IBM 5120

IBM 5120 Computing System

IBM 5120 Computing System (1980) — built-in 9-inch CRT and two 8-inch floppy drives
Specifications
Developer	IBM General Systems Division; industrial design by Tom Hardy
Manufacturer	IBM
Type	Desktop business computer (also designated IBM 5110 Model 3)
Released	February 1980
Discontinued	10 December 1982
CPU	IBM PALM (Put All Logic in Microcode) — 16-bit board-level processor (same as IBM 5100 and IBM 5110). 1.9 MHz, 530 ns 2-byte cycle
Memory	32 KB RAM standard; configurations 16–64 KB in 16 KB increments. 64 KB ROM holds the operating environment; both APL and BASIC interpreters are in ROM as standard; front-panel toggle selects the language
Storage	2 × 8-inch 1.2 MB floppy drives built-in. Optional IBM 5114 Diskette Unit adds 2 more drives (4 × 1.2 MB total online). No tape drive option. No hard disk from IBM (Core International offered an aftermarket Winchester subsystem from 1981)
Display	Integrated 9-inch monochrome CRT (significantly larger than the 5100 / 5110's 5-inch tube — major usability improvement for accounting work)
Sound	None
Dimensions	Desktop form factor with detached keyboard
Weight	45 kg (99 lb)
OS / Firmware	APL and BASIC interpreters as operating environment, both in ROM as standard, switched via front-panel toggle at power-on
Predecessor	IBM 5110 (1978); the 5120 is officially also designated IBM 5110 Model 3
Successor	IBM Datamaster (System/23) (1981), then IBM PC (5150) (August 1981)
Model no.	5120

The IBM 5120 Computing System is a desktop business computer announced and released in February 1980 by IBM General Systems Division, and discontinued on 10 December 1982. The 5120 is officially also designated IBM 5110 Model 3 — it shares the IBM 5110's PALM processor and core architecture, but in a substantially redesigned desktop chassis with a much larger 9-inch CRT, a detached keyboard with palm rest, and two built-in 8-inch floppy drives.

The 5120 was IBM marketing's "lowest-priced IBM business computer to date", launched at a base US$13,500 (approximately $51,500 in 2024 dollars) with configured systems from US$9,340 to US$23,990. Industrial designer Tom Hardy won the IDSA Gold IDEA award (1980) and an I.D. Magazine Product Design Award (1981) for the 5120's chassis.

The 5120 was effectively the last machine in the PALM / APL line; the IBM System/23 Datamaster followed in 1981, and the IBM PC (5150) arrived in August 1981 marking the architectural break to Intel 8088 and PC DOS.

Relationship to the IBM 5100 and 5110

The 5120 shares with the 5100 and 5110:

• The PALM board-level processor (same 16-bit, 1.9 MHz, 530 ns cycle).
• The APL and BASIC interpreters — but on the 5120 both are in ROM as standard.
• The PALM microcode emulators for IBM System/370 and System/3 instruction subsets.

The 5120 differs significantly:

• 9-inch CRT vs the 5100 / 5110's 5-inch CRT — usable for sustained accounting work.
• Detached keyboard with palm rest, vs the integrated keyboard on the 5100 / 5110.
• 2 × built-in 1.2 MB 8-inch floppy drives vs the 5100's DC300 tape and the 5110's external 5114.
• No tape drive — DC300 is dropped entirely.
• Desktop chassis (45 kg) vs the 5100 / 5110's "portable" suitcase.
• Both APL and BASIC in ROM as standard (no language-specific configurations).

Software Bundle

IBM released six accounting applications bundled with the 5120 launch in February 1980:

• Task Inventory
• Billing
• Payroll
• Accounts Payable
• Accounts Receivable
• General Ledger

This bundle was the heart of the 5120's small-business positioning.

Architecture

The 5120 uses the same processor as the IBM 5100 and IBM 5110 — IBM's manuals call it the processor or controller, never "PALM". The 5120 is a repackaged 5110: its power supply board is labelled "5110 Power Supply Board A2" in IBM's own logic manual, and the maintenance manual's theory section heads the power chapter "5110-3 Power Supply". Memory:

• Executable ROS — microcode + monitor in PALM's directly addressable 64 KB.
• Language ROS — both APL and BASIC interpreters in ROM as standard; front-panel toggle selects at power-on.
• RWS (RAM) — 16 / 32 / 48 / 64 KB; 32 KB standard.

Form Factor and Physical Design

The 5120 chassis is a substantial desktop:

• Dimensions — desktop form factor with detached keyboard.
• Weight — 45 kg (99 lb).
• Built-in 9-inch monochrome CRT — major usability improvement over the 5-inch tube of the 5100 / 5110.
• Detached keyboard with palm rest — IBM cited "reduced static loading in arms and shoulders" as a usability advantage.
• Recessed lift areas on the bottom for easier handling (industrial design).
• Reduced desktop footprint compared with the 5110 + 5114 combination.

IBM 5120 — front view showing the integrated 9-inch CRT and detached keyboard. (Image: Wikimedia Commons, CC BY 3.0)

IBM 5120 detached keyboard with palm rest. (Image: Wikimedia Commons, Public domain)

Storage

• 2 × 8-inch 1.2 MB floppy drives built-in — the defining 5120 feature. The drives accept IBM Diskette 2D media (double-sided double-density, 1.2 MB formatted).
• Optional IBM 5114 Diskette Unit — adds 2 more 8-inch drives for a total of 4 × 1.2 MB online.
• No tape drive — the DC300 system from the 5100 / 5110 Model 1 is not available on the 5120.
• No hard disk from IBM — though Core International offered an aftermarket Winchester subsystem and the CoreNet LAN from 1981.

I/O and Peripherals

• IBM 5103 Printer — matrix printer carried over from 5100 / 5110.
• IBM 5114 Diskette Unit — external 2-drive 8-inch floppy expansion.
• Communications — RS-232 and BSC / SDLC options carried over from the 5110.

Operating Languages

APL and BASIC are both in ROM as standard. The language is selected at power-on via a front-panel toggle (the same UX as the 5100 / 5110 dual-language models). The APL implementation is the same modified APLSV; the BASIC is the same System/3 port.

Service Documents

Both IBM service documents for the 5120 are hosted on this wiki:

• IBM 5120 Maintenance Information Manual — SY34-0192-0, December 1979. The CE service manual. The scan has no text layer, so it cannot be searched; read it by eye.
• IBM 5120 Computing System Logic Manual — SY34-0193-0, December 1979. Gate-level logic diagrams and the DC power distribution sheets.
• GH30-0232-1 — Joint 5110 / 5120 Bibliography, September 1980 — also lists 5120 publications.

The 5120 documentation set on Bitsavers is thinner than the 5110's — many user-facing manuals (APL, BASIC) for the 5120 are the 5110 manuals because the language ROMs are essentially the same. Use the 5110 references where applicable.

Common Faults

• Processor card — same unobtainable gate arrays as the 5100's G2 and the 5110's J2 cards. Reseat before condemning.
• 8-inch floppy drives (×2 built-in) — drive belt aging is the dominant failure mode; head load solenoid fatigue; spindle motor brush wear; media (IBM Diskette 2D) oxide shedding. Drives are typically Shugart-class 8-inch FDDs.
• 9-inch CRT and deflection board — separate monitor sub-chassis inside the 5120 case; high-voltage flyback and deflection electrolytics are concerns.
• Power supply — two supplies in one enclosure, and the difference matters. Board A2 is the same transistor switching regulator as the 5100's and 5110's, producing +5, −5, +8.5, +12 and −12 V for the logic. Board A1 is a ferroresonant supply for the diskette drives, and it is the part the 5120 adds. A1 is fused on each of its three DC outputs — F2 (+5 V), F3 (−5 V), F4 (+24 V) — and A2 is not fused at all: it shuts down instead. So a blown F2, F3 or F4 means the fault is on the diskette side and does not implicate the computer's supply; a dead machine with all three fuses intact points at A2. The enclosure also contains transformer T1, the resonant capacitor C4 (an AC-rated part, not an electrolytic and not a general recap candidate) and circuit breaker CB1. See IBM 5120 Capacitor Replacement Guide.
• The 60-second rule. Where the 5100 and 5110 manuals say five seconds, the 5120 manual says that after an automatic shutdown the system should be powered off for at least 60 seconds before being powered up again. Cycling a 5120 faster than that makes a healthy supply look dead. This is the single most common way to misdiagnose one.
• Keyboard — remove keycaps with the proper tool rather than by hand. The claim that the 5120 uses a Model F buckling-spring keyboard could not be confirmed in IBM's own documentation and has been removed.
• Industrial-design "carry handles" on the bottom — plastic fatigue reported by some collectors.

Pricing

• Base configuration: US$13,500 (≈$51,500 in 2024 dollars).
• Configured systems: US$9,340 to US$23,990 depending on RAM and peripherals.

John Titor Connection

The 5120 is not part of the John Titor lore — by 1980 the System/370 emulator was no longer novel. The 5120 is the desktop sibling of the machine in the legend (the IBM 5100 / IBM 5110) but not the legendary machine itself. See the IBM 5100 page for full coverage.

Gallery

• 
  IBM 5120 Computing System (1980)
• 
  IBM 5120 — front view
• 
  5120 detached keyboard with palm rest
• 
  IBM 5120 full configuration

Related Pages

• IBM 5120 Maintenance Guide
• IBM 5120 Troubleshooting Guide
• IBM 5120 Capacitor Replacement Guide
• IBM 5100 — 1975 PALM-line origin
• IBM 5110 — 1978 predecessor (5120 is officially also designated 5110 Model 3)
• IBM 5114 Diskette Unit — external 8-inch floppy expansion

References

Further reading

• Bitsavers — IBM 5120 documents — the same two manuals hosted here, plus the 5110/5120 bibliography GH30-0232.