob (because these can be assumed to be moving rarely
and slowly).
Refresh all CV Sample/Holds.
All of this interrupt processing takes about U ms. This leaves less than 1 ms until the
next interrupt, during which time the CPU resumes background tasks such as reading
the keyboard and figuring voice assignment.
TM600A 7/83
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