|
Combined Internal and External SCSI Cabling
Fast/Wide Combined SCSI Bus Cabling
Fast/Wide Dual (Seperate) SCSI Bus Cabling
IML Oddities
Mixing On-Planar Narrow SCSI and Fast/Wide SCSI Adapters
Mixing Narrow and Wide Devices
Terminating at the SCSI Adapter
Non-Autoterminating
Autoterminating
Orange vs Yellow Termpacks
SCSI ID
Assigning SCSI IDs
SCSI Adapter ID
Boot/IML Drive SCSI ID
Read and Write Device SCSI IDs
Removable-media Read and Write Device SCSI IDs
Read-only Device SCSI IDs
Duplicate Termination
Duplicate IDs
Combined Internal and External Device Cabling
Here is an example of a combined internal and external SCSI configuration.
Both SCSI cables have the last physical device terminated. All SCSI IDs are
unique.
What can go wrong? If you have devices that share
the same SCSI ID or you terminate more than one
SCSI device on a SCSI cable, then you may "loose" one or more devices.
Fast/Wide Combined SCSI Bus Cabling
The SCSI-2 Fast/Wide Adapter can be set to combine the internal and external
SCSI buses as far as SCSI IDs are concerned. Just like with the SCSI w/cache,
all SCSI IDs must be different.
Fast/Wide Dual (Separate) SCSI Bus Cabling
The SCSI-2 Fast/Wide Adapter can be set to separate the internal and
external SCSI buses as far as SCSI IDs are concerned. Not all operating systems
can handle the separate setting. Also, some systems might not support the
separate settings.
Using Both Internal Ports on Fast/Wide
Interesting, requires some thought. Does mixing a narrow cable and a wide
cable dumb down the internal port to narrow fast, or can the controller handle
negotiating between both cables?
IML Oddities
In older systems, the system looks for the IML (system) partition on ID6. On
all systems, the search for the IML partition starts with the SCSI adapter in
the lowest slot (or planar SCSI), looking at SCSI device ID6 first, then
down.
Do yourself a favor and put the system (or convenience) partition on ID6. If
you get a I999xxx or "no boot device" or a SCSI error, suspect improper
termination or SCSI IDs.
Note: If having the boot drive as ID6 seems wrong,
it isn't. IBM actually followed the ANSI standard. Everyone else is wrong.
Mixing On-Planar Narrow SCSI and Fast/Wide SCSI Adapters
If you have a 56 / 57 / 76 / 77 / 85 with on-planar SCSI, you will NEVER get
much more than 6MB/sec from a Fast/Wide SCSI adapter. During POST, the planar
SCSI loads it's ROM into memory, and it controls the maximum transfer
speed.
Mixing Narrow and Wide Devices
It can be done, since the SCSI adapter queries each device during boot and
sets the maximum speed it uses to communicate with each device.
BUT... You CANNOT use a narrow SCSI cable with wide
devices (even with 68 to 50 pin adapters!) and expect wide transfer speeds.
Also, for narrow devices on a wide cable, the upper bit must be properly
terminated by the 50 to 68 pin adapter.
Terminating at the SCSI Adapter
Electrically, an UNUSED SCSI connector on the adapter is the end of a SCSI cable.
So if you have a SCSI /A (long, uncached) with ONLY internal drives,
you would need an internal (DIP) termpack for it. If you have an earlier one or
two oscillator SCSI w/cache (no termpack) then you will need an external
terminator. For the question of replacing an orange termpack with a yellow one,
look at Orange vs Yellow Termpacks.
BUT... SCSI-1 (narrow, eg. 5 MHz) controllers may accept poor or no
termination and still work. If the SCSI bus has a few devices on it, and the
traffic is light, you will probably get away with it.
SCSI-2 (fast, narrow or wide, eg. 10 MHz) controllers really hate having
SCSI signals being reflected at the physical end of the SCSI cable. All IBM
SCSI-2 adapters (capable of fast transfers) are autoterminating.
Non-Autoterminating
The following SCSI adapters use the Adaptec AIC-6250EL Line
interface chip, and totally lack the electronics needed to autoterminate:
Autoterminating
The following SCSI adapters are autoterminating, either with the 84F8324
Line Interface or other integrated line interface.
Orange vs Yellow Termpacks
Tim Clarke wrote:
There is only one FRU for the "internal" termpack for the IBM SCSI
w/cache (FRU 85F0063) that has the appropriate 20-pin socket and the IBM SCSI
w/o cache (FRU 85F0002). The termination resistor pack (20-pin) is FRU P/N
57F2870 (Bourns 4120R-003-221/331).
Peter spake thus:
The auto-terminate function requires a little more hardware: a
switching transistor that disables the TermPwr wire from the T-RES and another
transistor / IC function that senses the voltage on the TermPwr line and the
voltage on the data lines to figure out whether the line is terminated or
not.
SCSI ID
The SCSI ID has two main functions:
- Allows the SCSI controller to distinguish one device from another.
Because each device (including the SCSI controller) has a unique ID, a
device cannot receive commands that are intended for another device.
- Allows the SCSI controller to determine the priority of each device.
Assigning SCSI IDs
When more than one physical device is connected to the same SCSI controller,
the devices compete for support from the SCSI controller. Because the SCSI
controller communicates with only one device at a time, it gives each device a
priority, based on its device ID. The range of IDs is from a high priority of 7
to a low priority of 0. The SCSI controller is usually preassigned to the
highest priority, ID 7, and the hard disk drive that starts the computer is
assigned to the next-highest priority, ID 6.
In general, low- or medium-performance devices should not be assigned to IDs
higher than high-performance devices. However, this can vary, depending on your
applications and your requirements.
SCSI Adapter ID
Set the SCSI controller to ID 7. Change to other IDs only for unique
configuration requirements. The SCSI Adapter's ID determines the priority of
SCSI devices on ONLY that SCSI Adapter.
Boot/IML Drive SCSI ID
The SCSI hard disk drive that starts your computer should be assigned to the
next highest ID (usually 6). If the SCSI controller for this drive is on an
adapter, and there are multiple SCSI adapters in your computer, the adapter to
which this hard disk drive is attached must be installed in the lowest-numbered
expansion slot, of all the SCSI adapters in the computer (lowest slot
limitation exists in older systems).
Read and Write Device SCSI IDs
Read and write devices such as hard disk drives, are high-performance
devices. These devices should be assigned to high-priority IDs, such as 6 or 5.
Unless you use the Selectable Drive-Startup feature, the ID also determines
which drive will start your computer, default is ID 6.
Removable-Media Read and Write Device SCSI IDs
Removable-media read and write devices such as optical drives, should be
assigned to mid-priority IDs, such as 4 or 3, below those of the hard disk
drives.
Read-only Device SCSI IDs
Read-only devices such as CD-ROM drives, usually should be assigned to
low-priority IDs, such as 1 or 0. However some applications might recommend
that you assign your CD-ROM drive to a higher ID (such as 4 or 3). Hard disks
usually should have higher IDs than the CD-ROM drives.
Duplicate Termination
In this example, it is VERY possible that the final device in the
chain, ID3, will "disappear" and not be seen during POST. If ID1 was
terminated, instead of ID2, you may loose ID2 and ID3. This is important if you
just installed another SCSI device and your system claims it can't find the
boot drive.
Duplicate IDs
In this example, ID3 is duplicated. The system will most likely hang, since
two devices will try to respond to ID3. You won't be able to boot, since your
HDs are unavailable.
|