Write-only memory

On April Fool’s Day, I usually look for interesting jokes of the past. And I’ve found one that is related to vintage technology. From the Write-only memory Wikipedia page:

Write-only memory (WOM) is the antithesis of read-only memory (ROM). By definition, a WOM is a memory device which can be written but never read. Since there seems to be no obvious use for such a memory circuit, from which data cannot be retrieved, the concept is most often used as a joke or a metaphor for a failed memory device.

The Signetics original
Out of frustration with the long and seemingly useless chain of approvals required of component specifications, during which no actual checking seemed to occur, an engineer at Signetics once created a specification for a write-only memory and included it with a bunch of other specifications to be approved. This inclusion came to the attention of Signetics management only when regular customers started calling and asking for pricing information. Signetics published a corrected edition of the data book and requested the return of the ‘erroneous’ ones.

Later, in 1972, Signetics bought a double-page spread in the April issue of Electronics and used the specification as an April Fool’s Day joke. Instead of the more conventional characteristic curves, the 25120 “fully encoded, 9046 x N, Random Access, write-only-memory” data sheet included diagrams of “bit capacity vs. Temp.”, “Iff vs. Vff”, “Number of pins remaining vs. number of socket insertions”, and “AQL vs. selling price”. The 25120 required a 6.3 VAC Vff (vacuum tube filament) supply, a +10 V Vcc (double the Vcc of standard TTL logic of the day), and Vdd of 0 V (ie. ground), ±2%.

At the bottom of the Wikipedia article, you’ll find links to a scan of the original data sheet in PDF format, but here they are anyway:

The footnotes made me chuckle.

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