Ted Fried Owns "the World's Slowest Apple III," Following an Unusual Troubleshooting Session
Having picked up a non-booting but otherwise excellent Apple III, Fried had to resort to an interesting underclock to get it working.
Vintage computing enthusiast Ted Fried has an Apple III with a difference: it may well be the world's slowest, having been modified to run at just 1MHz following debugging efforts to bring it back to life.
"I recently acquired an Apple III which was not able to boot so I thought it would be a good opportunity to drop-in my MCL65+ to replace the [Synertek] 6502 [CPU] to help debug the motherboard," Fried explains. "It is a beautiful machine which included the Apple III monitor and an external disk drive. It is the first model which contains a 12V memory card."
The MCL65+ is one of a range of drop-in devices Fried has designed, typically around a Teensy microcontroller, as replacements for the CPU in vintage computing systems. It's designed to either replace faulty parts or to provide functionality missing from stock hardware — usually meaning a faster operating speed, giving older hardware a much-welcome boost.
This time, though, the machine is being underclocked — running at just 1MHz, a little over half the stock speed of an Apple III as released in 1980. "Starting at the ROM, I traced the net back through every IC [Integrated Circuit] in the path, back to the source which ended up being an Apple PLD [Programmable Logic Device]," Fried explains. "It appeared to have a bad output driver which could drive to a logic '1' but not to a '0'."
To quickly solve the problem, and without a spare compatible PLD to hand, Fried simply added a 680 ohm pull-down resistor to lower the output's logical zero point. This brought the Apple III part-way back to operating condition, but not fully. "The machine could still not load the SOS operating system," Fried explains.
"It is my understanding that the Apple III will operate at a clock speed of 2MHz when accessing some address ranges and 1MHz for others, so I worry that my 680 ohm pull-down solution may not work at the faster clock speed. I looked over the schematic for a way to force the motherboard to run full-time at 1MHz."
That solution turned out to be a simple bent pin away, preventing the logic which would otherwise boost the clock to the full near-2MHz operating speed. "With the motherboard clock speed fixed to 1MHz," Fried notes, "it makes this the world's slowest Apple III.
"I don't think this will cause any compatibility issues because my observation was that anytime the CPU access I/O [Input/Output] such as the disk drive, the machine would switch to 1MHz. I think it only runs at 2MHz when the CPU is executing out of memory."
Fried's full project write-up is available on his website.