SCSI Layout

Internal -and- External SCSI Cabling, Single SCSI Bus Adapter
Fast/Wide Combined SCSI Bus Cabling
Fast/Wide Dual (Seperate) SCSI Bus Cabling
   Using Both Internal Narrow and Wide Ports
   IML Oddities
   Mixing On-Planar Narrow SCSI and Fast/Wide SCSI Adapters
   Mixing Narrow and Wide Devices
   Terminating at the SCSI Adapter
   Orange vs. Yellow Termpacks
Assigning SCSI IDs
   SCSI ID Functions
   SCSI Adapter IDs
   Boot/IML Drive SCSI IDs
   Read and Write Device SCSI IDs (Hard Drives)
   Removable-media Read and Write Device SCSI IDs (optical drives, tape)
   Read-only Device SCSI IDs (CD-ROM)
Duplicate Termination
Duplicate IDs

Internal and External Device Cabling, Single SCSI Bus Adapter

  Here is an example of a combined internal and external SCSI configuration. Both SCSI cables have the LAST PHYSICAL DEVICE TERMINATED. Hint, Hint.... 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.
  Of course, you would never be so stupid as to forget plugging the drive power cable back in, or forget to plug the SCSI cable in (either at the device OR at the SCSI adapter).

Fast/Wide Combined SCSI Bus Cabling

 The SCSI-2 Fast/Wide Adapter can be set to combine the internal and external SCSI busses as far as SCSI IDs are concerned. Just like with the SCSI w/cache, all SCSI IDs must be different.

Fast/Wide Dual (Seperate) SCSI Bus Cabling

 The SCSI-2 Fast/Wide Adapter can be set to seperate the internal and external SCSI busses as far as SCSI IDs are concerned. Not all operating systems can handle the seperate setting. Also, some systems might not support the seperate 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
     Here's where I'll probably be heavily sniped at from the tall grass...

   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 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 (slow, narrow, eg. 5MHz) 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. 10MHz) 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.

  The following SCSI adapters use the Adaptec AIC-6250EL Line interface chip, and totally lack the electronics needed to autoterminate.
SCSI /A (long uncached, DIP socket for a termpack)
SCSI w/cache (without the DIP socket for a termpack)

The following SCSI adapters are autoterminating, either with the 84F8324 Line Interface or other integrated line interface.
SCSI /A (short, with DIP socket for a termpack, uses the 84F8324 Line Interface)
SCSI w/cache (with DIP socket for a termpack, uses the 84F8324 Line Interface)
SCSI-2 (actually, you set a jumper to enable termination)
SCSI-2 F/W Streaming RAID

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 57F2870 (Bourn 4120R-003 -221/331).

 Peter spake thus:
    "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. 

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 ID Functions: 
o 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. 
o Allows the SCSI controller to determine the priority of each device. 

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
o 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 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 ID6.

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 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.

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