URGENT:SCSI TERMINATORS (Purpose and Usage)
ITEM: RTA000042033
QUESTION:
URGENT: Please define the various SCSI device terminators we have
and the purpose and use. A customer just called and said he has
a 320 to which he is trying to attach three external devices to the
SCSI adapter. He has a terminator, part #00g0968. He has been told
this terminator was made for only two external devicesą Is this true,
if so why? Do we still have terminators that are made for more than
two external devices? What is that part number?
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A: SCSI bus terminator questions
Your customer was correct. Originally, the internal/external SCSI bus
was limited to two external devices due to a passive termination
scheme. In June 1991, IBM introduced Forced Perfect Termination (FPT)
terminators. These terminators lifted the restriction so that the full
seven devices could be attached to the SCSI bus. All terminators
shipped since mid-1991 use this active termination technique.
Terminators:
non-FPT: a terminator that uses the older passive method of
termination. Internal/External SCSI bus was limited
to two external devices.
FPT-3 : a terminator that actively terminates only three of
the SCSI bus lines: ACK, REQ, SEL.
FPT-18 : a terminator that actively terminates 18 lines of the SCSI
bus, including the eight data lines. This terminator is no
longer manufactured.
FPT-18+: a terminator that actively terminates 18 lines of the SCSI
bus. This terminator replaces the FPT-18. This FPT
is functionally equivalent to the FPT-18 and is fully
interchangeable with the FPT-18.
The FPT-3 is generally used on the the internal end of the cable, and
the FPT-18 is used on the external end of the cable. Note these
terminators apply only to single-ended SCSI buses. Differential
SCSI buses utilize a completely different terminator (P/N 79X3795).
The part number 00G0968 refers to an FPT-3 50 pin external single-
ended SCSI-1 terminator. This terminator will allow seven devices
to be attached to the SCSI bus. It is not a non-FPT terminator.
You may wish to refer to FLASH 1149 for more information on the
problem and ECA028 which fixes the problem. To verify the EC is
applied to your system, inspect the internal and external SCSI
terminators. They will be labeled PN 00G0968 or PN 51G7736 if the
EC has been applied. (Since 00G0968 is the part number you specified
the necessary changes probably have been applied to your system).
Contact your CE if you have further questions on whether or not the
EC has been applied.
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QUESTION:
HERE IS A BETTER DESCRIPTION OF WHAT THE CUSTOMER IS FACING: HE
WAS RUNNING AIX 3.2.3 ON A 320. ON THE SCSI BUS, HE HAD ATTACHED
TWO IBM SCSI DEVICES AND TWO OEM, DOUBLE ENDED SCSI DRIVES. THE
SYSTEM WAS WORKING FINE. THE CUSTOMER DECIDED TO UPGRADE TO 3.2.5
AND FOUND THAT HE COULD NOT PREFORM THE UPGRADE UNLESS HE TOOK THE
TWO OEM DRIVES OFF THE SYSTEM. THEN HE TRIED TO RECONFIGURE THE
DRIVES, AND FOUND HE COULD NOT.(BY UPGRADE, I MEAN HE DID A COMPLETE
REINSTALL WITH A BOOTALBE TAPE TO 3.2.5 - NOT JUST PTFS.)
HERE ARE SOME QUESTIONS: 1. WHAT COULD BE THE CAUSE OF NOT BEING ABLE
TO INSTALL AIX ON A SYSTEM WITH OEM DRIVES ATTACHED TO THE SCSI BUS?
2. IN YOUR RESPONSE ABOVE, YOU SAY THAT TERMINATOR
PART NUMBER 00G0968 IS AN FPT-3 EXTERNAL SINGLE-ENDED SCSI TERMINATOR.
CAN YOU EXPLAIN THE DESCRIPTION YOU GAVE ME FOR FPT-3. WHAT IS THE
RESULT OF TERMINATING ONLY THREE LINE? I ASSUME ONE CAN STILL ATTACH
MORE THAT THREE EXTERNAL DEVICES -- CORRECT?
3. CAN YOU HAVE "DOULBE ENED" DEVICES WITH SINGLE
ENDED DEVICES ON ONE INTERNAL SCSI BUS OF THE 320 RS/6000? I BELIEVE
THE SCSI BUSES ARE SINGLE AND DIFFERENTIAL. ARE SCSI DEVICES ALSO
CLASSIFIED THIS WAY? PLEASE EXPAND ON YOUR RESPONSE TO THIS AS MUCH
AS YOU CAN.
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A: SCSI cabling questions (Background Notes)
SCSI-1 versus SCSI-2
--------------------
Probably the best place to begin is a discussion of SCSI-1 versus
SCSI-2 (the SCSI level). SCSI-1 or SCS-2 refers to a command protocol,
or a software level. SCSI-1 is the original SCSI command protocol and
can transfer at 5MB (megabytes) per second.
SCSI-2 is a faster protocol and can transfer information at 10MB per
second. SCSI-2 is also a "super-set" of SCSI-1. This means that a
SCSI-2 adapter can run SCSI-1 devices. However, if this is done, the
SCSI-1 device will not take advantage of the SCSI-2 adapter's more
powerful features and will only perform at SCSI-1 speeds (5MB versus
10MB). Note that this performance hit will only occur when the SCSI-2
adapter "talks" to the SCSI-1 device. If the SCSI-2 adapter "talks" to
SCSI-2 device, the data transfer rate will return to the proper 10MB
per second.
You could also attach a SCSI-2 device to a SCSI-1 bus. The SCSI-1
adapter would not be capable of the faster SCSI-2 features. Hence, the
SCSI-2 device would transfer data at SCSI-1 speeds. Note that the
issue of bus transfer rates is only valid for high-speed devices, such
as fixed disk drives. "Slow" devices, such as tape drives and CD-ROMs,
generally do not ever approach the data transfer limits of a SCSI-1 or
SCSI-2 bus.
Single-Ended versus Differential
--------------------------------
"Single-ended" and "Differential" define actual hardware differences in
the SCSI bussing (as opposed to the above software differences). In
single-ended SCSI busses, the voltage is determined relative to a
common ground. In differential busses, the voltage is determined by
the differences between the voltage in two wires. This makes a
differential bus less susceptible to electromagnetic interference
(noise) than a single-ended bus. Because these differences are on the
physical connection, single-ended and differential cannot be "mixed",
as opposed to SCSI-1 and SCS-2. (A single-ended device can only be
attached to single-ended SCSI adapter. A differential device can only
be attached to a differential SCSI adapter).
SCSI Performance Specifications
-------------------------------
The performance specifications are as follows:
Transfer rates: SCSI-1 - 5MB per second
SCSI-2 - 10MB per second
Bus length: SCSI-1 Single-ended - 6m
SCSI-2 Single-ended - 3m
SCSI-2 Differential - 19m
As can be seen, SCSI-2 differential can move data faster, over longer
distances.
Dual-Ported versus Single-Ported
--------------------------------
By "double-ended" and "single-ended", I think you mean "dual-ported"
and "single-ported" - please correct me if I am wrong.
"Dual-ported" and "Single-Ported" refer to different means of
connecting SCSI devices to a SCSI bus. Dual-ported SCSI devices (which
require dual-ported cables) have two connectors: one for the bus going
in to the device and one for the bus coming out to allow multiple
devices to be daisy-chained.
Dual-ported device Dual-Ported Cable
+-------------------------+ +----+
| | | || <--- connector
| ------ ------ | +----+
| Bus In Bus Out | | |
+-------------------------+ | | <--- cable
Single-ported devices only have one connector on the device. To allow
daisy-chaining, the cable itself has two connectors.
Single-ported device Single-Ported Cable
+-------------------------+ +-----+
| | connector ---> || || <--- connector
| ------ | +-----+
| Bus In | | |
+-------------------------+ | | <--- cable
As can be seen, the two cables are different; therefore, they have
different part numbers.
Note that connecting a dual-ported cable to a single-ported device will
not work. The bus will not be properly terminated.
But, you can connect a single-ported cable to a dual-ported device.
However, the "piggyback" or the second connector on the cable must be
left open and the second connector (Bus out) must be terminated.
-----------------------
Answer to your questions:
1. In light of the above discussion, are the cables on your customer's
SCSI bus connected correctly? If the SCSI bus is not terminated
properly or not connected properly, all sorts of errors can result.
(Including problems with configuring disk drives).
If all seems in order and you still cannot get the drives to work,
please give me a more detailed description of how they are hooked
up, any error messages, your procedure for reconfiguring the drive,
the state of the drives (available, undefined), etc.
2. As stated previously, the FPT-3 terminator removes the restriction
of only two external devices on an internal SCSI controller. You
can put the full SCSI bus load of seven devices on the bus with
this terminator.
Terminating three lines is all that is necessary for the slower
SCSI-1 bus. In addition, the FPT-3 terminator is usually used on
the internal SCSI bus. This bus is not that long, and the FPT-3
terminator is adequate for this role.
The FPT-18 or FPT-18+ terminator is used for the faster and/or
longer SCSI buses. The FPT-18 is generally used on the external
SCSI bus. The FPT-18 is also used almost exclusively on SCSI-2
buses.
As noted previously, differential SCSI-2 buses require a special
terminator. This terminator is not compatible with single-ended
SCSI-2 buses.
3. Yes, single and dual-ported devices can be attached to the 320's
internal controller. Refer to above discussion for SCSI bus
classifications and specifications.
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This item was created from library item Q658542 CRBDK
Additional search words:
CRBDK HARDWARE IX MAY94 OZNEW RISCSYSTEM RISCSYSU SCSI SYS SYSTEM
TERMINATORS UNIT URGENT
WWQA: ITEM: RTA000042033 ITEM: RTA000042033
Dated: 10/1996 Category: RISCSYSU
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