8600 IDC Boards
Intelligent Disk Controller

Outline of IPB IDC Slots

IDC Board

74F899QC 9-bit Latchable Transceiver with Parity Generator-Checker
SN75176B Differential Bus Transceivers
   SWAG: These might take SE output from the 53C700s and convert it to Differential.
NCR 53C700 SCSI I/O Processor Data Manual, Feb 1990
NCR 53C700 SCSI I/O Processor Data Manual, Nov 1992
NCR 53C700 SCSI I/O Processor Programmers Guide, Mar 1990

Channel A and B pads only
D1-2 Activity LEDs (green)
D3 Diagnostics LED (red)
J1 SCSI Port A
J2 SCSI Port B
Resistor SIPPs (lower right) unknown purpose, socketed

External SCSI Port Mounting
For some odd reason, the HPDB-50 ports have the usual catches for a HPDB-50 SCSI cable to latch on to. But oddly enough, there are short hex mounting posts, similar to those used with parallel or serial ports. These hex posts are right up against the HPDB-50 port catches, and would prevent anyone from using the pretty good cable connectors that clip on to the SCSI port.

3 Extra 74F899QC?
There are 16 74F899QC 9-Bit Latchable Transceiver with Parity Generator-Checkers on the IDC, compared with 13 on the other IPB cards. My SWAG is they have to do with the synchronization of the Orthogonal RAID function?

Select SE or Differential with PCBs

Parallan did some innovative design. Each 53C700 can be configured to feed SE or Differential signals to the external SCSI ports. There are three female headers per 53C700 on the IDC. There is a PCB with two male headers. To choose SE or Differential, simply move the PCB to select the SCSI mode.

Note the silkscreened "Single Ended" and "Differential" between the 50 pin headers.

This image shows the configuration for SE drives.

Now, I am familiar with the term pack on a SCSI/A needing to be removed if there are SCSI devices on the External -AND- Internal ports. So how does one remove the term resistors in this case? Some fiddly bit of work pulling these SIPPs out, quite cramped if you want to pull the three SE SIPPs out...

SE and Differential Terminators

SE and Diff Terminating Resistor Identities
The three orange resistor SIPPs (4608R-104-221/331) closest to the top of the lower 50 pin header are the terminating resistors for SE mode.  The FOUR red resistor SIPPs (L101C331 246) -AND- the three orange DIPs (4114R-001-151) below them are the terminating resistors for Differential mode. (Ed. Image typo, should be 4114R)

Note: Pin 1 on the SIPPs and DIPs are to the right!

Note: There is only ONE PTC (F1) between both SCSI headers.

Disks, Controllers & Bays

Two independent (dual channel) RISC-based SCSI I/II controllers on single card; 64 bit interface to IP-Bus. The sales stuff refers to 5MB/s versions of the 53C700, the 1990 version of the datasheet says 5MB/s, but that same datasheet also mentions that there was a planned 10MB/s improvement. The 53C700 on the IDC are date stamped 9245. Period ads mention synchronous rates of roughly 6.75 to 9.75 MB/s. So... how fast is it?

To run Orthogonal RAID, you need two IDCs, so there are four NCR 53C700 (2 per IDC). Both 53C700s on an IDC are fed from a 25.000000 MHz oscillator. No fooling, six places to the right. With normal RAID, all of the drives are on one controller, (all use one clock), but 2 independent IDCs NEED a very precise clock on-board to enable them to synchronize.

Note: The 53C700 has a "SCRIPTS" processor, which is the "RISC-based" controller. There is no separate RISC chip on the IDC.

"Factory" disks are the IBM 0661-467 (400 MB) and 0663 (1 GB) SCSI disks. The 295 does check the HD firmware for a text string.

Supports 1 to 28 hot insertion/extraction disks (all hot pluggable).

The removable trays are CRU DataPort I trays, which are on Flea-Buy...

Max of 9 GB internally (9 bays x 1 GB disk).

Max of 28 GB total (4 ) 53C700 x 7 disks (1GB) with Exp Cabinets.

Ten 3.5" half height bays internally (diskette uses one).  That leaves 9 bays, with one for the system disk, leaves 8 drive bays to support Orthogonal RAID-5 (with on-line spares). So without the SCSI expansion chassis, this limits one to 8 drives (two SCSI drives connected to each 53C700). I suppose you could run one SCSI drive on each 53C700 for the smallest OR-5 footprint, but you won't have the on-line spares to support automatic RAID array rebuilding...

Supports 1 to 3 optional External Expansion Cabinets each with ten 5.25" half height or five 5.25" full height bays.

OS/2 preload includes FTUTIL: allows disk pairs to be striped, mirrored, or duplexed. Also allows hot spare pooling, hot insertion/extraction, automatic data rebuild, and hot fix.

The disks in the PS/2 Servers 195/295 can either be used as individual disks, configured as hot-standby spare disks, configured as Parallel Disk Array (PDA) pairs using the FTAUTIL.EXE standard utility, or combined into RAID-5 PDAs using the optional Orthogonal RAID-5 Disk Array/2 software, which implements orthogonal RAID-5 fault tolerance. Data and parity information are spread across the disks in a PDA, thus both protecting the data and providing performance improvements for applications which mainly read data (rather than write data).  The PDA can be set up to be "orthogonal" by ensuring that the disks of the array are on separate disk controllers, thus increasing the availability of the system still further.

One or two disk controllers can be installed on the IP-Bus, providing two or four SCSI disk channels, respectively. Each channel can support up to seven SCSI disks, for a maximum of 28 disk drives.

When all four disk channels are available, then arrays can be configured across multiple SCSI channels. SCSI channels are defined vertically, arrays can be defined horizontally, thus giving an "orthogonal" configuration. This provides extra fault-tolerance (since a single SCSI disk channel, a single cable, or a single disk can fail without losing the array).

PDAs are set up using the Server 295 reference diskette.  Information on the PDAs can be displayed using MASS/2.  Configuring spare drives and reconstructing a PDA following a drive failure is done using the utility PDAUTIL, as well as automatically by MASS/2, if there is a hot-standby spare disk in the system.

Orthogonal RAID-5

The PS/2 Server 195/295 uses Orthogonal RAID-5, an enhancement on RAID-5.

The minimum full Orthogonal RAID-5 array is eight drives. Four for data, and four for online spares,

Multiple requests to one disk or across one adapter will typically take longer to satisfy than the same number of requests to multiple disks across multiple adapters. A standard RAID-5 system depends on just one disk adapter which all of the disks are connected.

Orthogonal RAID-5 Disk Array technology removes these bottlenecks, and provides an extremely high level of reliability.  Unlike previous RAID-5 implementations, the Orthogonal RAID-5 Disk Array provides redundant SCSI-adapters, SCSI buses, power supplies, power cables and fan assemblies. By multiplexing data access across all four SCSI controllers, the Orthogonal RAID-5 also significantly improves data access performance.

Any one component of a Orthogonal RAID-5 disk subsystem can fail with no loss of data and no interruption to system operation.

RAID Level Performance Characteristics

RAID Level Capacity Large Transfers High I/O Rate Data Avail.
Single Disk Fixed (100%) Good Good (1)
RAID-0 Excellent Very Good Very Good Poor (2)
RAID-1 Moderate (50%) Good Good Good
RAID-2 Very Good Good Poor Good
RAID-3 Very Good Very Good Poor Good
RAID-4 Very Good Very Good Poor Good
RAID-5 Very Good Very Good Good Good
Orthogonal RAID-5 Very Good Very Good Good Very Good
1) The MTBF for single disks can range from 10,000 to 800,000 hours.
2) Availability = MTBF of one disk divided by the number of disks in the array.

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