MFM---ESDI----SCSI----EXPLAINED

Section 1: General Messages
Message #14177  Dated 12-03-89 @ 19:57.
   From: SYSOP
     To:
Subject: scsi & esdi drives              (Reply to Message #14172)

Ed:
Well, you asked, so here goes. The main differences between the ST-506
interface, also known as MFM and the newer ones, such as SCSI and ESDI,
are the way they handle the disk Input/Output. MFM stored the data in 17
sectors per track, and had a data transfer rate of about 5mb ( that's
mega *bits*, not *bytes*. ESDI upped the anti by about a factor of 2.
The data transfer rate is about 10 Mbits, and generally uses 34 sectors
per track. The ESDI and SCSI drives can have 15 heads, 1500+ cylinders.

Since DOS doesn't know anything about more than 1024 cylinders, but is
quite happy with as many heads as you throw at it, the newer drives have
circuitry built into them that translates all of this into something
that DOS will tolerate. In addition, they "hide" one sector per track
from DOS, and really have 35 sectors per track. That way, you can mark
out a bad track and the drive and controller will use the spare sector
and DOS is none the wiser. That's how you can get an ESDI or SCSI drive
with no bad tracks. They are there, but DOS doesn't know about them. MFM
drives locked the bad tracks out, the ESDI/SCSI drives keep track of
them in an internal table, and can't be accessed by DOS. Not all ESDI
drives have this feature, but most do. As you can expect the
controllers need to be matched up to the drive for all of this to
happen, but if you get the right combination, it is a sweet deal, fast
too. Now you are probably asking, what are the differences between
ESDI and SCSI. Well, ESDI generally maintains the traditional format
of MFM, meaning that there is a controller, and a hard  disk. The
controller will handle the chores of the hard disk as well as the
floppy disk drives. SCSI, on the other hand is entirely different.

The SCSI controller isn't a controller at all, rather it is a "bus
master". The controller is on the hard disk itself. Since the
controller is on the hard disk, it could care less what machine it is
connected to, as long as the signals coming from the bus master
conform to the SCSI standards. The advantages of this are that the bus
master can control many "devices" (as they are known in SCSI jargon)
so that you can have more than 2 hard disks connected to one SCSI bus
master. In fact you can have up to 8, depending on the implemtation.
Both MFM and ESDI only allow 2 hard disks. In fact, since the SCSI bus
master just re-routes the requests, you can have another hard disk
controller, MFM, ESDI, RLL in the same machine, and all of the hard
disks will get along just fine. You can even have CD ROM drives, Tape
Drives, and 6 other SCSI hard disks, and a couple of ESDI disks, all
hooked up, running off of a single computer. Since the controller is
on the "device" (what we know as a hard disk) bizzare things are
possible. For instance, CDC/IMPRIMIS have a technique known as ZBR
which stands for Zone Bit Recording, and it violiates all of the rules
that we are used to.

What this technique does is to space the tracks evenly across the
surface of the disk. That means that the inner tracks will have less
tracks than the outer tracks, and the farther out you go, the more
tracks you will have. Since the controller is on the disk, it makes no
difference to DOS what is going on. It just requests what it wants,
and the SCSI controller/bus master requests it, and the drive and the
electronics on the drive comply.

The hard disk that I use here is a SCSI disk and it has 1536 cylinders,
15 heads, uses RLL formatting, but it appears to DOS as a hard disk
with 1013 cylinders, 34 heads, and 17 sectors per track. Just what DOS
wants to see, but nothing could be further from the truth. So which is
better? In a single machine, ESDI wins the race, but if you want to
have 3 or 4, or 5 disks all running off of one machine, with a tape
backup thrown in for good luck, then SCSI is the only way to go.

Unfortunately, the SCSI standard isn't a standard at all and as we all
know, standards are nice, because you have so many to choose from.
Seagate has a SCSI interface that only supports two hard disks, and
each vendor has it's own idea of what the SCSI standard is. I could go
on and on, but now you know "the rest of the story".