Subject: Re:large disk platters?

From: Tony Duell <>
Date: Fri Feb 21 14:01:46 2003

> In about 1976 University of Missouri Bioengineering Program acquired an "IBM
> surplus image system" from somebody "unknown" out west. They sent a couple


> There were two clamshell halves that were opened to access the platter, each
> track had a fixed head over it. Stored on each track was the image on a
> single display station. By switching between tracks you could access
> different images. There was a vacuum pump to remove the air if you opened
> the clamshells to adjust the heads. Each of the display stations had an
> integrated keyboard and a proximity or optical pen to select menu items.

One of the peripherals I have for my PDP11s is a PPL model 121 display

It uses a magnentic disk to store the video images. I think it rotates
once per field (the motor speed is electronically controlled with an eddy
current brake IIRC). Images are stored using analogue FM modulation on 3
tracks of the disk (one for each of R, G, B). There are several sets of
fixed heads which can be selected so you can store several images on the
disk and display them.

The disk is a lot smaller than the ones described here, though. I've not
taken the HDA apat (for obvious reasons), but it looks to be a normal 14"
platter. The HDA fits, disk horizontal, into a normal 19" rack module.
There's a separate PSU rack module, and a cardcage of electronics. It
connects to the Unibus via a DR11B interface I think.

Received on Fri Feb 21 2003 - 14:01:46 GMT

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