IBM PS/2 Model 55 LS

IBM Personal System/2 Model 55 SX

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

The Personal System/2 Model 55 SX represents a mid-tier desktop computer within IBM's Personal System/2 (PS/2) product line. Introduced in May 1989, it is powered by an Intel 386SX CPU clocked at 16 MHz. In October 1990, IBM released a diskless workstation version known as the Personal System/2 Model 55 LS. The Model 55 SX quickly became the top-selling machine in the PS/2 series, making up 23 percent of IBM's PC sales in just four months. By 1991, it held the distinction of being the best-selling x86-based PC globally.

Development and release

The PS/2 Model 55 SX was officially unveiled by IBM on May 9, 1989, alongside the portable IBM PS/2 Model P70. It marked IBM's initial adoption of Intel’s 386SX processor in a personal computer, which Intel had introduced nearly a year earlier in June 1988. The cost-effective 386SX processor performs internal operations on a 32-bit scale but utilizes a 16-bit data bus, leading to roughly a 30 percent decrease in performance compared to the full 386 running at the same frequency. The Model 55 SX was housed in a horizontal desktop chassis, identical to that of the earlier IBM PS/2 Model 30 released in 1987.

Both the Model 55 SX and the Model P70 were the first PS/2 computers manufactured entirely at IBM’s Raleigh, North Carolina facility after production shifted from Boca Raton, Florida, in 1989. However, prototype development continued at Boca Raton. Initial quality control challenges with the motherboard caused a delay of over one month prior to the Model 55 SX’s market debut as IBM refined its design to meet quality standards.

Model 55 SX

Every Model 55 SX is equipped with a 16 MHz Intel 386SX processor, comes standard with 2 MB of RAM, and includes integrated VGA display capabilities. Due to the processor’s 16-bit data bus, this model features three Micro Channel expansion slots that are all 16-bit, distinguishing it from other PS/2 systems that offered some slots with 32-bit capacity. One of these slots is designed to accommodate extensions for specific MCA graphics adapters such as the 8514/A. The initial releases included two variants: the 55 SX-031 equipped with a 30-MB hard disk and the 55 SX-061 featuring a 60-MB hard disk. In June of 1991, IBM broadened the range by adding models 55 SX-041 and 55 SX-081 with larger drives of 40 and 80 MB respectively. All hard drives utilized Enhanced Small Disk Interface (ESDI) technology, delivering faster transfer rates compared to older ST-506 interfaces. Internally, the system provides two bays for 3.5-inch drives—one occupied by the hard disk and one by a floppy disk drive; expanding storage required external devices connected via one of the Micro Channel slots.

Model 55 LS

IBM introduced the Model 55 LS (LAN Station) on October 9, 1990. This diskless workstation variant of the Model 55 SX omitted any internal drives entirely. These units were preconfigured with either an Ethernet or IBM’s proprietary Token Ring network adapter and booted their operating system over a network connection from a LAN server while storing all data remotely. Although available as a special order since 1989, this was IBM’s first computer explicitly promoted as a diskless workstation.

Sales

During the summer of 1989, sales of the Model 55 SX saw a significant increase, making up 23 percent of all IBM personal computer sales by September, a notable rise from just 4 percent two months earlier. This proportion remained relatively constant into 1990, with the Model 55 SX representing 22 percent of IBM's PC sales as of June. Throughout most of 1990, it held the position as the leading personal computer in the U.S. market. By 1991, the Model 55 SX had become the top-selling x86-based PC worldwide and was IBM’s most popular PS/2 model by a considerable margin, with 25,000 units sold in August 1990 alone. This figure exceeded the previous record set by the PS/2 Model 50 Z, which sold 18,000 units in a single month (October 1988). The success of the Model 55 SX marked a turnaround for the PS/2 series, which had previously lagged behind IBM's original PC lineup. Its high demand led to reported component shortages in April 1990.

In June 1991, Apple surpassed IBM as the leading computer vendor in the U.S. with its Macintosh Classic; however, IBM’s Model 55 SX continued to be the best-selling PC-compatible machine through late 1991 and into early 1992. IBM discontinued the PS/2 Model 55 SX in May 1992, substituting it with the similarly configured PS/2 Models 56 and 57.

Reception

Although sales were robust, reviews for the Model 55 SX were mixed to unfavorable. It received an Editors' Choice Award from PC Magazine in 1990; however, earlier critiques focused on its limited expansion capability with only three slots and a sluggish hard drive. InfoWorld observed that despite its higher cost, it performed slower than many other 16-MHz 386SX systems. Russ Lockwood of Personal Computing identified it as the slowest among four tested 386SX models but commended its motherboard design and competitive pricing. PC World rated it lowest in terms of performance and expansion potential.

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 w )	MCA, 16-bit	3	2	one 1.44 MB	60 MB ( ESDI)	2 MB	8 MB	VGA	optional	Desktop	Template:Date table sorting		-	55 SX	8555-081	Intel 386SX	16 (0–2 w )	MCA, 16-bit	3	2	one 1.44 MB	80 MB ( ESDI)	2 MB	8 MB	VGA	optional	Desktop	Template:Date table sorting		-	55 LS	8555-LE0	Intel 386SX	16 (0–2 w )	MCA, 16-bit	3	2	none	none	2 MB	8 MB	VGA	optional	Desktop	Template:Date table sorting		-	55 LS	8555-LT0	Intel 386SX	16 (0–2 w )	>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)