IBM PS/2 Model 55 SX

IBM Personal System/2 Model 55 SX

Company type	Product model
Industry	Computer hardware
Founded	May 1989 (1989-05)
Headquarters	Raleigh , United States
Products	Personal computer

The Personal System/2 Model 55 SX represents a mid-tier desktop computer within IBM's Personal System/2 series. Launched in May 1989, it is powered by an Intel 386SX processor clocked at 16 MHz. In October 1990, IBM unveiled a diskless workstation variant known as the Personal System/2 Model 55 LS. The Model 55 SX quickly became the top-selling machine in the PS/2 lineup, making up 23 percent of IBM's personal computer sales just four months post-release. By 1991, it held the distinction of being the leading x86-based PC globally.

Development and release

On May 9, 1989, IBM introduced the PS/2 Model 55 SX alongside the portable IBM PS/2 Model P70. This marked IBM’s initial adoption of Intel’s i386SX chip, a budget-friendly variant of the full 32-bit i386 that operates with a 16-bit data bus, resulting in roughly a 30 percent drop in performance at equivalent clock speeds. The Model 55 SX utilized a horizontal desktop chassis identical to that of the earlier IBM PS/2 Model 30 from 1987.

Both the Model 55 SX and P70 were the first PS/2 units manufactured at IBM’s Raleigh, North Carolina facility following the transition of production from Boca Raton, Florida in 1989. However, prototyping activities remained at the Boca Raton site. The announcement of the Model 55 SX was postponed by over a month due to initial quality control challenges with the motherboard, necessitating design adjustments.

Model 55 SX

Every Model 55 SX comes equipped with a 16 MHz Intel 386SX processor, 2 MB of RAM, and integrated VGA display capabilities. Given the processor’s 16-bit data bus, this model includes three Micro Channel expansion slots operating at 16 bits each, differing from other PS/2 models that feature some 32-bit slots. One slot is designed to support specific MCA graphics adapter expansions such as the 8514/A. Initial configurations were offered as Model 55 SX-031 with a 30-MB hard disk drive and Model 55 SX-061 with a 60-MB hard drive. In June 1991, IBM broadened the range by adding models 55 SX-041 and 55 SX-081 equipped with 40- and 80-MB drives respectively. All hard drives employed the faster Enhanced Small Disk Interface (ESDI), which was standard across PS/2 systems. The system contains two internal bays for drives: one housing the hard disk and another for a 3.5-inch floppy drive. Additional storage required external devices that occupied one Micro Channel slot.

Model 55 LS

The diskless version of the Model 55 SX, named Model 55 LS (LAN Station), was introduced on October 9, 1990. This model lacked internal storage drives and came pre-equipped with either an Ethernet or IBM’s proprietary Token Ring network adapter. It booted its operating system over a LAN connection from a server, with applications and data accessed remotely. Although diskless workstations had been available through special bids since 1989, the Model 55 LS was IBM’s first computer explicitly marketed as such.

Sales

During the summer of 1989, the Model 55 SX experienced a significant increase in sales, representing 23 percent of IBM's personal computer sales by September, a substantial rise from just 4 percent two months prior. This market share remained relatively stable into 1990, with the Model 55 SX accounting for 22 percent of IBM's PC sales by June. Throughout most of 1990, it held the position as the top-selling personal computer in the United States. By 1991, it had become the leading x86-based PC worldwide and was IBM's highest-selling PS/2 model, with sales reaching 25,000 units in August 1990 alone—exceeding the previous record set by the PS/2 Model 50 Z, which sold 18,000 units in one month in 1988. This success marked a recovery for the PS/2 series, which had previously lagged behind earlier IBM PC models. The popularity of the Model 55 SX also caused reported shortages of components in April 1990.

In June 1991, Apple surpassed IBM as the top computer vendor in the U.S. with its Macintosh Classic; nevertheless, IBM maintained its lead as the best-selling PC-compatible brand through late 1991 and into early 1992. The PS/2 Model 55 SX was phased out in May 1992 and succeeded by the similarly equipped PS/2 Models 56 and 57.

Reception

Although sales figures were strong, reviews for the Model 55 SX were mixed to unfavorable. It received an Editors' Choice Award from PC Magazine in 1990; however, earlier critiques pointed out its limited three expansion slots and relatively slow hard drive. Eugene Wong of InfoWorld remarked that despite its higher price, it performed slower than most other 16-MHz 386SX computers. Russ Lockwood from Personal Computing found it to be the slowest among four tested 386SX models and criticized the inefficient use of Micro Channel architecture for drive controller and graphics performance, though he commended the quality of the motherboard and its competitive cost. T.J. Byers of PC World ranked it lowest among comparable systems due to its sluggish speed and restricted expandability.

Submodels

IBM PS/2 Model 55 SX submodels

Model	IBM P/N	Processor	Clock speed (MHz)	Bus	No. of slots	No. of drive bays	FDD	HDD	Stock RAM	Maximum RAM	Video adapter	Monitor	Form factor	Date introduced	Notes	Ref(s).
55 SX	8555-031	Intel 386SX	16 (0–2 w)	MCA, 16-bit	3	2	one 1.44 MB	30 MB (ESDI)	2 MB	8 MB	VGA	optional	Desktop	Template:Date table sorting
55 SX	8555-041	Intel 386SX	16 (0–2 w)	MCA, 16-bit	3	2	one 1.44 MB	40 MB (ESDI)	2 MB	8 MB	VGA	optional	Desktop	Template:Date table sorting			-
55 SX	8555-061	Intel 386SX	16 (0–2 Template:Abbr w wait states)	Micro Channel architecture#MCA,16-bit	3	2	one 1.44 MB	60 MB {{small (Enhanced Small Disk Interface ESDI)}}	2 MB	8 MB	Video Graphics Array VGA	optional	Desktop	Template:Date table sorting/1989/05		-
55 SX	8555-081	i386SX Intel i386SX	16 (0–2 Template:Abbr w wait states)	Micro Channel architecture MCA,16-bit	3	2	one 1.44 MB	80 MB {{small (Enhanced Small Disk Interface ESDI)}}	2 MB	8 MB	Video Graphics Array VGA	optional	Desktop	Template:Date table sorting/1991/06		-
55 LS	8555-LE0	i386SX Intel i386SX	16 (0–2 Template:Abbr w wait states)	Micro Channel architecture MCA,16-bit	3	2	none	none	2 MB	8 MB	Video Graphics Array VGA	optional	Desktop	Template:Date table sorting/1990/10		-
55 LS	8555-LT0	i386SX Intel i386SX	16 (0–2 Template:Abbr w wait states)	Micro Channel architecture MCA,16-bit	3	2	none	none	2 MB	8 MB	Video Graphics Array VGA	optional	Desktop	Template:Date table sorting/1990/10