My fascination with relay-controlled systems begins in the mid-1970s when I stumble on two sets of bound documents for the PBX in an office that went out of business. The ledger-sized one (17″×11″) had circuit and logic diagrams; the page-sized one (8½”×11″) had descriptions of the diagrams and PBX operation (see SB #61: Tock for more).

I spent many hours studying those books but only ever figured out the basics. These telephone switches are among the most complex electro-mechanical machines we’ve designed.

This series of posts explores this last species of telephone switch controlled entirely by relay logic: the #5 Crossbar Switch.

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