Blinky lights WAS: The SR71 Blackbird was designed ENTIRELY with slide rules. fwd 591

The first clue is, while staring at the data bus, a bit slowly fades off and back on...intermittent contact varying by board temperature. The easy ones were depositing one bit on, but two lights come on...solder bridge. Or deposit a one but it shows a zero...data line shorted to ground. Then there was the case of many bits randomly changing...intermittent on an address line, so data flipped between two locations at random. While a memoy test would usually find these problems, the advantage of a front panel display is being able to extend the problem board out of the card cage and logic probe it to track down the errant solder joint or bad chip in situ.

Certain address-data patterns indicating a crash quickly become obvious too. One good indication is a nicely decrementing address bus, as it marches through memory destroying all in its path. This was a common occurrence on 8080-z80 systems since the FF opcode (typical data returned for non-existent memory) was a RST 7 (CALL 38H), which rapidly walked the stack pointer down from wherever it was set, then wrapped back to high memory and started over. Many memory tests, and even a front panel, would not catch the infamous M1 memory timing gotcha on Z80 processors, where the first byte of an instruction fetch required faster access than subsequent fetches. A symptom of this was a missed instruction which then fell into a FF type loop.

A highly active front panel can be used as a measure of processor activity, so the rapidly changing lights stereotype was probably derived from some screenwriter's short tour of an old mainframe site. The earliest versions of the blinkenlight syndrome I can recall are from some mid-50s movies, "Gog and Magog" (a real IBM 650?) and "The Invisibile Boy" (fake panel of light squares). "War of the Worlds" (Gene Barry version, 1953) featured a mechanical difference analyzer instead of a computer, so it must have started after that particular movie. Jack Peachicken