P82 — control environment

(7) P82 can do more than you think . . . .  —

Index — P82 series:  archive postP82 control — revival(a) — revival(b) — revival(c)

P82 (project 1982) was conceived and constructed more than 10 years after the moon landing, but there are some similarities with the build and logic.


Controlling a slide show is mainly a one-way flow of instructions — but extension via a control loop of data transfer and the return of status is similar to how the AGC or most other programmable real time controllers operate. Moon-landing included.

Design of the P82 controller allows direct control of slide projectors and other room devices with dedicated instructions, and also extended control by data exchange with semi-autonomous remote circuits.

Extended control — model railway

Key extension was a model railway where train movement, route selection and signalisation could be controlled manually or programmed by the HP9825 computer working through the main P82 control unit — to send commands and to receive real time status back. Such status is handled either through interrupt or by regular poling – typical example would be: make a train start, change the signals, clear the way via turnouts, check where the train is, stop it at the right place. Design was for 4 trains, unfortunately we managed only one, but both concept and operating design were validated.

One major physical problem with the small size of “n” gauge, is non-contact with the rails because of dust or other contaminant. So an essential development to guarantee smooth operation is a piece of rolling stock near to the locomotive containing a battery which provides pulsed voltage to the engine motor whenever there is no voltage pulsing detected through the normal pick-up, thus minimal battery usage. The principle assertion is that the convoy always moves forward until it finds contact.

Hammond and Leslie

An example of direct control is the “Leslie” rotating loudspeaker connected to the Hammond organ. This is a direct fall-out of the circuitry used for creating the sound and vision through slide projectors, lights and audio track music.

Tape tracks 1&3 carry stereo sound, tracks 2&4 have control signals for anything that the P82 has access to. Adding Leslie movement to organ recordings to spice up the room environment was part of the original design with six dedicated instructions.

Leslie instructions were ‘fast’ [B1], ‘slow'[B0], ‘break'[B4] and ‘resume’ [B5]. Two additional instructions [B2] and [B3] connected tape audio or Hammond audio to the Leslie at the correct time.

It was possible to record Hammond audio to tape via the P82 without need for cable changes. The famous Leslie cable was diverted via a relay box to switch any function between P82 and Hammond live. Both audio and control cables were permanently installed via the floor tubing.

P82 alongside the AGC ?

We are not pretending that any of this is comparable to landing on the moon (!) yet the fundamental principals of program, control and feedback in a real time environment do have much in common. Also we are using many techniques which were conceived or developed at the time of the AGC and of the HP2116.  P82 concept and logic design was, however, totally original.


[Note on audio technology at the time the P82 was conceived: analog tape was the only accessible multi-channel media and, although studio PCM equipment was available it was not mainstream. Also the CD was not commercialised until 1983, and Sony’s digital audio tape [DAT] only in 1987.]

Index — P82 series:  archive postP82 control — revival(a) — revival(b) — revival(c)