Classic computing fanatic Ted Fried has an Apple III with a distinction: it might be the world’s slowest, having been modified to run at simply 1MHz following debugging efforts to carry it again to life.
“I just lately acquired an Apple III which was not in a position to boot so I believed it will be alternative to drop-in my MCL65+ to exchange the [Synertek] 6502 [CPU] to assist debug the motherboard,” Fried explains. “It’s a stunning machine which included the Apple III monitor and an exterior disk drive. It’s the first mannequin which comprises a 12V reminiscence card.”
Confronted with a non-booting Apple III, Ted Fried went by a painstaking debugging course of — reaching an uncommon conclusion. (📷: Ted Fried)
The MCL65+ is one in all a spread of drop-in gadgets Fried has designed, usually round a Teensy microcontroller, as replacements for the CPU in classic computing methods. It is designed to both substitute defective elements or to supply performance lacking from inventory {hardware} — often that means a quicker working velocity, giving older {hardware} a much-welcome increase.
This time, although, the machine is being underclocked — operating at simply 1MHz, a little bit over half the inventory velocity of an Apple III as launched in 1980. “Beginning on the ROM, I traced the online again by each IC [Integrated Circuit] within the path, again to the supply which ended up being an Apple PLD [Programmable Logic Device],” Fried explains. “It appeared to have a foul output driver which may drive to a logic ‘1’ however to not a ‘0’.”
To rapidly resolve the issue, and and not using a spare appropriate PLD at hand, Fried merely added a 680 ohm pull-down resistor to decrease the output’s logical zero level. This introduced the Apple III part-way again to working situation, however not totally. “The machine may nonetheless not load the SOS working system,” Fried explains.
“It’s my understanding that the Apple III will function at a clock velocity of 2MHz when accessing some deal with ranges and 1MHz for others, so I fear that my 680 ohm pull-down resolution might not work on the quicker clock velocity. I seemed over the schematic for a method to power the motherboard to run full-time at 1MHz.”
Bending a single leg of a single chip seems to be sufficient to underclock an Apple III to 1MHz. (📷: Ted Fried)
That resolution turned out to be a easy bent pin away, stopping the logic which might in any other case increase the clock to the total near-2MHz working velocity. “With the motherboard clock velocity fastened to 1MHz,” Fried notes, “it makes this the world’s slowest Apple III.
“I do not suppose this can trigger any compatibility points as a result of my remark was that anytime the CPU entry I/O [Input/Output] such because the disk drive, the machine would swap to 1MHz. I feel it solely runs at 2MHz when the CPU is executing out of reminiscence.”
Fried’s full mission write-up is on the market on his web site.