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To connect a normal PC floppy drive as "drive B:" to the +3 you need a 34 way ribbon cable with an IDC card edge connector at one end, and an IDC socket connector at the other.
Modern floppy drives don't provide the drive ready signal, so the
!Ready line needs shorting to ground to signal to the spectrum that the drive is ready (adding a switch here so you can manually toggle the signal on and off is a good idea otherwise the spectrum won't start up unless there's a disk in the drive).
=====+3DOS format disks=====
3.5" disks can be formatted in a variety of different capacities as CP/M disks. The +3 ROM cannot format 3.5" disks itself so either a utility is required on the +
2 (such as [http://www.worldofspectrum.org/zxplus3e/software.html#mformat|The Spectrum +3 Multiformatter] by Garry Lancaster), or the disk must be prepared on the PC.
=====DOS format disks=====