P82 – 40 years revival (a)

(8) Bringing P82 back to life . . . turning it on

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

A very welcome discovery is that the P82, after all these years, seems in an excellent operable condition. Yet, even with the copious documentation prepared in those early nineteen eighties, there are some important memory gaps as to how parts of P82 were designed. The main unit was working around 1983, but modifications and extensions were still in progress as late as 1987. After that date, the rest of our life took over (!).

WRITE to TAPE — (1) byte transfer mode

Next step here is to verify the “write to tape” functionality, much easier on a digital audio recorder than on the TEAC A-3440. And even easier to visualise and verify using Adobe premiere to display the final result. From the P82 keyboard we can select what we write to tape — or rather lets say write to our audio device, as our Zoom F8. Write selection options are: memory, keyboard, internal clock, remote status. For the test we choose clock and keyboard. We set the 1 second clock running and recording, then key in some more instructions over it. This first test is in write byte mode.

The two sources are correctly identified and recorded. Here we see the normal clock interval, plus one of the individual extra instructions inserted between them — kind of time sharing, handled by the P82 logic.

Zooming in we can make out the sync timing and 8-bits of information of each instruction – one byte instruction is ≈ 55ms. The oscillations are counted up against a down counter. Count greater than space equates as “zero”, and count less than space equates to a “one”. So just for fun we can even check the values visually, this example (bit zero first on the left) is 1101 0101 = Hex10, Hex11 = AB

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Write timing

This concept of A-D and D-A conversion is very tolerant of tape speed changes and audio level settings. Timing comparisons are uniquely within the bit and it will work as long as most of the cycles are counted. P82 nevertheless has a visual set-up for levels, saturation, and threshold to accommodate different audio devices.

During construction in 1982 an oscilloscope was necessary to obtain such clarity; fortunately we had the HP 180A. Today Adobe Premiere does the job for us — display is set to time-units mode to see down to the individual cycle level of an audio waveform. Our waveform is using 48kHz, so there are 48000 time-units per second.

WRITE to TAPE — (2) word transfer mode

P82 keyboard instructions are all one byte (2 hexadecimal keys) and so writing in byte mode satisfies basic operation. However word mode (= double byte, 16 bits, or 4 hexadecimal digits) is required for the internal clock and for external date transfers and for use with the HP9825 — select the source we wish to write and “write word”. Byte and word modes should not be mixed.

4x 16-bit words — maxiumum transfer speed

Transferring memory data – display can show memory address, data, or both.

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