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January 2012 WRO - Cross Streets, Part II: Note 1

Permanent memory at this time was a Programmable Read Only Memory chip. The first generation was not erasable. You programmed a memory location by using very short pulses of higher-than-normal voltage to blow fuseable links inside the chip.  Hence the term “burning.”

If you made a mistake, there was no “undo” or “real easy” way to fix it.  The not-so-cheap chip was now trash and you had to start over.  Thus, burning a PROM was a task done in a careful, slow and deliberate fashion – with no one else at home and the phone off the hook.

An alternate approach, for a small bit of code, was to populate a board with 1N48, 1N914 or similar diodes, to create your own PROM.  If a diode was soldered in, the bit was a “1.”  If the connection point was open, the bit was “0.”

Some early CW ID encoders for RTTY and repeaters - many still working today - used such schemes, as well. - WA3UVV

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- Richard Fisher, KI6SN

Editor, WorldRadio Online

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More Photos of the Myers Crystal Set

Here are more pictures of the crystal set designed by Arlo Myers, WA6UDR, and featured in the June 2010 edition of WorldRadio Online magazine.

For an enlarged view, click on each image.