Handbook for Sound Engineers

928 Chapter 25

tive in this application, catch the very first input transi-
tion, and stuff out a uniform, clean, predictable clock
pulse for the flip-flop. Subsequent bounces merely
extend the output pulse slightly but don’t generate any
spurious output transitions. An alternative would be the
interspersing of Schmitt trigger buffers between the
switches and the flip-flops. These have a very wide
hysteresis, which in conjunction with some R/C
retarding can also provide surprise-free toggling.

Flip-flops can have their outputs jammed by stuffing
the required state-up set (making the Q output go posi-
tive) or reset (negative)—an invitation for remote
processor control.

25.15.8 Logic Sense

Some of the logic in this particular design is unconven-
tional, all done in the name of reducing component
count, largely obviating level-shifting transistors while
maintaining the inviolable ground-for-active law of
control interfacing. This is a common-sense rule that
simply means that any accessible control line should
just need to be taken to some reasonable ground in order
to activate whatever it’s supposed to—not to a specific
voltage above or below ground. This helps avoid the
“should this go to +5 V or 24 V” routine, while greatly
simplifying system design—grounds are omnipresent.

The main reason for the unusual logic powering, Fig.
25-97, stems from the use of a bipolar PROM in the
assignment logic. This needs a tightly controlled 5 V
supply, unlike CMOS ICs, which will run off nearly
anything with volts on it.

25.15.9 What Is a PROM?

PROMs (or programmable read-only memories) are

digital devices used extensively in computer tech-

nology for storing individual items of information or

sequences of information that are regularly referred to:

• Memory is self-explanatory.


• Read-only means that in normal operation it’s only
  possible to retrieve the information that’s stored. New
  information cannot be put in or the contents modi-
  fied.


• Programmable means, given the right gear and soft-
  ware, prepared information can be written into the
  PROM. The type used in this design can’t be
  restuffed though, since the programming is achieved
  by literally blowing tiny internal fuses in the shape of
  the data. This seeming inversatility is reasonable with
  such devices where the device cost is cheap
  compared with programming costs (human time).

The information stored is, of course, binary in

nature—a 0 or a 1, up or down, there or not, and so on.

The number of these binary bits contained in each

PROM can be in the millions. Four megabit proms are

now common. For this channel system control, the

PROM used stores 256 bits, which, in fact, is still a bit

of overkill, but they don’t really come much smaller.

This baby PROM, a Harris 7602, is much like most

adult PROMs in that the bits are organized internally in

chunks eight wide, as in a digital word (byte). Eight

happens to be the byte width of most popular microcon-

trollers. In the baby PROM there are 32 such bytes of

stored data (32 × 8 = 256), each being accessible with a

specific 5-bit-wide address code (given by the binary

Figure 25-99. Push button interface circuit.

+VE supply Monostable

debouncer

+VE supply

Flip flop

Switch output

(push on, push off)

(Note: +VE and VE supplies to ICs

not drawn for simplicity)

LED

1/2

4013

1/2

4098

Pushbutton

switch

VE supply

Preset control lines

(On)

(Off)

1 M 7

100 k 7

470 k (^7) 470 k 7
MF