Amstrad External Disk Drive

Amstrad made two different disk drives for the Amstrad/Schneider CPC range:

DDI-1 - a set, consisting of the real DDI-1 (= Disk Drive Interface) and a FD-1 3" disk drive, as well as the CP/M 2.2 license and disk.

This set was for the CPC464, which doesn't have the disk controller built-in.

FD-1 - a 3" disk drive as secondary disk drive unit for the CPC664, CPC6128 and CPC464 with DDI-1

Note that there was no official external disk drive for the 6128Plus or even the 464Plus.

While the difference between a CPC664/CPC6128 and a 6128Plus was simply a changed pin-out which can easily be solved by a new plug on the disk drive's cable.

The 464Plus was harder to equip with a disk drive: Although the pin-out of the expansion port was left unchanged, the DDI-1 cannot be operated on the 464Plus without modifications.

Technically Speaking

The two disk drives were identical, the only difference was that the DDI-1 shipped with controller, manual and CP/M 2.2 on one disk.

The disk drives were jumpered (rather: soldered) to react on both drive selects, A: and B:.

The cable decided which drive became A: and which one became B: :

On the CPC664 and CPC6128 computers, DRIVE SELECT 0 was not wired externally, so the drive could only become drive B: on these systems.
For the DDI-1, DRIVE SELECT 0 was only wired to the outmost connector of the attached cable, while the inner connector was wired to DRIVE SELECT 1 only, thus the drive at the cable's end always became A: while the other one became drive B: accordingly.

Note that the drive also powered the DDI-1 interface when used together with it: Certain pins documented as N/C in the CPC664/CPC6128 manual in fact carry +5V from the drive to the controller. The main benefit of this was, that you couldn't connect a standard drive with a more common format (5¼" in the beginning, 3½" later and today) and thus cheaper disks as drive A: without modifications, as the DDI-1 won't work without the power fed from the FD-1.