More LARC Design

There were two interesting IO devices on the LARC, The drums and the page recorder.

The LARC Drums

12 drums were delivered to Livermore. Each stored 250,000 words. They were each about the size of a washing machine—somewhat more than a meter cube. The drum rotated about a horizontal axis. The recording surface was a cylinder concentric with the axis. The read write heads traveled along a path parallel to the axis. Stepping time was 10’s of milliseconds. I think that the rotation speed was about 1000 rpm. The IO processor could move the heads on different drums independently and concurrently and also transfer data over two or three channels simultaneously. Transfers could begin as soon as the head reached the right track; it was not necessary to wait for the first of the blocks to be transferred. There was some built-in block size—about 100 words I recall.

Sperry devised a benchmark that showed the advantages of concurrent head motion. The benchmark did neutron diffusion to simulate a reactor. This was easily multi-tasked. IBM’s Stretch had a bigger faster disk, but could achieve only a few accesses per second, having only a single access mechanism.

The LARC Page Recorders

Most of this information is as recently (1999) recounted by George Michael. This was a device consisting of a sprocketed film magazine and a “charactron” CRT. The LARC IO processor would control the film motion explicitly, frame by frame, and on a frame, record an image of textual data by causing it to be painted on the CRT once. The lab already had quick turn-around film processing for a variety of purposes. Over-night computing runs would produce developed film the next morning.

The charactron appeared in several products at Livermore. It was composed of a CRT with extra beam steering capabilities and a focal plane near the electron gun as well as at the screen. In this extra plane there was a grid of about 64 stenciled characters. The beam would be guided to one of these characters by the first deflection circuits, and then passed on and steered to the phosphor by the second deflection circuits, where the beam was again focused. The selected character would appear at the selected place on the screen. On one charactron demonstration (not the LARC’s) I saw a large circular display with a few thousand characters. The owner handed me a magnifying glass and invited me to examine the period, which was well proportioned. I saw “This is a period” upon examining the period. The small text was very well formed.

A few years later Livermore acquired a machine from Stromberg Carlson which used a charactron to produce an image on a Xerographic drum which was then transferred to paper.

Here are some notes on the CPU design.