This is the last of a series of posts about the #5 Crossbar Switch, an electro-mechanical telephone switching system that uses relay logic.

The first post touched on the evolution of telephone switching and introduced the switch fabric comprised of many 10×10 crossbar switches. The second post discussed the line and marker circuits; the latter of which implements the switching logic.

This post — the point of the series — discusses the very clever design that allows a 10×10 crossbar to use only 20 relays (here called “magnets”) to control 100 potential connection points within the switch.

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In the first part of this series I touched on the evolution of (landline) telephone switching — which began with operators handling calls manually and which ultimately became the job of computers.

One of the last stages along the way was an electro-mechanical relay-logic marvel of unsurpassed engineering complexity, the #5 Crossbar Switch.

The last post introduced the switch fabric through which calls are routed. This post explores how that fabric is controlled and what happens when we pick up the phone, dial it, and are connected to the remote end.

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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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