A 'personal computer' from the 1950s

Feb. 20, 2017
by Mark Ollig

While yours truly was programming a Sinclair ZX81 computer at the start of the 1980s, other computer hobbyists had already been using personal computers during the previous decade.

Personal computers from the 1970s included the Apple II, Tandy TRS-80, and the Altair 8800, just to name a few.

Aside note: While writing this column, I sent out a message on Twitter saying; “Writing the next Bits & Bytes column about a 1950s ‘personal computer.’”

One of my faithful followers tweeted back; “Hahaha, was it as big as a house?”

“Not this one!” I replied.

In one 1956 glossy brochure, the Bendix G-15 computer was advertised as “a complete computational facility” and a “general purpose digital computer.”

This description sounds a lot like how one might possibly describe a personal computer.

The Bendix G-15 was manufactured by the Bendix Computer Division of the Bendix Aviation Corporation, headquartered in Los Angeles, CA.

After seeing the size of the Bendix G-15, it could easily fit in the corner of any office or living room, as would any other personal computer.

Its physical dimensions were approximately 2.5 feet deep by 2.3 feet wide, and a little over 5 feet tall.

The Bendix G-15 weight was listed at 850 pounds in their brochure, although I have seen 950 pounds referenced in other sources.

Remember folks, I said it was possibly the first personal – not portable – computer.

The main computer input console, called the “Master Writer,” was, in fact, an electric typewriter cabled to the Bendix G-15 computer. This was used for inputing data at a maximum typewriting speed of 8 characters per second.

Other ways to input programs into the Bendix G-15 included: Punched or perforated high-speed paper tape, standard IBM punched cards, and the use of peripheral magnetic tape units.

Up to four model MTA-2 magnetic tape units (each about the size of a small refrigerator) could be wired into a single Bendix G-15 computer.

One MTA-2 magnetic tape drive unit stored up to 300,000 words of information.

Word length used with the Bendix G-15 could be set for 29 or 58 binary digits.

A paper digital graph plotter (think printer) was used to see the output data at up to 12-inches-by-18-inches.

Its main memory was stored on a rotating magnetic drum, with general storage of 2,160 29-binary-bit words.

The central processing unit used vacuum tube technology.

The Bendix G-15 had a random memory access time of 14.5 milliseconds; its clocking speed was 0.069MHz.

The electronics used to operate this digital computer included 180 vacuum tube and 300 diode “packages” wired onto plug-in printed circuit boards.

The Bendix G-15 was powered by standard 110-120 volt AC using 60 cycles, single phase input and was cooled via internal forced air – however, I imagine the vacuum tubes and electronic components gave off some heat.

User commands were entered into the computer using the Bendix G-15’s programming system called INTERCOM.

Using a single command would result in a number of simultaneous internal operations being performed.

This sounds like how yours truly uses macro commands.

I build macros or shortcut command strings using the basic text editor within my WRQ Reflection for UNIX and OpenVMS (open virtual memory system) program.

Using a single macro command will execute strings of lengthy individual commands I would otherwise need to type when programming the digital telephone processing switches I maintain.

INTERCOM commands are single address access, and will reference information stored within the computer’s memory positions.

Users of the Bendix G-15 had access to over 1,000 pretested programs.

Its numbering system allowed decimal input and output, but used binary coding internally.

An important safeguard to verify maximum error-free operation of the G-15 included reliability checks using certain command strings.

The basic Bendix G-15 computer could be purchased for around $50,000 in 1956, which equals about $450,000 in 2017 dollars.

“What applications would a 1956 Bendix G-15 computer be used for?” you might ask.

Here are some of the applications:

• Numerical control of machine tool operations.

• Real-time data processing for the aircraft industry.

• Electrical transmission and pipe lines.

• Trajectories and missile performance.

• Mathematical analysis and academic research.

• A teaching tool for universities.

• Construction civil engineering for highway design.

• Petroleum exploration and refining.

The G-15 digital computer was built by the Bendix Aviation Corporation using the drawings and designs they purchased from American computer designer, Harry Douglas Huskey.

Huskey discussed the Bendix G-15 in this 2011 video provided by the Computer History Museum: http://bit.ly/2lkcVPI.

Harry Douglas Huskey worked at the National Physical Laboratories in Britain for a year with the famous mathematician, computer scientist, and cryptanalyst, Alan Turing, whom Huskey credits in the video.

During World War II, Turing cracked the secret Nazi Enigma code, which significantly aided the Allies in defeating Germany.

I learned Huskey is still with us today at age 101 years young.

An estimated 300 Bendix G-15 computers were sold in the US.

In 1963, Bendix Aviation Corporation sold their computer division to a well-known Minnesota company based in Minneapolis.

This Minneapolis company was the Control Data Corporation.

Be sure to follow me on Twitter at @bitsandbytes.

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