Cracking Open: IBM PS/2 Model-55sx

The PS/2 Model 55sx debuted in 1991. It was one of the later generations of the PS/2 line. The Model 55sx came with onboard VGA, 2MB of RAM, a 40MB hard drive and an Intel 386sx CPU.

Here's the front of the unit. As you can see, there is no CD-ROM drive. CD-ROMs were not standard on PCs of this time.

The bay on the left is a 1.44Mb 3.5" floppy disk. The PS/2 line introduced the 3.5" form factor to PC-compatibles.

The hard drive resides in the bay to the left.

Here's the built in case-lock. The PS/2's case is steel, unlike a lot of modern cases which are plastic.

The rear of the unit. You can see the available connectors, fan and expansion slots.

Beyond the VGA standard, the PS/2 mouse and keyboard ports are pretty much the only legacy of the PS/2 line that survived into the 21st century.

As you can see, this unit doesn't include a network card. Ethernet ports are standard on most machines today, but in the 90's the Ethernet/Token Ring battle was still raging. Network connectivity would have been an option.

The expansion cards are stacked horizontally to the right.

Across the bottom are keyboard and mouse ports, one parallel LPT port, one 25 pin RS232 serial port, and one VGA port.

Here's the FCC information for the unit.

On the right side of the case is the heat vents.

The underside of the unit. As you can see, more steel.

Chock full of IBM patents.

The PS/2 desktop cases were poorly designed and not easy to open. Two screws had to be released on the sides, and then the case slides backs and lifts up. Usually the cases were tight and didn't slide easily.

A side view of the case coming off.

The top of the case removed showing the inside.

Apple Macs sometimes had signatures inside the case of the people who assembled them. This Model 55sx has an identifying stamp and that's it.

A look inside.

One of the things that gets your attention first is the Rube Goldberg-esque device used to turn the unit on and off. A coat hanger style rod connects the power switch on the front to another switch inside of the case.

Here's a look at the MicroChannel riser card inside of the unit. The top 2 are 16 bit MicroChannel slots. The bottom is a 32 bit MicroChannel expansion slot.

A look at the floppy drive (left) and hard drive (right).

Here's a look at the motherboard. The yellow and black wire across the top connects the system's speaker.

The system's memory is buried deep inside the case, beneath some cables and underneath the support arm for the riser card.

A closer look after removing the riser card's support arm.

The riser card is now removed. As you can see, the riser card also connected the hard drive to the motherboard.

The MicroChannel riser card. At the bottom is the card edge connector that goes to the motherboard. The top connects to the hard drive.

The backside of the riser card. Nothing to see here.

The ribbon cable for the hard drive. As you can see, it's completely proprietary, different from any SCSI or IDE connector you've seen. It also sends power to the drive as well as transfer data.

The rear of the hard drive. Note the lack of molex power connector. Power is delivered through the card edge connector you see here.

Disconnecting the power switch arm.

Another look at the power switch arm.

Starting to remove the power supply.

The P7 and P14 connectors connect power from the power supply to the motherboard.

The power supply removed from the unit.

Danger! Don't take the power supply apart!

Naturally, we take the power supply apart. This was safe as the unit hasn't been on in months. You can see the transformers and capacitors for the power supply clearly.

Disconnecting the floppy drive.

Here's the Model 55sx's floppy cable. Like the hard drive cable, it also delivers power.

Viewing the hard drive and floppy drive from the rear.

A closeup of the floppy drive. Again, no power connector.

The drives are removed by sliding out the front. First, this shielding must be removed.

To remove the shielding, there are two screws. This on the right is first.

The second shielding screw is on the left. It also holds down the case lock.

The case lock unscrewed.

The drive shielding removed.

There are tabs hidden under the case which must be released to let the drives out.

The floppy and hard drive covers removed.

The hard drive and floppy drives slide out of the front.

Here's the mounting bracket for the hard drive. A simple locking slide out unit.

The underside of the hard drive.

Here's the system's hard drive. The IBM WD-L40S. It is a 40Mb ESDI drive.

ESDI was an high-performance drive interface standard which was an alternative to SCSI at the time. IDE drives were cheaper, and ultimately became standard. At the time, they were just becoming popular.

The top of the floppy drive.

Here's the back of the drive. Again, you'll notice the lack of a power connector.

The front of the floppy drive. As you can see, there's no floppy door which is traditional today.

Here's the right side of the drive. The drive was built for IBM by ALPS.

The underside of the drive showing the slide-on mounting bracket.

The motherboard removed showing just the bottom of the case.

Here's the system board out of the unit.

Here's the memory from the unit. It's a 72-pin 4MB SIMM 80ns produced by Toshiba. There's only one in this unit.

Here's the system board without memory installed.

Here's the onboard BIOS chips and the real time clock.

The Model 55sx features an Intel 386sx-16 CPU. The SX is a 32-bit internal CPU with a 16-bit databus. It was seen as a cheaper alternative to the 80386DX processor.

The open socket to the left was for an 80387sx math coprocessor.

The two gold color chips are oscillators which help control timing on the motherboard. The chip on the right is an IBM 90X8941. This is the VGA controller for the motherboard.

The chip on the left is an 37F0728. Based on the location, I assume it also has to do with the VGA card.

A set of 8 NEC D4146L-10 chips soldered to the motherboard. I couldn't find information about them, but I suspect they're Video RAM chips. Can anyone give more info?

These chips are the oscillators controlling the clock speed for the motherboard.

The chip on the left is a 23F856. The chip on the right is a NEC D765BC. I couldn't find specific information about these, but based on location, they probably are floppy controllers or have something to do with memory. Any help?

The IMGS171S-35. Once again, I can find pricing for parts, but no explanation.

The large golden chip in the upper left is the 90X8134ESD. This is a DMA controller, which controls access to the RAM chips. (Thus blowing the theory a few slides back.)

The other two major chips are a 27F4659 and a 27F4619. I could find no information on them other than pricing once more. By process of elimination, I assume they have something to do with the hard drive interface.