RetroTechCollection

IBM 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.[1] 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.

IBM 5120 Computing System
IBM 5120 Computing System (1980) — built-in 9-inch CRT and two 8-inch floppy drives
Specifications
DeveloperIBM General Systems Division; industrial design by Tom Hardy
ManufacturerIBM
TypeDesktop business computer (also designated IBM 5110 Model 3)
ReleasedFebruary 1980
Discontinued10 December 1982
CPUIBM 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
Memory32 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
Storage2 × 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)
DisplayIntegrated 9-inch monochrome CRT (significantly larger than the 5100 / 5110's 5-inch tube — major usability improvement for accounting work)
SoundNone
DimensionsDesktop form factor with detached keyboard
Weight45 kg (99 lb)
OS / FirmwareAPL and BASIC interpreters as operating environment, both in ROM as standard, switched via front-panel toggle at power-on
PredecessorIBM 5110 (1978); the 5120 is officially also designated IBM 5110 Model 3
SuccessorIBM Datamaster (System/23) (1981), then IBM PC (5150) (August 1981)
Model no.5120

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.[1] 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

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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

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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.[1]

Architecture

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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".[2][3] 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

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The 5120 chassis is a substantial desktop:

  • Dimensions — desktop form factor with detached keyboard.
  • Weight — 45 kg (99 lb).[1]
  • 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

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  • 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.[4]

I/O and Peripherals

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  • 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

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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

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

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

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  • 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.[2] 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.[2] 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.[2] 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

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  • Base configuration: US$13,500 (≈$51,500 in 2024 dollars).
  • Configured systems: US$9,340 to US$23,990 depending on RAM and peripherals.[1]

John Titor Connection

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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.

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

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Wikipedia, "IBM 5120". Tertiary; used for the launch chronology, the pricing, the software bundle and the industrial-design awards. The withdrawal date is sourced there to the IBM Hursley Museum's Hardware Announcements to 1987 list, whose URL no longer serves the PDF, so it could not be confirmed against the underlying document.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 IBM, IBM 5120 Maintenance Information Manual, SY34-0192-0, December 1979 — hosted on this wiki as IBM 5120 Maintenance Information Manual. Section 2 items 207 (AC box) and 208 (power supply locations: PC boards A1 and A2, resonant capacitor C4, transformer T1, circuit breaker CB1, fuses F2, F3 and F4); item 272 (Y1 socket pin assignments and the DC rail tolerance table); items 273, 274 and 284 (AC, DC and internal diskette DC voltage distribution); and the "5110-3 Power Supply", "DC Power Distribution", "DC Voltage Tolerance" and "Reference Voltage" pages of the Theory section. The scan carries no text layer and has to be read by eye.
  3. ↑ 3.0 3.1 IBM, IBM 5120 Computing System Logic Manual, SY34-0193-0, December 1979 — hosted on this wiki as IBM 5120 Computing System Logic Manual. The DC power distribution sheets, which show the supply as "5110 Power Supply Board A2" and carry the rail distribution and cable net lists.
  4. ↑ Core International, 1981 5110 Disk brochure. The publisher's site is long gone and the brochure survives only in the Internet Archive's Wayback Machine, which this wiki's verification tooling cannot reach, so its contents could not be checked. Recorded as attributed, not verified.

Further reading

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