Disk Protector Unit

The Disk Protector Unit or DPU for short is a small circuit I designed to cure the SAM's disk corruption problem whenever the SAM is reset or turned on with a disk in the floppydrive.

The problem is caused by the ASICs 8 MHz clock which is halted during reset and then causes the floppydiskcontroller go out of control. Because the floppydiskcontroller needs a constant 8 MHz clock all the time.

All that the DPU does is generate a constant 8MHz clock to replace the ASIC's 8 MHz clock. The design is simple and easy to build. Only half of the 74LS04 IC is required for the DPU. The other half of the 74LS04 IC can be used to make a debounced NMI switch.

Schematic

Notes

connecting DPU wires Before fitting the DPU somewhere on the SAMs PCB you must cut the ASICs 8 MHz clock PCB track at the same place as shown at the right. The Figure also shows the places where to connect the 5 Volt, 0 Volt ( GND symbol ) and 8 MHz clock wires.
Doublesided Foamtape can be used to mount the DPU on top of the ASIC or anywhere elso on the PCB. When doing this make sure the component ends at the solderingside are flatened to prevent then from comming through the foamtape causing shortcircuits.
Instead of foamtape the middle screw at the SAMs PCB could also be used in which case you need a 5mm plastic spacer and a 12 mm long M3 bolt. When using this method make sure the hole in the PCB is not covered otherwise you can't put the keyboard back !

Components

1	74LS04
2	470 Ohm resistors
1	1 uF/10V tantalum capacitor
1	33 pF capacitor
1	22 nF ceramic capacitor
2	8.000 MHz Xtal

For NMI debounce

2	330 Ohm resistor
1	3 terminal pushbutton/switch

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