                      SCSI BUS & TERMINATION REQUIREMENTS
                      ___________________________________


IBM OEM STORAGE PRODUCTS                                DATA SHEET GEN001
_________________________________________________________________________


ALL FAST-SCSI DISK DRIVES


Outline

This application note describes the necessary physical bus
techniques for using Fast-SCSI drives such as the IBM 0662,
0663, and 0664 ranges.

Some customers have reported problems in workstation applications,
often in machines which have Fast-SCSI controllers such as the
Sun SPARC and others.  In many cases, these have turned out
to be errors in cabling or termination violating the
ANSI SCSI specification requirements.


Fast-SCSI

One of the ANSI SCSI-2 specification features is a 10 MHz
option--this is Fast-SCSI.  With an 8-bit bus this can
transfer 10 MB/S (50-way cable), and with a 16-bit bus it will
support 20 MB/S (68-way cable).

Single-ended or differential bus configurations can be used.
Single-ended signals are relative to ground while differential
signals are transferred as a pair of signals of opposite
polarity using differential drivers and receivers to
achieve better noise immunity than single-ended.

Single-ended and differential devices cannot be connected to the
same bus, although the physical cables and connectors are the same.


Physical Cable Limitations

It is important to realize that cables carrying signals
at these frequencies are acting as transmission lines, that
is they have a characteristic impedance like other radio-frequency
cables.  In addition, if the cables are of the wrong impedance,
or are terminated incorrectly, signal reflections can occur which
will cause false data clocking and hence bus errors.

Fast-SCSI busses MUST use either twisted-pair "flat" cable or
twisted and shielded external cable.  Short lengths of the cheap
flat ribbon cable can be used, but are not recommended except in
minimal systems.

Single ended Fast-SCSI busses have a total length limitation of
3 meters.  In addition, care must be taken to minimize stubs.

Stubs are lengths of cable connected to the main bus in a "Y"
or "T" fashion.  Devices should instead be connected in a "V"
configuration.

Stubs are a common error in external SCSI boxes.
The cable should run from one of the bulkhead connectors to the
drive, and then back to the other bulkhead connector.
A common error is to link the bulkhead connectors by
a length of cable which then is connected by a stub to the drive(s).

The ANSI SCSI committee recommend stubs be minimized, 5cms
maximum length and at least 0.3 m apart.




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Passive Terminators (S-E)

The so-called "passive" older style terminators used 330 and
220 ohm resistor networks with the terminator power supplied
directly from pin 26 of the 50-way cable.

This design of terminator is designed ONLY FOR USE AT 5MHz.

They are found in several forms:
1.  "Inline" terminators used at the end of a cable to connect
a device which does not have on-board terminators.

2.  Bulkhead or external terminators--these may be found in
either Centronics plug format or in 50 or 68-pin Micro-D
designs.

3.  On-board terminator R-packs fitted to disk drives, CD-ROM
drives, etc.



Active Terminators

Active terminators contain 110 ohm resistor pull-up/termination
resistors for each bus line, and also an active voltage regulator
to supply 2.85 volts for these resistors.  The regulator uses the
4-5 V terminator power line in the cable for input power.

******************************************************
Fast-SCSI devices MUST BE USED WITH ACTIVE TERMINATORS
This is NOT optional!!
******************************************************

Active terminators are found in several forms--

1.  "Inline" terminators used at the end of a cable to
connect a device which does not have on board terminators,
eg, IBM 0664 drives.

S-E examples:  DataMate DM550-06-ADR and Amphenol 33770.
These can be distinguished from the passive versions by the small
box-shaped moulding on one of the long sides.

This contains the regulator components.

Inline terminators are only available for 50-pin cables.

2.  Bulkhead or external terminators--these may be found in
either Centronics plug format or in 50 and 68-pin Micro-D
designs.

Examples of S-E active versions are:

   Amphenol 22431 (Centronics plug)
   Amphenol 28705 & 16708 (50-pin Micro-D)
   Amphenol 16715 & 33321 (68-pin Micro-D)

3.  FPT and Slick terminators--These are an active pull-up
design originated by IBM, and are extremely effective in
eliminating signal overshoot.  They are available in Centronics
and Micro-D format.

Examples of S-E FPT terminators are:

   Aeronics FPT-18 (Centronics)
   DataMate Slick DMT2050-02-68SK (68-pin Micro-D)

4.  On-board active terminators fitted to target devices, such as IBM
0662.  These are usually selectable electrically by a jumper.
Another jumper allows the drive to supply terminator power instead
of (or as well as) the controller.



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Recommendations for Fast-SCSI Good Practice

1.  In Fast-SCSI or mixed Fast/Standard configurations,
MAKE SURE that the last device on each end of the cable has
an ACTIVE TERMINATOR.

This means that external devices such as CD-ROM and tape
drives must have their on-board passive terminators removed,
and an active terminator then fitted to the spare bulkhead
connector on the last device on the bus.

2.  In systems with an external SCSI port, if this is not
connected to external devices, MAKE SURE there is an active
terminator fitted to it.  Often systems will work without
one, but this contravenes the ANSI SCSI spec and WILL NOT
run reliably in all environments.

3.  In some systems with mixed Fast/Standard devices, particularly
with external cables, it is often beneficial to supply terminator
power to the last device on the external cable
FROM THAT DEVICE.  This improves term power decoupling and
reduces noise.

4.  Keep total cable length below 3 meters, avoid any stubs,
and use good-quality twisted-pair flat or shielded cable.

Twisted-pair cable should only be used inside enclosures and
should not be laid onto metal chassis as this affects the
impedance.


All trademarks are acknowledged.  This information is provided
for the use of OEM Manufactures attaching IBM disk drives & is
believed to be accurate but is supplied without guarantee.
