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 floppy drive.
The problem is caused by the ASICs generation of the 8 MHz clock which is halted during reset and then causes the floppy disk controller go out of control. Because the floppy disk controller 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
See attached.
Notes
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) and 8 MHz clock wires.
Double-sided Foam tape can be used to mount the DPU on top of the ASIC or anywhere else on the PCB. When doing this make sure the component ends at the soldering-side are flatenned to prevent then from coming through the foam tape causing short circuits.
Instead of foam tape 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 |
PCB and Gerbers available on GitHub
Also see Edwin's original DPU Page
