Sorry About the Delay
P-Lab built a demo board that makes it easy to understand how delay-line memory worked in some of the earliest computing systems.
For those that find the origins of modern digital computing fascinating, the technologies that powered the earliest machines are of great interest — even if actually using those technologies on a daily basis would be maddening. It is undoubtedly for the best that delay-line memory is now a distant memory, but at one time it served an important role in the world of computing. Unlike modern random-access memory, delay-line memory was accessed sequentially, rendering it both slow and complex.
However, these memories were less expensive than vacuum tube-based flip-flops — and that was a big deal when a kilobyte cost as much as a car. And compared to electromechanical options like relays, delay-line memory was a speed demon, so it filled an important niche until better technologies came along.
Being such an odd and archaic type of memory to the modern computer user, it can be difficult to understand. A project recently built by P-Lab makes it much easier to visualize delay-line memory in operation, which makes it a great resource for those interested in the earliest days of computing. P-Lab made a demo board from standard TTL chips that are still readily available today that visually depicts how data was written and read by the computers that made use of delay-line memory.
The board has a look somewhat reminiscent of early computers like the KIM-1. The interface consists of switches that specify a 3-bit address and 8 bits of data. Small displays represent the values in hexadecimal. Values can then be written to, or read from, a memory space of 64 bits — 8 words of 8 bits each. Modern solid-state flip-flops stand in for traditional delay-line storage methods.
A ring of LEDs spins, with one representing each bit of stored data. The rate at which it spins is controlled by a clock generator that can operate between 1hz and 150Hz. This rate represents the delay of the simulated delay-line memory. Any given bit can only be read or written when it loops around into the one position where that is possible.
The demo board looks really cool, but you are probably better off skipping the process of reproducing it and just watching the video. That alone will go a long way toward helping you understand the technology. But if you do want to build your own copy, full schematics of the circuit are available for download.
R&D, creativity, and building the next big thing you never knew you wanted are my specialties.