Friday Notes (May 15, 2026)

It has been a while — three weeks exactly — since the last post here. I haven't been idle, though; quite the opposite. I reached three-score-and-ten last fall and have taken it as a mile-marker indicating it's time to get some stuff done. I hibernated through the winter, but now that spring … Continue reading →

Telco X-Bar Switch (part 3)

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 … Continue reading →

Telco X-Bar Switch (part 2)

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 … Continue reading →

Telco X-Bar Switch (part 1)

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 … Continue reading →

Friday Notes (Apr 10, 2026)

I started the Friday Notes series in March of 2021. (I would have mentioned the five-year anniversary in last month's edition had I noticed it.) Since then (61 posts), I have managed to whittle down long-standing, and in some cases ancient, piles of notes. The piles aren't all vanquished; likely … Continue reading →