Here's a picture of the upper and lower sides of the FCT as assembled by Vince.

Flip Chips are inserted into the card slots (one for single-width, both for double-width). The ICs are expanders for interfacing between the testing computer (originally from Windows 95 over parallel, but in this version between an Rpi0 and the FCT over GPIO pins). Single pins are for attaching probes if desired, and LEDs provide some status information. (Most of the information is provided by the application that interfaces with the FCT).

#umdpdp12 #FlipChips

Today's task: get our Flip Chip Tester (FCT) up and running. For those who don't know, Flip Chips (in DEC parlance) are small circuit boards with a handle opposite an edge connector, about the size of a 3x5 card, that have a small number of discrete components on it.

Each Flip Chip provides a specific set of components to a machine. The one pictured here is an M617, which provides six four-input NAND gates (two per IC). The Flip Chip would be inserted into a slot, and a wire-wrap backplane would connect it to power, ground, and upstream and downstream components.

One of my favorite things to explain to students is how the PDP-12's CPU can quite literally be repaired. When FCs go bad, it's usually because some IC on the FC has failed. Of course, you can just swap out the whole FC (if you have a spare). However, they can also be easily repaired if you know what to replace and you have equivalent IC packages.

Enter the Flip Chip Tester. Before he died, Warren Stearns prototyped a device that, given a set of test vectors for a particular FC, could tell you which ICs were failing. His tester worked for most of the single- and double-width discrete logic boards.

After Warren's death, several folks worked to make Warren's tester more robust and replicable, including Mike Thompson at RICM (https://www.ricomputermuseum.org/) and Vince Slyngstad (https://so-much-stuff.com/pdp8/repair/fc-tester.php, including photos of Warren's prototype).

Previously, we built a tester using Mike's modifications for FTDI/USB use, but for a variety of reasons that one failed (none Mike's fault). So now, we're setting up one with Vince's modifications that enable control using a Raspberry Pi 0. Vince did the complex assembly for us, which frankly was a huge lifesaver.

Friends @neale and @ckape donated Pi 0s, and I have the time between semesters to hopefully get this one up and running.

My hope is that by the end of the day, I'll have a working tester. I'll document my progress here in this thread.

#umdpdp12 #dec #pdp12 #pdp #FlipChips #retrocomputing #vintagecomputing