ISA MFM disk controllers (From 19Jun09)

To be pedantic. you are confusing interfaces and data encoding methods here. The ST412 interface (developed from the ST506 interface, the ST412 allows buffered seems) uses a 34 pin control cable and a 20 pin data cable. The control signals are very like those on a floppy drive (there are step/diorection/track 0 signals for moving the heads, for example). The data encoding/decoding is done in the _controller_, what appears on the interface connector is essentially the bitstream to/from the head. The controller determins whether you use MFM or RLL encoding (or something else, I guess). Some drives could handle RLL, some apparently couldn;t (but all could handle MFM encoding). The ESDI interface uses the same cables. The control signals are less like a floppy drive (there's a bit serial command/status interface for sending seek commands, etc). The clock and data signals are combined (writing) and separated (reading) in the drive. At the interface connecto you get the data signal and the clock that goes with it. The drive internally may use RLL or MFM encoding -- to most users it doesn't matter (it does to me at the moment, having got various drives in bits on the bench). One minor difference in (some) cabling. The ST412 interface sues a 1-ofn (actually 1 of 4) drive select scheme (4 wires, one is asserted to select a particular drive), like floppy drives. Some PC hard disk cables for 2 drives had a twist in them so you set all the drives to the same drive select number, ut the signals appears on different pins at the controller -- rater like the IBM twist in a floppy cable, but different wires are involved> The ESDI interface has 3 binary-coded select lines for a maximum of 7 drives (all deasserted selects no drive at all). You can use the twisted cable with ESDI. -tony