Handbook for Sound Engineers

Consoles 929

numbers from 0 to 31). For any of up to 32 command
states, preprogrammed responses for eight output lines
are immediately accessible.
This particular type of baby PROM is usually used at
the top end of microprocessor memory maps where a
page (256 bytes) is given over to the function of the
processor vectors, such as interrupts. As an example, if
the processor receives a nonmaskable interrupt (NMI),
it usually means “Panic! The power is collapsing!” or
some other similar situation. NMI makes the processor
look at a certain address in the page of the baby PROM,
which tells it where to find in memory a program to
save the environment (i.e., hide safely all the crucial
operating data, quickly).
In the context of this channel system, the PROM
outputs drive the analog switches (organized per Fig.
25-96) to route and control the channel and monitor
signal paths through the system elements. This occurs in
accordance with and under the command of the PROM
address inputs, which are indicators of selected channel
function (record/mixdown/overdub), local or remote
fader reverse commands, and, importantly, mute and
solo status.
Most of the control logic is still done in hardware,
largely consisting of jammable debouncer/flip-flops. For
the channel function control, a single pushbutton that
steps through the four functions is realized by a simple 2
bit counter (IC23 in Fig. 25-97). This generates a 2 bit
code that feeds both the PROM control inputs and a
4028 binary to decimal decoder IC25, which drives the
relative status indicating front-panel LEDs.
Solo, solo-free, and solo-safe are dealt with in IC16,
IC20, and IC24, but the relevant action on the analog
circuitry is still executed via the PROM. It can be
deduced that the solo command and mute of the PROM
do just the same thing, resulting in a fair number of
duplicated and redundant program codes within the
PROM. At least this gives room for expansion or func-
tion modification (if and when required) by simple card
link changes and a differently programmed PROM.
Here is one of digital’s great strengths—the future capa-
bility of “chameleoning” a system simply by software
changes, not by hardware: a built-in upgrade path.

25.15.10 Logic Meets Analog

The 7602 PROM hangs between logic ground and 5V
(of the split ±5 V logic supply), thus necessitating all
input feeds to be similar in swing—0 to 5 V. All the
drive logic flip-flops, debouncer, and master bus logic
are similarly powered.

Analog transmission gates, such as the design of Fig.

25-96, are required to pass (and stop) analog signals

referred to ground and, therefore, of both polarities, so

the gates have to be fed from a split supply (in this

instance, the ±5 V logic supply).

Converting between the 0 and 5 V logic and the

±5 V control voltage swing needed by the gates is done

by using the open-collector output drives of the PROM,

Fig. 25-100. Open-collector is exactly that—there is no

positive output pull-up internal to this PROM. The idea

is that it may be paralleled with other open-collector

devices in a wired-OR bus configuration. When the

output transistor is turned off, the collector is at a

high-impedance state. The collector is pulled up an

extra 5 V above the internal supply of the PROM. When

the transistor turns on, the collector dutifully zaps down

to the 5 V supply. It doesn’t care what is at the other

end of the load pulling resistor provided it isn’t of

excessive potential (12 V is safe; the output ports are, in

fact, the programming path with these devices and

much above that may induce some involuntary repro-

gramming).

Some of the analog switches are driven directly off

the PROM outputs, while others have the necessary

inverse-switching feed provided by conventional

inverters.

As a note to the unwary, bipolar memories such as

the 7602 use a lot of power when being switched. This

explains the large amount of decoupling festooned

around it and the logic supply generally. Needless to

say, the analog transmission gates are referred to audio

ground, not the click-infested logic ground, despite the

fact that they are powered off the logic supply.

Figure 25-100. Input/output termination (unipolar to bipo-

lar control swing conversion utilizing PROM open- collec-

tor output).

0 V

Input

swing

0V/ 5

5 V

Open

collector

output

10 k 7

Output

swing

+5 V/ 5

PROM

+5 V