BBS in a Box 11/AMUG BBS in a Box Volume XI (April 1994) (MacWizards).iso/Files/Publish/M/MicroNetSCSI Info.sit/MicroNetSCSI Info

Text File | 1990-11-13 | 13.3 KB | 249 lines | [TEXT/GEOL]

Macintosh SCSI Termination
by the Engineering Department of
MicroNet Technology, Inc.

Dated:  11/1/90


What is Termination?

Termination involves the placement of impedance matching circuits on the bus.
In the case of SCSI, these circuits are typically placed at each end of the
SCSI Bus (cable). These circuits are known as "Thevenin Terminators" and are
simple resistor networks in the case of SCSI.  These resistor circuits are
composed of a 220 ohm resistor connected to Vcc and the signal node, while a
330 ohm resistor is connected to ground and the signal node.  One of these
resistor pairs is connected to each of the 18 active signal lines.  The Vcc (+5
nominal) for the terminators is typically provided by line 26 on the SCSI bus
cable.  In practice, however, this voltage is sometimes supplied by the SCSI
device (hard disk)  directly to the terminator circuit if the circuit resides
on the device itself.  In this case, Vcc on line 26 of the SCSI would not be
required.

Why Terminate?

Terminating a SCSI bus preserves high transition speeds, and when properly
placed will clean up the signal along the entire length of the line.  They also
provide a reasonable degree of noise immunity.  The most important reason to
terminate an SCSI bus is that termination is required for the bus to work!  The
ANSI SCSI specification calls for the bus to be terminated in the way described
above.

How to Properly Terminate a SCSI Bus

A SCSI bus is properly terminated by placing bus terminators at each end of the
bus.  There are a couple of exceptions.  If the bus is very short (ie. 18" or
less) then generally it is best to terminate just one end.  Also, if the cable
length (6 meters maximum) between two devices is greater than 10 feet then the
cable should be terminated at the 10 foot point in addition to each end.  When
daisy chaining three or more SCSI devices the bus will often exceed this 10
foot specification and will require three terminators in the chain to work
properly.

Physically, terminators generally have three forms in the Macintosh world.
Electrically, these three types of terminators are equivalent and vary only in
where and how they are installed.

First, are the ON-DRIVE terminators which are known as resistor packs (or sips,
or dips).  They reside right on the device (hard disk) itself and are almost
always removable.  Always note the orientation of these parts before attempting
to remove these!  They have a polarity and must not be inserted backwards.

Second, there are the terminator blocks or plugs.  These are short "plug like"
devices and are inserted between a hard drive cabinet’s SCSI connector and the
SCSI cable or on the second cabinet connector if one exists.  Don’t confuse
these with the ON-DRIVE terminators described above.  ON-DRIVE means literally
that, on the drive (HDA) not on the cabinet.

Third, are the "Mother Board" Terminators.  These may look like a SIMM or they
may look like a narrow plug.  They are used ONLY when there is no ‘internal’
hard drive in the Macintosh AND you are connecting an external SCSI device
(hard drive) with an SCSI cable longer than 18".

They are inserted into the 50 pin SCSI connector inside the Macintosh where the
cable for an internal drive would normally connect.  These Mother Board
Terminators are keyed (a polarity notch) and must never be inserted backwards!

Never use the second type of terminator when the drive inside a cabinet has
terminators installed on it.

Common Termination Problems

Poor Quality Cables.  Always use cables which are double shielded (foil and
braid).  Apple and MicroNet brand SCSI cables are built with specifications
that include twisted pair and double shielding.  Never use simple printer type
RS232 cables!  Make sure the connector hoods (or shrouds) are each connected to
the shield braid!

Too Many SCSI Devices Terminated.  Only the last SCSI device (disk drive/tape
drive/scanner/printer) should be terminated.  If the Macintosh has no hard disk
drive installed inside it, then a Mother Board Terminator should be installed
if the cable to the external device is longer than 18". MicroNet Technology
developed what is called the "MBT", which can be used to terminate the
Macintosh SE, SE/30, II, IIx, IIcx, IIci when an internal hard disk drive is
not present and is available through MicroNet resellers.

Total SCSI Cabling Exceeds 6 Meters (19.6').  The total length of all cables
used (added together) must not be greater than 6 meters.  Don’t forget to add
the internal cabinet wiring which is generally about 1 foot.

Never use a plug type terminator designed for installation on a cabinet
connector if the SCSI device (hard disk drive or tape drive) inside the cabinet
already has terminators installed on the drive!

Never Exceed the Maximum Cabling Length of 6 Meters (19.6 feet) Total.  Never
mix cable brands, types or styles.  This is a common source of trouble.  Each
cable has a different type of construction, impedance and wire placement which
can result in bus reflections.  In mixed cable configurations some devices may
simply not work even if all other guidelines are followed!

Macintosh and The SCSI Reset Line and On-Drive and Off-Drive Termination

Pin 40 of the 50 pin SCSI connector is the hardware reset line.  When the
Macintosh is first turned on (cold boot time) it repeatedly asserts this reset
line in order to reset all attached SCSI devices.  Generally, this poses no
problem.  Some non-block devices can have trouble recovering from this reset
condition and cause other attached devices to have trouble.  Most notable are
older SCSI scanners and tape drives.  The most common symptom of this problem
is a Macintosh that won’t boot from a hard drive when a scanner or tape drive
is attached to the same cable. This problem can also occur when using some of
the first generation SCSI hard disks.  They will enter a ‘Unit Attention’ mode
and will not respond to read or write commands until a ‘Request Sense’ SCSI
command is issued by the Macintosh.  Newer drives will simply abort the Unit
Attention condition after it receives several read or write commands and
eventually will respond to the data requests.  Symptoms of this later problem
will be a drive which will not show up on the desktop the first time the
Macintosh is booted but will show up fine the second time around. It should
also be noted that whenever a SCSI device is first powered on it also enters
this Unit Attention condition.  The symptom for this is again a drive which
will not show up on the desktop (mount) the first time the Macintosh is booted.
The purpose of the Unit Attention condition is to alert the Host (Macintosh)
that an important event has occurred which the host should be aware of before
accessing that device.  The Host is then supposed to ask the SCSI device what
occurred by using the Request Sense command.  The Host would then act take an
appropriate action before making data requests of the device.  A good example
of this scenario is the resultant Unit Attention condition which follows the
swapping of media on a removable media device, such as an optical cartridge
device.  It should be obvious why the host should be alerted to this swap
before it "writes data to the newly inserted cartridge"!  The Unit Attention
condition following the swap would prompt the Host (Macintosh) to determine
what event has caused the device to respond with a Unit Attention condition.
When the Host (Macintosh) follows up with a Request Sense command it then
learns of the cartridge swap and takes appropriate action.  In this case it
grays out or removes the icon on the desktop and mounts the new cartridge.

Hardware resets and drive power being turned off and then on again are
considered important events. That is why they cause most SCSI devices to enter
the Unit Attention condition. Older Macintoshes have trouble with devices which
enter the Unit Attention condition because they don’t follow up and  query the
device to find out the reason for the condition. Therefore, if the device
doesn’t clear the condition itself, the Macintosh’s data requests will not be
honored and the result can be a drive which won’t boot reliably or at all.  The
fix for these types of problems is simply to cut the reset line somewhere in
the cabling (line 40 at the 50 pin end).

Many subsystem manufacturers have taken this approach successfully.  While this
approach remains debatable it can be the only remedy for severe cases.  While
it won’t stop devices from entering Unit Attention conditions at power up time,
it will stop the hardware resets issued by Macintoshes at boot time from
reaching the device.  This can make the difference in booting and not booting!

The reason this topic is being presented here in a discussion on termination is
that it is all important that the reset line (pin 40) always be terminated!  If
it is not, the device will be constantly resetting itself and will not work!
Normally this is never a problem since line 40 is one of the 18 active signal
lines which normally get terminated.  But if you are working with a device
which has line 40 cut inside its cabinet, AND you remove the ON-DRIVE
terminators in favor of an ON CABINET (external) terminator then line 40 won’t
be terminated since the cabling inside the cabinet has this line cut.
Therefore, pin 40 on the drive won’t be terminated since this line is open and
won’t reach the terminator which is now on the outside of the cabinet!

The warning here is simply to verify continuity of line 40 on a cabinet’s
internal cabling before determinating the drive inside it.  If line 40 has been
cut on the cabling inside a drive cabinet YOU CAN’T DETERMINATE THE DRIVE
ITSELF unless you change the internal cabling to pass line 40 through to the
outside connector of the cabinet!

Enough of these type of cabinets exist in the world that it is a real problem
when reconfiguring bus termination.  Another approach to handling a case like
this is to put this drive (or device) on the end of the SCSI which should be
terminated anyway and don’t use an external terminator on the bus at all. If,
however, you have more than one of these devices with line 40 cut inside its
cabinet then you must consider changing their internal cable if you wish to
determinate the drive (device) inside it and use external ‘plug’ type
terminators.

Macintosh Configurations and Termination Issues

Point #1)  Don’t Be a Worry Wart

There is a lot of bad advice floating around.  The purist will tell you that if
you have more than two SCSI devices terminated on the bus you will blow up your
Macintosh. This simply isn’t true. If you have a third terminated device on the
bus or even (heaven forbid) a fourth you are not going to pop the SCSI chip in
your Macintosh. In some cases with older SCSI drives it may be necessary to
leave the ON-DRIVE terminators in place in order for them to power up and to
work at all.

Another purist might tell you that both ends of a SCSI bus MUST be terminated
in order for anything to work.  This again is junk talking.  Take for example
the fact that Apple doesn’t terminate the mother board of certain Macintosh
computers!  If there is no internal drive in these Macintosh computers it
simply never gets terminated like it should be (hence the  recent availability
of mother board terminators from MicroNet Technology, Inc., and Apple
themselves).  And since these Macintosh computers are always one end of the
SCSI bus one might ask the purist why any SCSI devices work at all if there is
no internal drive! The fact is that if the cable is fairly short (18" to 6')
the Macintosh SCSI bus will generally function fine even if the Macintosh isn’t
terminated.  However, the faster Macintosh computers, like the IIci and the
IIfx may need internal termination inorder for them to work properly.

The pursuit of the Perfect SCSI bus can be elusive.  As soon as you add SCSI
devices from different vendors and use the different cables supplied by each,
you have a potential problem.

Mixing cable types, lengths, and styles is an invitation to trouble.  But in
the real world, there isn’t much choice!  This is largely why switching cables
around and trying cables of different lengths has solved many problems.  A part
of configuring SCSI peripherals will always be determined empirically.


Questions and Answers

Q)  Should I ever remove the ON-DRIVE terminators from a drive which is
installed inside a Macintosh?
A)  NO.


Q)  Should I ever use a Mother Board Terminator if there is a drive installed
inside the Macintosh?
A)  NO. You can’t !  Each plugs into the same 50 pin connector on the mother
board and you know what they say about two pieces of matter occupying the same
space!


Q)  What is a ‘Black terminator’ and when should I use it?
A)  The black terminator is a product from Apple and is for use only with the
Macintosh IIfx.  It is an external plug type terminator and would only be used
‘where and when’ a normal external plug type terminator would be used!  This
black terminator differs (electrically) only slightly from a normal plug type
terminator and not physically.  It’s purpose in life is to compensate for some
changes in the new SCSI chip used in the Macintosh IIfx.  If later models of
the Macintosh use this new SCSI chip then it is likely to be recommended for
those machines as well.


   Prepared by the Engineering Department of
   MicroNet Technology, Inc.
   20 Mason
   Irvine, CA  92718
   TEL: [phone]
   FAX: [phone]



Copyright 1990 by MicroNet Technology, Inc.