[Ken Shirriff] Takes a bite of the Apple-I

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The Apple-I was far removed from Apple’s later products. A $666 single-board computer, the product had a number of unique design features, including the use of a video memory shift register to save money. The shift registers of the day required high current clock pulses ranging from -11 to 5V and there was a DS0025 clock driver chip to get the job done. [Ken Shirriff] breaks down the unusual chip for us in a recent blog post.

Using a shift register as memory is not a new idea. Really old computers like EDSAC used mercury delay lines as memory, which was essentially a physical shift register. In those cases, the ALU and other processing only needed to work a little bit at a time, further simplifying things. Before the Apple, there were seven shift registers to store 6-bit display data and a cursor position. The 6 bits of character data drove – indirectly – a character generator ROM to convert the data into dots for the display.

Driving all those shift register flip-flops requires a lot of clock current, so the DS0025 uses an unusual transistor design. There are 24 separate channels in two groups. It works like a large transistor, but you could also think of it as two 12-emitter transistors or 24 separate transistors in parallel. Interestingly, the metal wiring is tapered because at the beginning of the conductor the current flows for all 12 subtransistors, but towards the end it is only the current for the last subtransistor, so the conductor doesn’t have the same width. In addition, the two transistors must have the same resistance, which requires careful design so that the transistors turn on at the same time.

The end result is an inverter that can deliver 1.5 amps. This current helps to overcome the relatively large capacitance in the clock line of the shift register. The clock speed was 1 MHz and the load capacity was about 150 picofarads.

we enjoy [Ken’s] messages ranging from mysteries to space hardware. It’s always interesting to see what’s in these devices, or at least what was in the old devices that we’ve all seen.

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