51-N (and Y) is SCSI model and is an IML system. Reference and Diagnostic images are stored in the system partition in a hard drive which is set to ID #6.
Rear I/O Panel
1x parallel, 2x serial (25-pin and 9-pin), mouse, keyboard, 1x RS/6000 style SCSI connector.
Slot #1 is assigned for onboard SPOCK SCSI.
Base clock is fixed to 33MHz (both -N/Y).
A stock 51-N does not accept any higher OSC other than 66MHz. It will stop with 102 error code. My 5551-NAB has a modified BIOS ROM and it accepts 80Mhz OSC (84Mhz is its limit). With the modified BIOS ROM and AMD 5x86, it runs like a champ in 160MHz inner clock without 102 error.
72-pin SIMM socket x4. Total RAM is limited to 32MB. The planar takes
FirstPage (Fast?), True Parity, 70ns SIMM only (Standard Industrial Parity SIMM).
ECC function can be activated only when total RAM size in bank A and bank B
Total 5 slots are available in a small slim body using a short height riser card. On the riser card, there are 3 slots for full length adapters on the left side (looking from the front panel) and 2 slots for half length adapters on the right side. 1 slot (left bottom) is already occupied with a PS/55 Japanese Display Adapter /J or XGA-2 /A according to model and type.
This part is commonly used for 5551-W (20Mhz) and 5551-R/L (33Mhz, EIDE) models.
No video chip is provided on the planar itself. Two types of display
adapters were available:
DBCS/Display Adapter-J, FRU 07G0446 For models except below.
4 Mode FDD: Star marked 2.88MB 4 mode drive x1.
The drive detects 2.88MB(2ED), 1.44MB(2HD), 740KB(2DD) and 1.2MB(2HD) Japanese special mode.
Something about Japanese special 1.2 MB mode
In order to read and write 1.2MB formatted floppy, special device drivers should be installed for DOS/V, JDOS and OS/2 2.00 and higher environment. These drivers are contained in a "Support Diskette for 1.2MB Diskette".
For DOS/V and JDOS:
$FDD35IA.SYS is in Support Diskette Ver. 1.20 and 1.21.
OS/2 2.00 and higher:
OS/2 drivers and utilities: OS2DASD.DMD, IBM1FLPY.ADD, FORMATA.COM, DISKCOPY.COM. CONFIG.SYS will be automatically amended and following two lines will be added:
BASEDEV=IBMFLPY.ADD /PS2 REM --DrvInst--BASEDEV=IBM2FLPY.ADD
FORAMTA.COM under OS/2:
Hard Drive interface (Spock)
32 bit SCSI I/F is onboard. SCSI micro code set is 92F2246/47 and bus master control code chip is 10G4891. 512KB cache is ready on the planar. One 50 pin SCSI Header (not a card edge type) is on the planar and one RS6000 style 60 pin connector is on the rear I/O panel.
I couldn't configure F/W SCSI /A, AKA Corvette along with the integrated SPOCK SCSI.
HDDDepending on model-type, 200MB to 540MB SCSI hard drive was equipped. IML drive should be set to ID=6 and the drive capacity is limited <1GB. IBM DPES1080 can't be used as an IML drive. Actual capacity of the drive exceeds BIOS limit a bit. Conner CFP1060 might be the biggest one which can be used as an IML drive. (Actually I used it on my N). I don't know why a HDD >1GB can't be used as ID=6 though it has 46/47 micro code combo.
If you can't find appropriate drive with 1G capacity, I recommend to use a small drive for IML and choose larger drive, say 2G or something, and give ID=5 to such a drive and set it to "C drive" in set configuration (I mean "changing boot order").
Hard Drive bay
Two for 3.5" drives (The bay beside FDD is already populated with a stock SCSI HD and an optional drive bay locates below FDD).
Unable to install a regular CD-ROM drive within the case because a full length 5" drive will hit the PSU. If you can get a drive for laptop models with SCSI interface and an appropriate connector, and if you don't care for cosmetic appearance, maybe you can put it in the case.
Reference and Diagnostic Diskettes
Ref: Ver. 1.00/1.01/1.11 are available. Ver. 1.11 is something weird.
Basic design for this machine was established in the model 5551-Wxx which has 486SX-20. Drive structures used in 50-W were commonly used for 51-N/Y/R/L models. It is impossible to identify model/type at a glance. You need to check the label at the left bottom of the front cover.
Pic #1: Top to bottom: 8570-A21, 5551-NAB, PC720 6860, PC750 6886
Appearance is completely same with 5551-N/Y except rear I/O panel. Main difference between 51-N/Y and 51-R/L is that 51-N/Y have onboard SCSI I/F and IML function while 51-R/L have E-IDE I/F and *NO* IML function (reference partition is not used for 51-R/L. "Set Configuration" should always be done with the reference diskette).
*1 SCSI Adapter II/A32 (SPOCK SCSI with 512KB cache) is equipped for RXE and RED.
RXE: SCSI HD is hooked to the SCSI /A. Onboard IDE is left unused.
RED: 340MB IDE HD is connected to onboard IDE interface and a SCSI CD-ROM is connected to SCSI/A.
Rear I/O Panel
1x parallel, 2x serial (9-pin and 25-pin), mouse, keyboard.
CPU and Socket type
R planar has solder pads for 486 QFP package, and a 486DX2-66 is loaded on a ZIF socket 3.
5551-R is capable to run PODP5V83.
BIOS should be updated to use a PODP. BIOS ROM is EEPROM (Flash ROM) and only
one revision (for PODfying) was released by IBM Japan.
L planar can't take a PODP because it has ZIF Socket 1 which does not fit to a POD (P24T). AMD and/or Cyrix on an interposer works fine.
Genuine Option for 5551-L
70MHz OSC could be used instead of 66.666MHz OSC. Any OSC higher than 70MHz will cause 102 error.
Same as 51-N/Y. Support ECC function with true parity 70ns SIMMs.
Depends on machine type, two variations were available:
Power Supply Unit is slightly different from that of 51-N in it's dimension, but electrical specification is almost same. PSU for 51-R/L is shorter than that of 51-N/Y. R/L model has power connectors, J25 and J26, on the planar and at the same time the planar has edge connector J1 at the front right of the planar. Both connectors are electrically connected. Both 5551-R/L use AT-like power connectors on the planar.
Exception: L planar used in 5530-L lacks J25/J26 connectors. There are solder pads on L planar used in 5530-L. 5530-L uses edge connector J1 to get DC from PSU.
Hard Disk Interface (EIDE) (sort of...)
R and L models have EIDE interface on the planar. I once used 8.4G 16 heads drive in my 51-R and 30-L.
51-R /L was released before EIDE specification was fixed. It could handle drive larger than 524MB. I do not know its upper limit. As far as I can see, 51R recognized a Seagate 8.4G 16 heads drive which PC750 6886 could not recognize. (The very same drive once FDISKed using 51-R could be used for 6886 but not as a primary *boot* drive but as a secondary drive).
Reference and Diagnostic Diskettes
Following Ref/Diag diskettes are commonly used for 5551-R/L and 5530-L.
Windows 9x and 5551-R/L
To solve this problem, replace AMD CPU to Cyrix 5x86. This symptom commonly observed on 51-N and other SCSI models using genuine IBM SCSI /A.
Don Peter reported that he succeeded to run Win98SE on his IML machine. But (sorry Peter) I can't believe that any of IML machine could run Win98(SE) in 32-bit disk access with built in SPOCK driver. Anyway I don't use Win98/98SE on my PS/55's. Win98 is too "heavy" for my good old PS/55's.
Additional information (2002.01.06)
Am I correct?
This model is almost same with 5551-R. CPU is degraded to i486SX-33. The most distinctive feature of 5521 is that a Cirrus GD5428 1MB SVGA /A was given for YSB and YWB models instead of PS/55 Japanese Display Adapter.
Modified R/L planar is used for
this model (QFP IBM486DL2-66 is omitted from U1). 486SX33 sits on a Socket-1
at U2. This planar has a jumper block at the location of
J6 on R /L planar. According to quick reference
the jumper should be turned on when a double-clock processor option is used
instead of a stock 486SX33 CPU.
Cirrus GD5428 SVGA /A (adapter ID: 917A)
Cirrus GD5428 SVGA /A (1MB VRAM standard but upgradeable to 2MB) was not available as an "optional part". It's different from Cirrus SVGA used in Server 500 and is free from "flicker disease". There are solder pads for additional VRAM and the circuit is ready for adding SOJ VRAM's. Win95 built-in driver can use full 2MB but NT4 detects only 1MB.
Click here for more information about Cirrus SVGA.