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.
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 () and 8 MHz clock wires.
|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|