Connecting drives to the ATR8000

From: Michael Current (aa700@cleveland.Freenet.Edu)
Date: 05/23/99-10:03:06 PM Z

From: aa700@cleveland.Freenet.Edu (Michael Current)
Subject: Connecting drives to the ATR8000
Date: Sun May 23 22:03:06 1999

From: Technoid Mutant <>
Date: Sat, 15 May 1999 13:02:48 -0400

I recall in a previous message I am too lazy to dig up, that one person
had some trouble with the Jumper settings on floppy drive mechanisms.

At risk of offending anyone: Here is what needs to be done to a drive to
connect it to the ATR8000:

My example is an older and very common Teac FD-55B 5.25" double-sided,
40 track drive mech.

First you need to set the drive id.  As there are only four possible
drives in the SA400 spec, you have four choices.  My drive has these
choices labeled:


DS0 would correspond to D1: and DS3 to D4:

Some drives are labeled differently but this is the jist of it.  No two
drives can have the same id.

A controller such as the Percom and ATR8000 are terminated.  They belong
at one end of the cable.

New PC-type controllers have done away with the requirement for
termination and id settings by flipping a portion of the cable to
autoselect the drive by position on the cable.  Atari 8-bit controllers
can't take these type cables.  A straight cable must be used.

All drive mechs which will work with Atari8 controllers must have a
socketed 'terminator' resistor network chip.  Usually 'RN1' on the
drive's mech.  All drives but the drive connected to the endmost
connector on the cable must have this chip REMOVED.  THe end drive MUST
have the chip installed in the socket.

The terminator acts as a baffle, absorbing signals that would otherwise
reflect off the end of the cable and generate 'noise' which would
interfere with operation.


Controller (Terminated
permanantly)                                              Last drive

Drive Select setting has NOTHING TO DO with termination.  Drives can be
out-of-order on the cable such as 3, 2, 1, 0 or 3,1,0,2, or 3,0,2,1  or
0,1,2,3.  By 'first in the chain' I mean the drive connected to the
connector after the controller's connector.  The terminated drive for
each of the four examples previous would be 1) Drive 0,  2) Drive 2,  3)
Drive 1  4) Drive 3 as they happen to have ended up as the  last drive
on the cable but not neccessarily the last drive id in the chain.

SCSI, "MFM" (ST506/412) compatible drives and controllers (bridge
controllers like OMTI, ADAPTEC, XEBEC  etc) use EXACTLY method of
configuration as the above.  This 34 pin cable was flipped for the PC
clones a long time ago also.  Atari uses a straight cable just like
floppies above. Same rules and procedures for termination etc.  Only
difference is that these 'mfm' mechs only have two available id's
instead of four as in SA400 floppys.

SCSI Drives follow the exact same rules for termination and id except
that the host adapter takes an id (usually 6 or 7) and there are eight
available id's (seven available for controllers).

If you are following then you will realize that the controllers or host
adapters in the case of scsi have thier own terminator.  Often the
Terminators are socketed and can be removed.  A "V" configuration with
the controller unterminated in the middle of the chain with two drives
terminated (the two at the end of each leg of the "V") is perfectly
legal in all the above configurations and interfaces.

Thus, if you have a CSS Black Box, you will look in the corner of the
board nearest the scsi connector and note a 16pin DIP chip which may be
a wierd color such as red, yellow, blue, white, or shiny black.  This is
your terminator pack.  If you pull this chip, and then plug your BB into
the middle connector on your scsi cable you then Must have a drive
connected to the end -connectors on the  scsi cable and those two drives
must be terminated.

Some new scsi drives have a jumper-enabled termination settings.  No
more terminating resistor packs to remove, lose and then later need
again.  Now you can just lose the jumper instead.  This jumper might be
labled "TE" for Terminate enable.

SCSI drives for the 8-bit on the Black Box are required to power the
termination.  I actually ran a wire from +5volts to pin 20 on the scsi
connector of a CDC drive I bought because it had no jumper for powered
termination.  This allowed me to use it as drive 0 without having to
have another drive that did have powered termination at the end of the
cable just to make the drive useable.

No scsi/sasi host adapter for the 8-bit I am aware of will tolerate a
drive set to use a Parity Bit.  This is usually a jumper on the drive
labeled "P" or "PE" for parity enable disable.  This jumper should be
installed or removed as appropriate to provide a Parity OFF condition or
the drive will simply not exist to the Atari's host adapter.

All SCSI host adapters for the 8-bit I am aware of use id 7.  A drive
set to id 7 will not exist on the chain and the host adapter may not

SA400 - Shugart Associates specification for controlling floppy drives
which has been and is still universal.

SASI - Shugart Associates Systems Interface.  The precursure of SCSI.
Hard disks only.  No tapes etc.  Drives are dumb and don't map bad
sectors without help from the computer operating system.

SCSI - Small Computer Systems Interface.   A superset of SASI developed
by a consortium of drive and controller manufacturers.  Development of
SCSI adds Tape drives and just about any other device to run on a SCSI
host adapter.  Atari 8's use small word scsi and don't provide all of
SCSI's features but those related to storeage.

ST506/412 - A 'dumb' hard disk interface developed by Seagate
Technologies.  The hard drives had no controller onboard, just the bare
minimum.  Things such as format and encoding are decided by a controller
such as  an OMTI or Xebec or Adaptec which would be cabled to the drive
by the installer.  That the controller is a separate device and that the
controller decides the encoding (RLL, MFM are most popular) proves that
useing the description "MFM" for this type of hard disk drive is
inaccurate.  Mechs made to this interface spec are encoding neutral.
"RLL Certified" drives are a little faster and have better media.
Faster because of 26 physically shorter sectors per track and better
media to satisfy the linear density required by 40% more sectors in the
same amount of space.  You might call many older IDE drives "RLL" drives
as they use the RLL encoding method by a different interface scheme.

All of the disks, interfaces and encoding schemes above assume that
there will be a fixed number of sectors in a track.  A 17 sector per
track format means that the innermost track (the shortest track on the
platter) can store 17 sectors.  The outer tracks have more area to
record on but fixed-sectors per track makes for less complicated

This brings me to an example encoding method called Zone Bit Recording
or ZBR.

Lots of ide and scsi disks now use this method.  With cheaper and faster
electronics, we now have the luxury of electronics smart enough to fully
utilize the disk.  This is accomplished by dividing the disk into
logical "Zones"  The outermost zone might store 80 sectors per track,
the next inner zone only 64 and so on down to 17 for the innermost zone
for example.  The controller onboard the drive determins which zone it
will write in and configures it'self accordingly.  The encoding method
is often a modification of RLL.

The above means that your hard disk 'lies' to your IBM PC compatible and
lets the machine think that there are a fixed number of sectors per
track when there are not.  Useing an older hard disk formatter intended
on a ZBR encoded drive that has bad sectors may result in the wrong
sectors being marked out as these formatters mark according to head and
cylinder and relative sector number as in 1, 620, 17 for the last sector
on an ST225 disk drive versus a linear sector number (21080).  In this
case of a 620,17,2heads drive, a zbr drive would be confused because to
it, you have marked an arbitrary sector leaving the bad one at the end
of the disk free for use.

I don't know what got into me with this message.  I hope it is useful
and doesn't bug anybody with the lecture....

OH, One BIG thing to remember:

Say you only have one drive, that drive must be terminated and on the
last connector on the cable.  There allways allways must be a device
which is terminated and connected to each end for anything to happen.
You can't leave an unconnected "tail" longer than 1/2 inch under any
circumstances or the system may not work.

Michael Current,
8-bit Atari FAQ and Vendor Lists,
Cleveland Free-Net Atari SIG, telnet:// (go atari)
St. Paul Atari Computer Enthusiasts,

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