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.
This browser could not start the VRML viewer.
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 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.
Logic notation and the eliminated AND chip
The original design notes used these Boolean symbols:
| Function | Notation | Meaning |
|---|---|---|
| AND | A · B | Both conditions are true. |
| OR | A + B | Either condition is true. |
| NOT | ~A | The condition is inverted. |
The first logic plan called for AND, OR, and NOT gates. Because the available IC packages contained unused inverter gates, De Morgan's theorem allowed the separate AND-gate package to be removed. In the notation used for this project:
In other words, invert both inputs, combine them with an OR gate, and invert the result. The spare NOT gates and an existing OR gate therefore performed the required AND function, reducing the number of IC packages on the board.
The two standard De Morgan relationships are:
~(A · B) = ~A + ~B
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, so this reconstruction should be treated as historical documentation rather than a build-ready circuit. The schematic may still survive in the project archive or with Bill.