Monday, October 12, 2009

On Sunday, Oct 11, 2009 I decided to put together an expansion card to plug into the Apple 1 motherboard. This card provides the ability to send a "reset" signal to the CPU, read the address bus, and provide +5v and ground pins for a logic probe or oscilloscope probe. The above image shows the card with power applied, and the logic probe connected. The card is connected using the exact same 44 pin connector that is on the motherboard.

Lights off, first test. The green LED is the power indicator letting me know the board is getting power from the motherboard : +5 volts

Here you can see the 16 address LEDs, with sibling 300 ohm resistors. Note just to the left of the "+ 5v" label (bottom left) and the ground label (top left) there is a single molex pin. These 2 pins are for debugging the motherboard (i.e. the pins are used to hook up a logic probe).
The LED for the reset will be removed -- mainly because I wired it up wrong, and the signal for reset was blocked. Cool to have, but not necessary. Just above the switch is a .1 uF ceramic capacitor - for debouncing. The cap is connected to each pin on the switch.

Rat's nest on the back of the board. What you see here are the address lines, reset, +5 volts running across the top of the board from connector pin 1, ground across the bottom of the board from pin 20. The reset line is held high with a 10k ohm resistor. When the momentary switch is pressed the line is forced low, sending a reset signal to the motherboard / CPU. Testing with a logic probe shows we are good to go.

I attached the 44 pin header to the card using super-glue and a couple of C-clamps. What you see taped onto the board are (6) 16 pin DIP sockets which were soldered into place later on. These will eventually be used for hex displays.

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