rf5565a.exe Model 55/65 Reference Disk v1.05 (zipped image)
189-076 PS/2 Model 55 SX and 80387SX Math Co-Processor (8555-031 and 8550-061)
189-089 IBM PS/2 Model 55 SX (8555-R31 and 8555-R61)
190-171 IBM PS/2 Model 55 LS (8555-LT0 and 8555-LE0)
191-077 IBM PS/2 Model 55 SX (8555-041 and 8555-081) and Memory Upgrade for 55 LS
SHS15F2195 IBM PS/2 Model 55 SX HMS
SHS15F2250 IBM PS/2 Model 55SX HMR
Extract of 8555 from HMM
Model 55 Technical Reference, 2d Ed (Oct 90) (Thanks to Peter Wendt)
Model 55 Technical Reference, 1st Ed (Feb 89) Internet Archive
Reworking the DS1287 / DS1387 RTC chip
IBM PS/2 Model 55SX Hangs at Start Up with Windows 3.1 (archived)
8555 Procomm Riser
ADF Sections for Integrated Fixed Disk and Controller
Early 8555SX Planar
F1 3A fuse
J1 Mouse port
J2 Keyboard port
J3 DB25 Serial port
J4 Parallel port
J5 HDD15 Video connector
J7 Power cable P7 header
J8 40-pin floppy header
J9 Bus Riser
J14 Power cable P14 header
ZM23,24 72-pin SIMM sockets
MMI 33F5102 hot glued to planar
ZM25-28, 32-35 NEC D4146L-10
ZM40 Dallas DS1287
ZM43 33F8145 BIOS Odd
ZM44 33F8146 BIOS Even
ZM70 NEC D765BC
ZM80 90X8134ESD DMA
Y1 25.175 MHz osc
Y2 28.32200 MHz osc
Y3 14.31818 MHz osc
Y4 32.00000 MHz osc
Peter wrote (edited):
Early Model 55SX had a riser card connector with solder pins that
were too long. If you applied pressure to the top of the case, it would
propagate through the riser card to the planar, which would come in contact
with the grounded chassis - shorting some of the data bus signals. [snip]
(IIRC there was an ECA "IBM dealer confidential" on this topic)
The "dead cockroach" board with the many patch wires and the glued,
silver-capped patch chip. Needs the almost bare riser card, does not work with
the later ones (or not always). Had various problems from which the non-functional
pin to shut off the 386SX is the worst. Does not work with "clip-on" processor
expansions. Fails with 8514/A + network cards.
8555 System Board FRU P/Ns
27F4667 Model 031 AND 061 (55SX). Bus Adapter, 55SXS FRU P/N 27F4666
57F3003 Model LT0 AND LE0 (55LS). Bus Adapter, 55LS FRU P/N 64F3732
33F8159 Model B00, C00, P00 (55SX MEDIALESS)
Late 8555SX Planar 27F4667 / 57F1536
F4 KB Fuse
J1 HDD-15 Video
J2 DB25 Serial
J6 Bus Riser
J7 Power header P7
J8,9 72-pin SIMM sockets
J11 40-pin floppy header
J14 Power header P14
ZM2 inmos IMSG171P-35R
ZM20 15F6864 or 90X8941 VGA
ZM21 37F0728 or 23F9177
ZM25-31 NEC D4146L-10
ZM33 CMOS RAM Extension chip
ZM36 NEC D765BC
ZM39 Dallas DS1287
ZM40 33F8145 BIOS Odd
ZM41 33F8146 BIOS Even
ZM64 90X8134(ESD) DMA controller
Y1 28.3220 MHz osc
Y3 25.1750 MHz osc
Y4 32.00000 MHz osc
Y5 14.318181 MHz
The revised 55SX board FRU 27F4667. Components slightly
rearranged. Riser cards have more capacitors and / or the
74LS-something that fixes a problem with 8514/A-style
The 55LS "netcomputer" planar FRU 57F3003. Derived from
the 27F4667. Uses the 74LS riserboard, different ROM, the
2K RAM extension (if -LEO or -LTO model).
55LS "netcomputer" planar FRU 57F3003 / 84F6889
>What is the unpopulated 28-pin DIP socket by the RTC? None of my
55 boards have it filled in. RIPL ROM if the network
board didn't have it?
24-pin - as you noted in your follow up posting.
It is for a 2K CMOS Ram extension chip (6116 or
something) that adds to the one in the Dallas RTC -
needed for some purpose on totally medialess 55SXs (no
FDD, no HD, only NIC). I think it is intended to hold
some system status / shutdown infos that are usually
written to disk.
P/N is FRU 64F0722 "2KB NV RAM Module"
> Also has anybody ever seen the resisters soldered on top
the IMGS171S-35, it's labeled as ZM2 on Louis' page.
On this machine they have soldered 6
resistors in connecting some of the pins on that chip.
I'm wondering if this is a custom job or something
It's something IBM did. I don't know what the exact
purpose is, but it circumvents problems that IBM
experienced with the RAMDAC. Model 30-286 planars may also
have this fix.
They are diodes. Only present for
some plastic versions of the RAMDAC. The ceramic version
System Firmware (POST & BIOS)
Firmware stored in EPROM.
Model 55 SX
33F8146 / 33F8145 - 02 Nov 1988, rev. 0, 2x 27C512 (ZM44 / ZM43)
92F0626 / 92F0627 - same parameters and contents as above
Model 55 LS
84F6896 / 84F6897 - 08 Feb 1990, rev. 0, 2x 27C512 (ZM41 / ZM40)
Early Riser P/N 27F4625 / 27F4630
This riser is bare of anything to the right of the MCA
slots but the battery/speaker connector.
Newer Riser FRU 64F3732, P/N 27F4625, 64F0774
This riser has an addition of a resistor, a 74F08, and a bit of wiring on the back.
Late Riser FRU 27F4666, P/N 64F0808, 64F0809
The battery/speaker header is moved down and a few resistors and capacitors are added.
The IC (ZM1) is still the same - 74F08.
64F0809 takes care of hangs with the 8514/A card.
Supports 1 MB and 2 MB 100 ns and 85 ns SIMMs.
Model 55 Tech Ref, 2nd Ed. claims 4 MB / 80 ns support.
[55SX -041 / -081, 55LS -LE0 / -LT0]
The PS/2 Models 55 SX and LS are compatible with:
DESCRIPTION FEATURE PART
SYSTEM BOARD MEMORY NUMBER NUMBER
PS/2 1 MB Memory Module Kit 85 ns 5212 6450603
PS/2 2 MB Memory Module Kit 85 ns 5213 6450604
PS/2 4 MB Memory Module Kit 80 ns 3933 87F9977
8555 -031 / -061
o 1 MB Memory Module Kit (85 ns) (#5212) (6450603)
o 2 MB Memory Module Kit (85 ns) (#5213) (6450604)
8555 Diskette Drive Connector
Not the same as in the HITR. Figure 3-5, page 3-8
of the Model 55 Tech Reference.
Type 2 Serial Port Controller
||-High Density Select
||-Head 1 Select
||+ 12 Vdc
Some Type 2 serial controllers used on the Model 55
system board do not respond as described in the Hardware
Interface Technical Reference.
After the FIFO mode is enabled, bit 6 of the Interrupt
Identification register is erroneously set to 0,
indicating a Type 1 serial controller is installed. Any
application program or operating system that uses bit 6
of the Interrupt Identification register as an indicator
to determine FIFO support will default to the character
mode. This indicator is ignored by the Model 55 ABIOS.
Therefore, programs that operate through ABIOS can use
the FIFO mode. The ABIOS routines also clear any error
indications remaining after a mode change.
Some application programs reset the received-data-ready
indication by writing bit 0 of the Line Status register
as a O. This method can cause compatibility problems and
is not supported by Type 1 or Type 2 serial controllers.
To avoid compatibility problems, bit 0 of the Line
Status register can be reset to 0 by reading the data
and discarding the data if it is not used.
• The Model 55 supports FIFO through ABIOS calls only.
Type 1 Parallel Port Controller (?)
Bi-Directional, no DMA.
> No, it doesn't get to the menu. It just says that
the battery is dead then starts the Automatic
Configuration (or, at least, attempts to). I tried
unplugging everything and the plugging them one by one,
but without any luck. It does the same thing every
Recently it may happen that the battery is *that* drained,
that the CMOS cannot be properly written and the next
reboot reads corrupted data and hangs the system.
After a power cycle and reboot from the reference
diskette the silly game repeats.
You will have to swap out the DS1287 chip for another one.
If you cannot get the 1287 you may use the DS12887, which
is the pin- and function compatible successor of the 1287.
CMOS Clear Method
Aron Eisenpress comes up with:
"I have exactly the same problem with a 55SX.
Only in my case I was trying to install OS/2 on
it. The install progressed to the first reboot and
then couldn't boot off the drive. What's weird is
that it manages to copy all the files to the drive
during the installation, so it does see the drive, it
just won't boot from it.
Aron suggested the following, but I haven't had a
chance to try it yet. I have to look for a DOS
floppy to boot from. If you try this and it works
please post it here.
"I did some searching on the 55sx problem after
replacing the Dallas RTC chip, and found one suggestion
that says you need to clear the CMOS before the
configuration will take.
The method is to boot from a floppy, run DEBUG, and type:
o 70 13
o 71 00
Then run the reference diskette and autoconfigure again.""
Color Changes After Memory
I keep losing half my color settings in
Windows whenever I increase the ram chips from 2 meg to
four meg. Does anyone know what settings are being changed
(INI files, resolution)? Any tips welcomed.
I have seen this twice in the past that a
Mod. 55SX "reacts funny" on memory upgrade. In both
cases it was a *board* failure: #1 had a micro-crack in
one copper-line at the underside of the board #2 had the
pins of the riser-card connector too long and these
touch the grounded chassis after re-insertion of the
riser card. Both errors showed up only with 2 memory
modules installed. Pretty strange.
I would suggest to try out a "bread board"
installation. Remove the board from the chassis, remove
the power-supply also and place it on a non-conductive
surface (Warning: underside pins may scratch furniture).
Install the memory modules, the riser board and lay HD
and FDD in a place where it can be connected but do not
touch board or power-supply. When the system runs fine
without the chassis inspect the underside of the planar
if there are cracks or too long component pins,
especially on the riser-card connector and around all
places, where the board fixing screws go.
And: there are some combinations of planars /
riser-cards known as non working. If the planar FRU is
27F4667 the appropriate riser-card must be 27F4666 (P/N
64F0809 on a white sticker, contains a 74F08-chip and
some condensors / resistors. The almost totally empty
riser-card (P/N 274630 on white sticker / 6 condensors
between slots / 2 larger tinned areas at the rear)
belongs to an early down-level 55SX-planar and is known
to cause problems. The 2nd series risercard .
But .... some early Mod. 55SX suffer on a
sort of systemboard bug that changes the LPT timing a
bit and they choke on faster transmissions in
bi-directional mode - like with using EPP devices
(CD-ROM, Iomega Zip drive) or Interlink. Those EPP
devices run but rather slow and if you connect that 55SX
to another computer with LapLink you will get a hell of
time-out errors when copying larger files (until the
It is pretty hard to tell which boards are
affected. I found out during testings, that mainly those
old boards tend to fail that have the "dead cockroach"
chip at the mid/front. It is sitting between the two
bigger square chips and is accompanied by a silver
capped chip glued to the board. Both are wired to the
system with patch-wires. These machines have the old
Level-1 riser card 27F4630 (printed on a decal), which
is almost bare apart from six small condensors between
the MCA connectors. The Level-1 riser board has no chips
and no "component printing" in white. The board P/N is
33F5064 (sticker over 33F5060 P/N - which was the buggy,
unfixed original board P/N).
There was a revised systemboard without
the patch-on chip, which came with the Level-2 riser
(several resistors, some el-co's and the small
condensors - came with an ECA from IBM due to problems
running 8514/A style cards), which has been replaced by
the Level-3 riser 64F0809 with a 74F08 chip (ZM1). The
early 55SX planar cannot be used with the Level-3 risers
... the later systemboard should not be used with the
bare Level-1 riser.
The later sysboard is FRU P/N 27F4667,
P/N 85F0419 (55SX) or 57F3003 (55LS / LEO/LTO - which
have the additional 2K CMOS NV-RAM), the riser is FRU
P/N 27F4666, P/N 64F0809 (55SX) or 64F3732 (55LS). Sadly
the FRU alone does not tell, which P/N is behind ... IBM
always delivered the latest P/N at a FRU request.
I installed about 350 - 400 Mod. 55SX in the early '90s
for the german Automobile Club ..... so I guess I knew
them quite well ... :-)
The harddisk is in fact a camouflaged MCA-adapter with
a harddisk mechanism atop.
The entire MCA-bus is fed over the cable to the drive.
This drive is the fourth MCA slot, which shows up in the
configuration. In case the contacts between board and
riser, riser and cable, cable and harddisk are oxydated
-or- in case the cable is slightly damaged there will be
a communication problem between board and harddisk -
which in the end leads to 1048x HD-error or "hang
condition" during Power On Self Test (boot-up).
The maximum system board memory capacity is 8MB (4MB
memory module kits installed in memory-module connector
1 and 2). A memory-module kit must be installed in
module connector 1. The system can support a total
of 16 MB of memory.
My experience is that the 55SX is fine with one SIMM,
but it must go in the slot closer to the power supply.
The 55SX takes PS/2 SIMMs, 1 MB, 2 MB, or 4 MB, either 85 ns
or 100 ns. I'm pretty sure it can also take 80 ns
SIMMs (definitely can in the 4mb size), but I'm not
-- Aron Eisenpress,
486 CPU Upgrade: IBM PS/2
Model 56, 57
>Any reason it won't work on the 8555?
Older 55SX had a slightly buggy mainboard / CPU where
the "CPU disable" pin did not work as supposed. You
could use "clip-over" upgrades only on the 55SX anyway
since it has no "upgrade" socket of any sort (the 387SX
socket does not feed all required lines through ...).
If your 55SX planar has the "dead cockroach" fix
(silver capped chip with patchwires around) suspect it
as one of the earlier ones that might or might not work.
The early models with the "bare" riser card without any
TTL-logic chip and without lots of capacitors had
multiple troubles - not only with the CPU.
>But could you ground out a pin directly on the cpu?
Ahem ... as far as I recall IBM reported having got a
quantity of 386SX where the disable pin does not work
*at all* - and therefore any clip-over upgrades won't
The "dealer confidential" paper I have in mind
mentioned the 55SX / 65SX only - none of the other
SX-machines (like L40, N33 etc.) where the case forbids
to use any updates anyways. The L40 however had been on
the "CPU upgradeable" list at Hantz & Partner
(www.upgrade.de) years ago, which was a "send-in
upgrade" with soldering on board level. So it seems as
if only very early 16MHz 386SX were affected by this
The corresponding pin is -FLOAT ("FLT#", active low -
pin 28), which "floats all Intel 386SX bidirectional and
output signals, including HDLA. Asserting FLD# isolates
the Intel 386SX from the surrounding circuitry." (Intel
Datasheet 24018708.PDF, Page 60) Originally this pin was
added to allow in-circuit emulation without the need to
unsolder / remove the chip from the board.
Simple test: ground pin 28 and the system may not
POST. If it does the chip is one of those faulty
386SX/16 CPUs in general do not have a (working)
disable pin, except they bear a 'C STEP' writing (most
of the ones I've seen don't...). All 386SX CPUs
with 20 MHz or more use at least the C stepping mask, so
that's not a issue for them...
Best regards, Alfred
To remove the password:
- open the cover (hardest part of the operation...)
- locate the speaker cable from the front to the
vertical bus riser card
- unplug the cable at the bus riser card
- turn it around by 180 degrees
- plug it back to the bus riser card
- close the cover.
Password is gone.
305 ERROR CODE In The IBM PS2 Model 55SX
This generally results from an easily replaced fuse on the
motherboard, right near the Keyboard and mouse connectors. The fuse is in a
holder. No soldering is required to remove it. The fuse is made by BEL FUSE -
5MF, 3 AMPS, 250 VOLTS [5MF(P) 3-R].
Radio Shack equivalent is part number 270-1054. It's also
called a type GGS 5x20 mm fuse. An exact replacement from
another source may be substituted.
This fuse often goes when a serial mouse is used with a
PS2 adapter. Only use a true PS2 style mouse in the 55SX.
The keyboard or the mouse could also be bad.
160 POST Error
A 160 POST error will be displayed if the Bus Adapter Card
(FRU P/N27F4666) is not installed in an 8555.
If the Bus Adapter is removed for problem determination,
or not re-installed after replacing the system board, the
system will not complete POST successfully. The Bus
Adapter Card is required by the 8555 for system
160 POST error may also be caused by a defective bus
168 POST ERROR ON 8555
If a 168 POST error code is displayed after replacing the
system board in an 8555, verify that the correct system
board is installed. If the correct system board is
installed, a 168 error usually represents a Token Ring
The 8555 LAN SYSTEM BOARD P/N 33F8159 is intended for use
in special bid 55SX MEDIALESS* Workstations. If
installed in an 8555 MODEL 031 OR 061, the machine will
POST a 168 error and will not boot from drive A.
* NOTE: 8555 "MEDIALESS" Models may be upgraded with
diskette and fixed disk drive options. The model number
should be used to determine the correct system board FRU.
104XX ERRORS In a PS/2 55SX
(Model 061 ONLY) H035587
If a 104XX error occurs in an 8555 Model 061, check for a
power supply barcode label K1DLXXXXXXX (X = any
number). If the bar code matches, replace the power supply
(this power supply can also be identified by a plastic
power supply fan grille). If the power supply does not
match the above description, follow the normal problem
IMPORTANT: This power supply should be replaced in 8555
Model 061's only.
201 POST Error On 8555
MEM Location H063320
If an 8555 displays a 201 POST error, verify that a SIMM
has been installed in the connector closest to the edge of
the system board, next to the power supply.
If the memory is installed correctly, and the error
remains, follow FRU isolation procedures in the hardware
maintenance service pamphlet.
8555 System Board Damage And
Related Handling H021594
Failure analysis of returned PS/2 8555 system boards has
revealed that some boards have physical damage to the
phase lock loop (PLL) SIP component. The PLL SIP is a
small ceramic circuit board, mounted vertically on the
system board. It is approximately one inch wide and 3/4
inch tall, and is soldered to the system board very close
to the power supply cable connectors.
The reported damage is a result of the PLL SIP being bent
at its system board connectors. The PLL SIP is
supposed to be mounted at a 90 degree angle to the system
board. Its design will permit some connector bending (up
to a 20 degree angle from its intended perpendicular
mount). Once bent, however, the PLL SIP must not be
*** IMPORTANT ***
- On the replacement system board FRU, if the PLL SIP is
bent toward the power supply connector, and interferes
with plugging in the power supply cables, do not
straighten the PLL SIP. Return the system board as new
- If bent away from the power supply connector, and if the
system board operates, the system board should be
considered good. Do not straighten the PLL SIP.
ADF Sections for @DF9Fh "Integrated Fixed Disk and Controller"
DMA Arbitration Level
DMA channel adapter uses to transfer data
< "Level 5">, 6, 7, 0, 1,
DMA Burst Pacing Interval
Time interval between DMA transfer bursts
during which the Micro Channel is released by the fixed
disk controller for use by the CPU. Under normal
circumstances, select <24 Microseconds>
<"24 Microseconds">, 31,
16, Burst Disabled
DMA Pacing Control
Enables or disables the 'DMA Burst Pacing
Interval.' If set to <Burst Disabled>, the DMA
Pacing Control will be disabled. Under normal
circumstances, select <Disabled>.
Time to Release
The amount of time that the fixed disk
controller will keep the Micro Channel after being
preempted. If the 'DMA Pacing Control' is set to
<Enabled>, the Time to Release will default to
immediate. Under normal circumstances, select <6
<"6 Microseconds">, 3,
Whether the adapter will release control of
the bus when it has been using it exclusively. Under
normal circumstances, select <On>.
Primary/Alternate Port Addresses
Port addresses used by the adapter.
Either <Primary> or <Alternate> will work
equally well. If there are two integrated
fixed disks, then select <Primary> for one and
<Alternate> for the other.
(3510h-3517h), Alternate (3518h-351fh)