# IBM PS/2 Model 55 SX

The Personal System/2 Model 55 SX is a midrange desktop computer in IBM's Personal System/2 (PS/2) family of personal computers. First released in May 1989, the Model 55 SX features an Intel 386SX processor running at a clock speed of 16 MHz. In October 1990, IBM introduced a diskless workstation version of the Model 55 SX, called the Personal System/2 Model 55 LS. The Model 55 SX was the best-ever selling computer in the PS/2 range, accounting for 23 percent of IBM's PC sales within four months of its introduction. By 1991, the PS/2 Model 55 SX was the best-selling x86-based PC in the world.

## Development and release

IBM announced the PS/2 Model 55 SX in May 1989, alongside the portable PS/2 Model P70. The Model 55 SX was IBM's first personal computer to use the 386SX processor by Intel, released almost a year earlier in June 1988. The 386SX was a cost-reduced version of the 32-bit 386 processor, also by Intel. Internally, the i386SX supports 32-bit operations, but its data bus could only access RAM 16 bits at a time, leading to a performance reduction of around 30 percent compared to a 386 processor with the same clock speed. The PS/2 Model 55 SX sports a horizontal desktop case exactly identical to that of the PS/2 Model 30, released earlier in 1987.
The Model 55 SX, along with the Model P70, were the first new models of PS/2 to be assembled from the beginning at IBM's factory in Raleigh, North Carolina. Previously, models of PS/2 were manufactured at IBM's facility in Boca Raton, Florida, before the company moved personal computer production to Raleigh in 1989. Prototyping of the computers were still performed in Boca Raton, however. Owing to initial runs of the 55 SX's motherboard failing to meet IBM's quality-control standards, announcement of the computer was delayed by over a month, according to InfoWorld, pending motherboard design revisions.$^{:1}$

## Specifications
### Model 55 SX

All models of the Model 55 SX features a 386SX processor clocked at 16 MHz and have 2 MB of RAM stock and on-board VGA graphics. As a consequence of the processor's 16-bit data path, the Model 55 SX sports only three 16-bit Micro Channel expansion slots, whereas other true 32-bit offerings in the PS/2 line carried at least some 32-bit slots. One of the Model 55 SX's 16-bit slots features an extension for specific MCA graphics adapters such as the 8514/A.$^{:36}$ The initial two models in the PS/2 Model 55 SX range were the Model 55 SX-031, featuring a 30-MB hard disk drive, and the Model 55 SX-061, featuring a 60-MB hard drive. In June 1991, IBM introduced two more models of Model 55 SX, with 40- and 80-MB drives (the Model 55 SX-041 and Model 55 SX-081, respectively). All hard drives were of Enhanced Small Disk Interface (ESDI) specification, which IBM used extensively in the PS/2 range and which was faster than the ST-506 interface of older IBM PCs. The computer has two 3.5-inch drive bays, both occupied as stock by the included hard drives and the 3.5-inch floppy disk drives. Users had to make use of external drives if they wanted more storage options, with the associated controllers taking up one of the three Micro Channel slots.

### Model 55 LS

In October 1990, IBM introduced the Model 55 LS (LAN Station), which were diskless-workstation versions of the Model 55 SX featuring no installed drives—not even a floppy drive. Instead, the computers came preinstalled with either an Ethernet adapter or a Token Ring adapter (IBM's own networking standard), with the machines booting their operating system from a LAN server and having all applications and date remotely served and saved. IBM had been offering the Model 55 LS as a special bid version of the Model 55 SX for customers who specifically requested since its release in 1989, but the Model 55 LS was IBM's first computer specifically branded as a diskless workstation.

## Sales

Sales of the Model 55 SX ramped up significantly over the summer of 1989, with the machine representing 23 percent of all IBM personal computers sold by September 1989, up from 4 percent only two months earlier. This share of IBM's PC sales remained steady into the next year, the Model 55 SX representing 22 percent of IBM's PC sales in June 1990. For the majority of 1990, it was the best-selling personal computer in the United States. By 1991, it was the best selling x86-based PC globally. It was IBM's best-selling PS/2 by a significant margin, with the company selling 25,000 units of the computer in August 1990 alone. The heretofore best-selling PS/2 Model 50 Z, meanwhile, sold only 18,000 in a single month (October 1988). This represented a reversal of fortunes for the PS/2 line, which had underperformed as a whole compared to IBM's earlier PC line. The Model 55 SX proved so popular that IBM reported parts shortages in April 1990.
Apple Computer eventually overtook IBM for the number one spot in U.S. computer sales with the Macintosh Classic in June 1991. However, it retained its status as the best-selling PC-compatible for the remainder of 1991 and the first half of 1992. IBM officially withdrew the PS/2 Model 55 SX in May 1992, replacing it with the similarly specced PS/2 Models 56 and 57.

## Reception

Despite its massive sales success, the Model 55 SX received mixed, generally negative, reviews in the press. Despite earning an Editors' Choice Award in PC Magazine in 1990, the magazine's Tom Unger had earlier wrote that, "Even with the loaded motherboard, some users may feel constrained by getting just three expansion slots", praising the solid construction and motherboard integration while panning the hard drive for being slow.$^{:136}$ InfoWorld's Eugene Wong called the Model 55 SX slower than most other 16-MHz 386SX machines despite being significantly more expensive. Russ Lockwood of Personal Computing found it the slowest of four 386SX machines reviewed and wrote that the Micro Channel architecture's potential for bus mastering was wasted on the drive controller and on-board graphics which were not capable of such. However, unlike Wong, he praised the motherboard's construction and called it price-competitive. In a review of several 386SX machines, PC World's T. J. Byers put it at the bottom of the list owing to its slow performance and limited potential for expansion.