Two series pass devices?

From the Jargon File: ":HCF: /H·C·F/, n.

Mnemonic for `Halt and Catch Fire', any of several undocumented and semi-mythical machine instructions with destructive side-effects, supposedly included for test purposes on several well-known architectures going as far back as the IBM 360. The MC6800 microprocessor was the first for which an HCF opcode became widely known. This instruction caused the processor to {toggle} a subset of the bus lines as rapidly as it could; in some configurations this could actually cause lines to burn up. Compare {killer poke}."

Cheers

Phil Hobbs

I used 6800s and never heard of that. They were depletion-load PMOS parts, not CMOS, so overheating was unlikely.

The original PDP-11, the 11/20, was known to have a short program, about 10 instructions, that would tease a metastability and quickly hang the CPU. A power cycle would fix that. DEC machines in those days were hairball async logic, a mess of gates and RC's and delay lines. I don't think the PDP8/I or the 11/20 actually had a clock.

søndag den 3. september 2023 kl. 21.20.59 UTC+2 skrev John Larkin:

I did not know that! Fascinating.

The Jargon File is a good read, but more for the humor and general atmosphere than for technical details. As JL says, it's fairly hard to imagine a 6800 catching fire unless it was running an oven controller when it locked up.

Cheers

Phil Hobbs

I don't think it is do with settling time or steps - simply that when the output is shorted (or otherwise much lower than the set output voltage) then the op-amp clamp diodes will let Vset from the DAC flow into the voltage divider measurement node and cause the wrong output voltage to be displayed. What is the 2kR resistor you refer to? I see R23 which is 10Megaohm.

The D4 kludge is truly appalling.

piglet