Recovered early-1980s project

Audio Simplex System Module

The recovered dualASSM001.wrl model combines one motherboard with two populated daughter boards. “ASSM” was the project abbreviation for Audio Simplex System Module.

Original single-microphone TTL version

The first ASSM was a single-microphone version using TTL logic. One of those original units has now been found and photographed, and the original circuit board and photographic PCB artwork used in producing the boards also survive.

These photographs preserve an earlier stage of the project than the later two-station motherboard-and-daughter-board assembly shown in the interactive VRML model below.

Original single-microphone TTL ASSM unit inside its metal enclosure — Surviving first-generation single-microphone ASSM unit.

Original photographic PCB artwork or negative for the first ASSM — Original photographic PCB artwork / negative.

Second original photographic PCB artwork or negative for the first ASSM — Companion photographic PCB artwork / negative.

Original etched circuit board from the first ASSM — Original circuit board.

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The original recovered assembly file is left unchanged. The package adds a compatible pcb-003.wrl alias so its two original daughter-board links resolve correctly.

Project history

These modules were installed beneath the service counters in Burger King restaurants. Each daughter board served a counter station equipped with a microphone, a talk switch, and red/green status indication. The motherboard joined the station circuits to the shared audio and control wiring.

The center motherboard connector carried the daisy-chain cable from one module to the next and eventually back to an amplifier in the kitchen. A separate kitchen board accepted that cable and also served as the system power supply.

How the simplex system worked

With the audio line available, the microphone-stand indicator was green.
Pressing a station's talk switch changed the state of the shared control wire.
An electronically controlled analog switch connected that station's microphone audio to the common line.
The same control state locked out the other microphones and changed their indicators to red.
Releasing the talk switch returned the shared line to its available state.

It was therefore a simplex system: several stations shared the wiring, but only one microphone could transmit at a time.

Two documented design generations

The first version used one circuit per unit, TTL logic, and an NE5534 operational amplifier. It used an open-collector control line so a station could assert the shared busy condition without logic outputs fighting one another.

This second version changed the control logic to CMOS and used a transistor to provide the open-collector function. The audio path was selected by a semiconductor analog switch—an electrically controlled switch that acted somewhat like a solid-state relay for the audio.

Reconstructing the first ASSM logic — one step at a time

The surviving first-generation board gives us four important ICs to work from: the NE5534 audio amplifier, a 7405N open-collector inverter, a 7413 NAND-gate package, and a 4016 quad bilateral switch. We know one 4016 section switched the microphone audio. The pushbutton Talk switch and the shared control line were the two outside conditions the logic had to watch.

Historical-reconstruction note: the original schematic has not yet been recovered. The lesson below reconstructs the required logical behavior from the known operation and the identified parts. The surviving board, photographs, negatives, and later models may represent different revisions, so they are valuable historical evidence but are not being treated as one exact pin-to-pin record.

Step 1 — Give each condition a one-letter name

T = 1 when this station's Talk button is pressed.

F = 1 when the shared control line says the line is Free.

Q = 1 when this station already owns the line.

Q_next = the state this station should have after the logic is evaluated.

The 4016 microphone switch can then follow Q: when Q = 1 the microphone path is connected; when Q = 0 it is disconnected.

Throughout this lesson we will use the older, compact notation ~A to mean NOT A.

Step 2 — Start with the obvious idea

At first it seems that the microphone should turn on only when the Talk button is pressed and the line is free:

Q = T \cdot F

First idea: Talk AND Free

The first attempt is exactly the Boolean expression Q = T · F.

That sounds right, but it contains a trap. As soon as this station takes the line, its open-collector output makes the shared line busy. In our notation that changes F from 1 to 0. The simple equation would therefore turn the microphone straight back off.

Design lesson: when an output changes one of its own input conditions, ordinary combinational logic may not be enough. The circuit needs to remember that this station is the one that made the line busy.

Step 3 — Add the “already active” condition

While the Talk button remains pressed, this station should stay active when either:

the line was free, or
this station already owned the line.

That gives us the working Boolean expression:

Q_{next} = T \cdot (F + Q)

Add the “already active” feedback path

Now F + Q says “the line is free OR this station already owns it.” Talk still has to be pressed, so Q_next = T(F + Q).

Read it in plain English as: “Talk must be pressed, and the line must either be free or already belong to me.”

T	F	Q	Q_next	What happens
0	X	X	0	Talk released — this station lets go.
1	1	0	1	Line free — this station takes it.
1	0	0	0	Another station has it — this station is locked out.
1	0	1	1	This station already has it — keep talking.

The X means “don't care.” If the Talk button is not pressed, the station must be off regardless of the other two conditions.

Step 4 — The straightforward gate drawing would contain an OR

Break the equation into two smaller jobs. First make an intermediate signal S:

S = F + Q

Then combine that result with the Talk switch:

Q_{next} = T \cdot S

Straightforward gate implementation

This is the easy-to-read version: an OR gate creates S = F + Q, then an AND gate creates Q = T · S.

If we built that expression literally, we would reach for an OR gate followed by an AND gate. But the surviving first-generation unit does not contain a separate OR-gate IC. This is where De Morgan's theorem earns its keep.

Step 5 — Use De Morgan to make the OR from NAND and NOT

One of De Morgan's identities is:

F + Q = ~ (~ F \cdot ~ Q)

So our intermediate OR signal can be rewritten as:

S = ~ (~ F \cdot ~ Q)

Replace the OR gate using De Morgan

De Morgan gives S = ~(~F · ~Q) = F + Q. Two inverters and one NAND perform the job of the missing OR gate.

Now the hardware match becomes very attractive: two 7405 inverter sections can produce ~F and ~Q, and a 7413 NAND section can perform the inversion of their AND. The result is exactly F + Q — without a separate OR chip.

Because the 7413 NAND gate has more inputs than this two-variable expression needs, the unused inputs can simply be held at logic HIGH so they do not change the NAND result.

Step 6 — The final AND can also be made from NAND plus an inverter

We still need:

Q_{next} = T \cdot S

A NAND gate gives the inverted answer first:

N = ~ (T \cdot S)

One more 7405 inverter restores the desired polarity:

Q_{next} = ~ N = T \cdot S

Make the final AND with NAND + NOT

The NAND first produces N = ~(T · S). Inverting N gives Q = ~N = T · S.

In other words, the 7413 NAND gates plus spare 7405 inverters can reproduce the AND/OR/NOT logic without needing a separate OR-gate package. That is exactly the kind of parts-count reduction designers commonly looked for when board space and IC count mattered.

Step 7 — Add the red and green indicators

Our present recollection is that green meant the shared line was available, while red meant that another microphone was already using it. If that memory is correct, a useful first model is:

G = F

R = ~ F \cdot ~ Q

Use Q to control the microphone; derive the indicators separately

One 4016 section is enough to represent the microphone switch. The LED logic is shown separately because the exact historical driver wiring may have changed between revisions.

The second equation is important: the red lamp means someone else owns the line, not merely that the shared line is busy. If this station itself owns the line, Q = 1 and the red indication is suppressed.

A later two-color bilateral LED would not change these logical states; it would only change how the two indicator outputs were wired to the lamp.

A spare 4016 section could have been used somewhere in the indicator path, because three switch sections remained after one was assigned to the microphone. The photographs alone do not yet prove that, so for now it remains a useful trace-following question rather than part of the reconstructed Boolean logic.

Step 8 — Put the whole idea into one sentence

Q_{next} = T \cdot (F + Q)

Press Talk to seize a free line; once this station has seized it, keep it until Talk is released; if another station already owns it, stay locked out.

Once the required behavior has been expressed this way, we no longer need to depend on one particular old board revision to determine the functional logic. The Boolean expression itself is enough to draw a gate-level schematic.

Step 9 — Turn the Boolean expression into a functional schematic

We can now assign each part of the expression to a logic function. This is a functional reconstruction: it tells us what must connect to what logically, even though resistor values, exact IC pin numbers, power-supply details, and the precise LED driver wiring still belong to the later component-level schematic.

As we build that drawing in the next steps, schematic illustrations will use the shared light-blue / dark-red LearnTronics schematic style so they remain visually distinct from the beige equation panels and the pink truth tables.

Stage	Function	Signal produced
7405 inverter	Invert the shared Free signal	~F
7405 inverter	Invert this station's active state	~Q
7413 NAND A	Use De Morgan to make the OR term	S = ~(~F · ~Q) = F + Q
7413 NAND B	Combine Talk with the OR term	N = ~(T · S)
7405 inverter	Restore the desired active-high state	Q = ~N = T(F + Q)
4016 switch	Use Q as the control for the microphone audio path	Microphone connected when Q = 1
7405 open-collector inverter	When Q is active, pull the shared control line to its Busy state	The other stations see F = 0

In compact form, the logic path is therefore:

F \to ~F Q \to ~Q \to S = ~(~F \cdot ~Q) \to Q = ~(~(T·S))

The shared control line needs a pull-up because a 7405 output can pull the line LOW but does not actively drive it HIGH. The exact pull-up value is an electrical-design choice, not something Boolean algebra can tell us.

What the logic alone does not determine: which spare 4016 section, if any, was used for the indicators; the LED current-limiting resistors; the exact supply arrangement; the NE5534 audio component values; or the physical pin assignments chosen on a particular board revision. Those can be added after the gate-level schematic is settled.

At this point we have enough information to draw a clean schematic from first principles, rather than trying to reverse-engineer one particular photograph.

Present status of the reconstruction

The VRML files preserve the mechanical arrangement and much of the component placement, but the daughter-board traces were never completely finished in the surviving model. The original schematic is not currently available. We can reconstruct a functional logic schematic from the required behavior, but a historically exact component-level schematic should still be labeled as a reconstruction unless an original drawing is recovered.