Four floppy drives with Amstrad disc interface

Four floppy drives with Amstrad disc interface

The Amstrad disc interface (DDI-1 or the disc hardware within CPC664, CPC6128 and 6128+) supports a maximum of 2 drives, although the NEC765 used within the interface supports up to 4 drives.

NOTE: The Vortex disc interface, the version which contains the NEC765 can access up to 4 drives directly.

Four drives hardware modification

The NEC765 floppy disc controller used by Amstrad can support up to 4 drives. However, in the Amstrad disc interface, only the US0 output is connected. This then decodes into two drive selects (/DRIVE SELECT 0 and /DRIVE SELECT 1).

The CPC6128 uses a 74LS38 (Quad 2-input NAND) to generate the select for drive 1, the select for drive 0 is generated from this using VCC, resistor and KTC1815.

With a hardware modification, internal in the CPC664, CPC6128 or 6128+, or internal to the DDI-1, the US1 output can also be connected, and now four drives can be accessed using the NEC765 directly.

To do this a 2-to-4 decoder should be used instead. (i.e. 74LS139)

Note that most Amstrad software, including AMSDOS supports only 2 drives. So to use the 4 drives you need to access the NEC765 directly.

FutureOS supports this hardware modification and can use the 4 floppy drives. (As a side note, FutureOS supports 8 floppy drives, but the other 4 are provided by adding a Vortex Disc Interface in addition to an Amstrad Disc interface).

Alternatives

It is possible to use 4 drives without any internal modifications. The NEC765 will still see 2 drives and this method is compatible with all existing software. Using switches you can choose 1 drive out of a collection of drives to act as A, and with a second switch choose 1 drive out of another collection to act as B.

e.g. 2 drives (A1 and A2) are in the collection for drive A. A switch chooses which A1 or A2 is drive A. 2 drives (B1 and B2) are in the collection for drive B. A switch chooses which B1 or B2 is drive B.