A Brief Look at the RCA 501

From 1958, a transistorised computer supporting up to 63 magnetic tape drives. This video cued to footage of operating the impressive console:

Some snappy slogans in the marketing materials here and here:

Compact, reliable, efficient.
Easy to install… easy to operate… easy to maintain…

It turns out this machine crossed the pond: “English Electric-Leo-Marconi Computers worked with RCA to re-engineer the RCA501 to become the KDP10 (later the KDF8), aimed entirely at the emerging commercial marketplace” according to Grace’s Guide to British Industrial History

Some more glimpses of the 501, and other early machines, in this 1962 Navy training film:

via a mastodon thread, inspiring me to identify a stock photo used to illustrate a news story.

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A bit more on the machine, from the 1958 Programmers’ Reference Manual. It’s core-based, with 28 bit words but addressed at the 6 bit character level. An instruction is 56 bits (eight characters) and takes two cycles, or 30 microseconds, to fetch. Addressing is 3 characters (18 bits) and each instruction has two addresses. Instructions may operate on characters or on words, or on records.

The machine’s arithmetic can be binary or BCD, in excess 3 style.

There’s a drum store for random access, paper tape, and lots of mag tape units can be attached.

The arithmetic unit does everything twice, to be sure: operating on the true and on the complement of the inputs. And the mag tape also stores every bit twice (it’s 16 tracks wide.) And there’s parity everywhere, at the character level.

Eight lowercase letters (ahqrstvx) are available on the Monitor Printer, but not on paper tape. The Online Printer offers 51 printable characters, and prints a 00 as an overprinted = + and @. None of which are individually printable.

The instructions allow addresses to be indexed, which offers some 5 index registers, implemented in particular triplets of locations in the core.

There’s an arrangement for slow I/O instructions to continue to completion while other instructions continue: one write and one read can be in progress while execution proceeds.

There’s a list of installations and their costs and experience with the RCA 501 (and many other machines!) at Ed Thelen’s site. For example, State Farm spent $2 million on their setup. And while RCA recommend a staff of at least 9, plus analysts, programmers and coders, for a single shift operation, we see that the U. S. Naval Propellant Plant recommended only 5 staff, plus 2 analysts, 4 programmers and a single coder. Scott Air Force Base could keep 4 analysts and 10 programmers busy - presumably these programmers had to do their own coding. Atlantic City Electric Company “anticipated that the RCA 501 equipment will be adequate for present applications and expected growth for at least the next ten years” and kept a total staff of 26 running just one shift.

The price mentioned in the video, US$ 121,698 in 1959, can’t be accurate. This would have been quite a bargain for a raised floor mainframe!

It is surprising, especially as it supposedly includes the cost of the facility too. The CPU alone is priced at twice that in Ed Thelen’s lists. This may be the source.

From the link:

In fact, two 12.5 ton refrigeration and heating units were installed to keep the computer running. The original cost estimate for the device (in 1959 dollars) was $211,066, which included soundproofing, ventilation, lighting and air conditioning. However, the cost estimate was revised downward, and the final total was only $121,698!


I’d conclude, this was the cost of the auxiliary cooling/heating units alone.
($120,000 was the price of a PDP-1 in 1961, which was already considered cheap and right in the upper range of drum computers. The LGP-30, a true drum computer with just 113 vacuum tubes, was $47,000, the comparable Bendix G-50 in workable configuration was about $60,000. And on top of this, I wouldn’t suppose that the Air Force didn’t pay a premium price.)