PS/2 Models 55 SX and 55 LS

Type 8555

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, 1st Ed. (Feb 89) (Internet Archive)
Model 55 Technical Reference, 2nd Ed. (Oct 90) (scan by Peter Wendt)

Reworking the DS1287 / DS1387 RTC chip
IBM PS/2 Model 55SX Hangs at Start Up with Windows 3.1 (archived)

8555 Power
8555 Procomm Riser

Early Planar
Late Planar
Riser Cards
   Early Riser
   Newer Riser
   Late Riser
System Firmware
   ROM Images
Known Problems
ADF Sections for Integrated Fixed Disk and Controller

Early 8555SX Planar

F1 3 A 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
ZM2 IMGS171S-35
ZM16 23F8566
ZM17 23F9177
ZM18 15F6864
ZM23,24 72-pin SIMM sockets
MMI 33F5102 hot glued to planar
ZM25-28, 32-35 NEC D4146L-10
ZM40 Dallas DS1287 RTC/CMOS
ZM43 33F8145 BIOS Odd
ZM44 33F8146 BIOS Even
ZM49 27F4620
ZM53 27F4659
ZM54 27F4619
ZM62 80386SX-16
ZM61 80387SX-16
ZM69 P8042AH
ZM80 90X8134(ESD) DMA Controller
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, 55SX FRU P/N 27F4666
57F3003 Model LT0 AND LE0 (55LS). Bus Adapter, 55LS FRU P/N 64F3732
33F8159 Model B00, C00, P00 (55SX MEDIALESS)

Later 8555SX Planar

SX: FRU P/N 27F4667 / 57F1536 / 57F1538 [P]
LS: FRU P/N 57F3003 / 84F6889 [P]

F4 Keyboard/mouse fuse
J1 HDD15 video connector
J2 DB25 serial port
J3 Parallel port
J4 Mouse port
J5 Keyboard port
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
ZM10 P8042AH
ZM20 15F6864 or 90X8941 VGA
ZM21 37F0728 or 23F9177
ZM22 23F8566
ZM25-31 NEC D4146L-10
ZM33 Ext. NVRAM socket (55 LS)
ZM38 27F4620
ZM39 Dallas DS1287 RTC/CMOS
ZM40 33F8145 BIOS Odd
ZM41 33F8146 BIOS Even
ZM51 57F1139
ZM60 80387SX-16
ZM61 80386SX-16
ZM63 27F4619
ZM64 90X8134(ESD) DMA Controller
ZM67 57F1505 PAL or pads only
Y1 28.3220 MHz osc
Y3 25.1750 MHz osc
Y4 32.00000 MHz osc
Y5 14.318181 MHz

The revised 55SX board FRU P/N 27F4667. Components slightly rearranged. Riser cards have more capacitors and / or the 74LSxx glue that fixes a problem with 8514/A-style cards.

The 55LS "netcomputer" planar FRU P/N 57F3003 is derived from the 27F4667. It uses the 74LSxx riserboard, different ROM, and has the 2K NVRAM populated (-LEO/-LTO models).

> What is the unpopulated 24-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?

Peter says:
   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.

FRU P/N is 64F0722 "2KB NV RAM Module"

William Walsh:
   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 IBM did??

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.

David Beem:
   They are diodes. Only present for some plastic versions of the RAMDAC. The ceramic version was fine.

System Firmware (POST & BIOS)

Firmware stored in EPROM.

ROM Images

Model 55 SX

33F8146 / 33F8145 - 02 Nov 1988, rev. 0, 2x 27C512 (ZM41 / ZM40 or ZM44 / ZM43)
92F0626 / 92F0627 - same contents as above
33F8102 / 33F8103 - 07 Jan 1989, rev. 1, 2x 27C512 (ZM41 / ZM40 or ZM44 / ZM43)

Model 55 LS

84F6896 / 84F6897 - 08 Feb 1990, rev. 0, 2x 27C512 (ZM41 / ZM40)
   requires later planar w/ extended NVRAM (ZM33) installed

Riser Cards

Early Riser P/N 27F4625 / 27F4630 [P] [P] [P]

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 [P] [P] [P]

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 [P] [P] [P]

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)

Type 2 Serial Port Controller

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 time...

Peter says:
   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 Upgrade

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.

Peter replies (edited):
   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 capacitors / resistors. The almost totally empty riser-card (P/N 274630 on white sticker / 6 capacitors 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.

LPT File Transfer Issues

William said:
   I noticed that it will not establish a "InterLink" connection with a model 70 via the parallel port...

My question: Is there a major wiring difference between the 55 and 70 ports? I know my cable will work with two 70s...and that the 55 will print, so I assume that the port must be OK.

I'm more curious than anything else now...I've since decided to just use two model 70s to transfer the contents of one hard drive to the other.

Peter wrote (edited):
   No - [they] are both the same layout. 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 system hangs).

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 capacitors 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 capacitors - 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 ... :-)

DBA Hard Drive (from Peter)

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).

Planar Memory

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 memory module connector 1. The system can support a total of 16 MB of memory.

Aron Eisenpress:
   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 certain.

486 CPU Upgrade (In)compatibility

>Any reason why the 486 upgrade for 56/57 it won't work on the 8555?

Peter replies:
   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?

Peter replies:
   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 work.

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 ( 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 general fault.

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 few.

Alfred Arnold:
   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...

Remove Password

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.

Known Problems


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/N 27F4666) 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 self-identification.

160 POST error may also be caused by a defective bus adapter.

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 adapter problem.

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 determination procedure.

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 straightened.


  • 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 defective.

  • 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, 3, 4

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>.
          <"Disabled>, Enabled

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 Microseconds>.
          <"6 Microseconds">, 3, Immediate

Fairness On/Off
   Whether the adapter will release control of the bus when it has been using it exclusively. Under normal circumstances, select <On>.
          <"On">, Off

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.
          <"Primary">  (3510h-3517h), Alternate (3518h-351fh)

Content created and/or collected by:
Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

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