The Atom Disk Pack is a standalone unit comprising a disk drive, interface and power supply - it connects to the Atom via the 1MHz bus.
The disk drive is a full height 5.25" floppy drive. The floppy disk interface uses the 8271 controller, the circuit is in fact almost identical to that used on the Acorn System 3 though the interface board also carries the DOS EPROM and an additional 3K of RAM. 2K of the RAM provides space for the DOS catalogue and workspace whilst 1K extends the top of BASIC text space from $3BFF o $3FFF.
The unit connects to the Atom's ACORN BUS EXTENSION socket on the rear of the computer via a short length of ribbon cable. The Disk Pack is mains powered and is also capable of powering the Atom, the Atom's internal 5V regulator must first be bypassed and then a cable is connected between the 4-pin socket on the Disk Pack and the Atom's DC IN socket.
The Disk Pack manual includes full details of the unit including parts lists and schematic
Atom Disk Pack Floppy Disk Interface
Atom Floppy Disk Controller Board PCB Layout Issue 1
Atom DOS
details to follow.
I am working on a replica Atom FDC Board.