MFM vs RLL or ST-225 vs ST-238R (was Re: 3.5" floppy drive in

Actually, all the signals on the data cable are digital. The Read/Write data signals are differential pairs of similar-to-TTL level signals, normally using a 26LS31 driver and 26LS32 receiver. But in a sense they're analogue in _time_. In write mode, the drive writes a flux transition to the disk for every pulse on the write data line. In read mode, the drive sends a pulse on the read data line for every flux transition on the disk. There are some restrictions on allowable frequencies of transisitions, of course. Therefore it's up to the controller to handle the raw data from the disk, to sort it out into sectors, check the sector headers and checksums, and so on. And when writing, the controller has to pack the user's data into sectors, add the correct headers, etc. The choice of encoding scheme (FM/MFM/RLL) is up to the controller. RLL drives should really be called RLL _capable_ drives. They are capable of handling the somewhat tighter timing requirements of the RLL encoding scheme. They'll work fine with MFM encoding too (just as double-denisty -- MFM -- floppy disks will work on a single density -- FM -- controller). -tony