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IBM 3270 PC

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The IBM 3270 PC (machine type 5271) is a mainframe-terminal-emulation workstation announced by IBM on 18 October 1983 (announcement letter 183-129).[1] The system unit is essentially an IBM PC XT (5160) motherboard, chassis, and PSU, but with the eight ISA expansion slots almost entirely occupied by specialist 3270-terminal-emulation cards. The combination of these cards plus the 3270 PC Control Program allows the workstation to host up to four mainframe terminal sessions, one DOS task, and two notepads simultaneously in a windowed display — a unique capability in 1983.[2]

IBM 3270 PC
IBM 3270 PC (machine type 5271) — XT-based mainframe-terminal-emulation workstation, announced 18 October 1983
Specifications
DeveloperIBM Entry Systems Division, Boca Raton, in cooperation with IBM 3270 terminal groups
ManufacturerIBM
TypeMainframe-terminal-emulation workstation (PC XT chassis + 3270 hardware)
Released18 October 1983 (announcement letter 183-129)
Discontinued1987 (superseded by IBM 3270 Workstation Program software running on standard PCs and PS/2s)
CPUIntel 8088 @ 4.77 MHz — identical to the IBM PC XT planar
Memory256 KB minimum (Model 2 fixed); 512–640 KB on Models 4 / 6 / 24 / 26 / 30 / 50 / 70; up to 2 MB on 31 / 51 / 71 with the Expanded Memory Adapter (XMA)
Storage5.25" 360 KB DSDD floppy (1 or 2); optional 10 MB ST-412 hard drive (Model 6) or 20 MB hard drive (Models 30 / 50 / 70 from 1986)
DisplayIBM 5272 Color Display (14" tilt/swivel, 720×350, 7 colours, 24 kHz hsync), or IBM 5151 monochrome, or IBM 3295 Plasma Monitor (Models 24/26)
SoundPC speaker
OS / Firmware3270 PC Control Program + PC DOS 2.0 / 2.1. Control Program v3.0 required for Models 31/51/71 and all P-models. Up to 4 mainframe sessions + 1 DOS task + 2 notepads simultaneously in a windowed display. Single DOS task only. Superseded in 1987 by the 3270 Workstation Program
PredecessorIBM PC XT (5160) (chassis) and IBM 3278 / 3279 terminals (function)
SuccessorIBM 3270 Workstation Program (1987) — same multi-session experience on a standard XT/AT with only a 3278/79 emulation card
Model no.5271 (system unit machine type). Submodels: 2, 4, 6, 24, 26, 30, 31, 50, 51, 70, 71, and P-prefix variants (P30, P31, P50, P51, P70, P71)

The 3270 PC was discontinued in 1987 when IBM released the 3270 Workstation Program (a software-only product that ran on standard PCs/PS/2s with only a 3278/79 Emulation Adapter), removing the need for the 3270 PC's specialised cards.

Note on Machine-Type Confusion

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The "3270 PC" family includes:

  • IBM 3270 PC (this page) — system unit 5271, the XT-based original.
  • IBM 3270 PC/G — system unit 5371, graphics version with IBM 5278 Display Attachment Unit and IBM 5279 14" display.
  • IBM 3270 PC/GX — also system unit 5371 (not 5372 — both /G and /GX share the same system unit type; the /GX is differentiated by the IBM 5378 Display Attachment Unit and IBM 5379 19" display).
  • IBM PC AT/G and AT/GX — system unit 5373, AT-based equivalents.

Models

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IBM 3270 PC (5271) submodels
Submodel Storage RAM Notes
2 1 × floppy 256 KB fixed Entry; not RAM-expandable
4 2 × floppy up to 640 KB
6 floppy + 10 MB HDD up to 640 KB Most common configuration
24 / 26 2 × floppy / floppy + HDD up to 640 KB Added 25 Oct 1984 (announcement 184-136); IBM 3295 Plasma Monitor support
30 / 50 / 70 various 640 KB on planar, 20 MB HDD Added 1986 (announcement 186-053)
31 / 51 / 71 various 256 KB planar + 1 MB on Expanded Memory Adapter card 1986 generation
P30 / P31 / P50 / P51 / P70 / P71 various various Use 101-key Enhanced AT-style keyboard instead of the 122-key

The 1984 expansion announcement (184-136, 25 October 1984) added IBM 3295 Plasma Monitor support — an orange-plasma display capable of 62 × 160 or 46 × 106 text grids, far beyond the 5272's text capabilities.[3]

The 1986 expansion (186-053) added 640 KB on the planar and the XMA Expanded Memory Adapter (up to 1 MB of EMS-style expansion memory) on the 31/51/71 submodels.[4]

Launch Pricing

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  • Model 6 (512 KB + 10 MB HDD): $6,210 system unit only.
  • Typical working colour-monitor configuration: ~$8,465 including software and cabling.[5]

Specialist Cards

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The 3270 PC system unit is fundamentally an XT 5160 motherboard / chassis / PSU; the difference from a stock XT is in the cards that occupy nearly all eight ISA slots:

Keyboard Adapter Card

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  • Intercepts the 122-key IBM 5271 Keyboard Element scancodes and translates them to XT scancodes.
  • Two variants:
Simple (ROM 6323581)
Complicated (ROM 6323582) — adds NMI button and serial port.
  • Critical: this card holds the video BIOS for the 3270 PC Display Adapter — the video cards do not work without the keyboard adapter card installed.[6]

3270 PC Display Adapter

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  • Main video card, full-length 8-bit ISA.
  • SCN2672 Programmable Video Timing Controller (PVTC) at I/O ports 0180h–019Bh.
  • 8 KB framebuffer split: 4 KB for the 3270-emulation layer / 4 KB for an emulated CGA-or-MDA layer.
  • Drives a unique 720 × 350 mode on the IBM 5272 colour monitor.

All Points Addressable (APA) Card

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  • Optional add-on.
  • Adds CGA-compatible 320 × 200 / 640 × 200 modes plus two 3270-PC-unique graphics modes.
  • Required for graphics-capable mainframe applications.

Programmed Symbols (PSS) Card

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  • Optional add-on.
  • RAM-loaded character sets (up to seven fonts) for 3270 graphics applications.
  • Three of the fonts (4, 5, 7) are tri-plane allowing 8-colour characters.

3278/79 Host Connect / Emulation Adapter

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  • BNC twinax connection to the IBM 3174 or 3274 cluster controller.
  • Two variants exist:
Older — 4 KB buffer, I/O at 2D0h–2D6h.
Newer — adds register at 2D7h; reconfigurable buffer size.
  • Technical reference: IBM 1502336 — PC 3278/79 Emulation Adapter Technical Reference (October 1983).[7]
 
IBM 5271 (3270 PC) physical layout diagram showing slot placement for the keyboard adapter, display adapter, host-connect card, and the optional APA / PSS expansion cards. (Image: Wikimedia Commons, public domain)

Operating Environment

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The 3270 PC runs PC DOS 2.0 or 2.1 overlaid with the 3270 PC Control Program:

  • Up to 4 mainframe sessions running concurrently.
  • 1 DOS task running concurrently with the mainframe sessions (only one DOS task can be active at a time).
  • 2 notepads for cross-session text manipulation.
  • All seven of these contexts visible in a windowed display on the 5272 monitor.

PC Magazine (January 1985) called the Control Program "a memory hog" — it consumes approximately 200 KB resident, leaving limited free RAM for DOS applications.

Control Program v3.0 is required for the 1986 Models 31/51/71 and all P-models.

In 1987 the 3270 Workstation Program (IBM 84X0982) replaced the Control Program, providing the same multi-session experience on a standard XT or AT with only a 3278/79 Emulation Adapter — and adding DOS 3.3 support plus up to six DOS sessions.[8]

File Transfer

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The 3270 PC's stand-out practical capability is its SEND / RECEIVE file transfer between CMS (or TSO) on the mainframe and PC DOS on the workstation, with automatic EBCDIC↔ASCII conversion.

Display

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  • IBM 5272 Color Display — 14" tilt/swivel, 720 × 350 in 7 colours (no high-intensity; bright and standard CGA colours render identically). Pin 4 of the DE-9 selects mono vs colour. Color timings: hsync 24 kHz negative, vsync 63 Hz negative. Mono timings: 18.6 kHz / 50 Hz.[9]
  • IBM 5151 monochrome — also supported (the graphics card auto-detects monitor type via pin 4).
  • IBM 3295 Plasma Monitor — orange plasma, 62 × 160 or 46 × 106 text grids, supported on Models 24/26.

Keyboard

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The defining accessory of the 3270 PC family is the IBM 5271 Keyboard Element — IBM part 6110344. It is the 122-key Model F terminal keyboard:

  • 24 PF (Program Function) keys.
  • Weight: 9.3 lb (much heavier than a standard PC keyboard).
  • AT protocol, scan code set 3.
  • First announced October 1983; shipped Q1 1984.[10][11]

The keyboard connects to the keyboard adapter dongle — a small cable that splits a DE-9 on the keyboard controller card into two 5-pin DIN connectors (one for the 3270 keyboard, one passing through to the planar XT keyboard socket).

A plain XT 83-key keyboard works but produces POST error 302 (press F1 to continue). P-model variants use the 101-key Enhanced AT-style keyboard instead.

 
The IBM 122-key Model F keyboard (IBM part 6110344) — the 5271 Keyboard Element used on all 3270 PC family machines. (Image: Wikimedia Commons, CC BY-SA 4.0)

Common Faults

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The 3270 PC chassis is identical to a stock IBM PC XT, so chassis faults are:

  • RIFA-branded X / Y mains-suppression caps in the PSU — vent and produce smoke / fish odour. Replace immediately.
  • Tantalum bypass cap shorts on the planar — pull the +5 V rail.
  • Hard disk failure on Model 6 / 30 / 50 / 70 — original ST-412 / ST-225 drives are end-of-life.

3270-PC-specific faults:

  • Keyboard adapter dongle cable — wear, missing, or pin damage. Pinout is documented at seasip.info and reproducible.
  • Custom cards — scarce in 2024. The Display Adapter + APA + PSS trio can be transplanted into other XT-class machines provided three adjacent full-length 8-bit ISA slots are available and any onboard video is disabled.
  • IBM 5272 monitor — dedicated tube and timings unique to the 3270 PC; failure means substituting a 5151 mono display (losing colour) is the only practical recovery.
  • Capacitor failure — seasip.info documents a real-world case of a blown capacitor taking out a 5271 planar.
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  •  
    IBM 3270 PC (5271)
  •  
    3270 PC with display
  •  
    3270 PC XT colour configuration
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    5271 physical layout diagram
  •  
    5271 Keyboard Element (122-key Model F)
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    122-key keyboard alternate angle
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References

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  1. ↑ https://sharktastica.co.uk/resources/docs/IBM_183-129_3270PC_83.pdf
  2. ↑ https://en.wikipedia.org/wiki/IBM_3270_PC
  3. ↑ http://www-01.ibm.com/common/ssi/rep_ca/6/897/ENUS184-136/index.html
  4. ↑ http://www-01.ibm.com/common/ssi/printableversion.wss?docURL=/common/ssi/rep_ca/3/897/ENUS186-053/index.html
  5. ↑ PC Magazine, 22 January 1985 — Valenzuela review.
  6. ↑ http://www.seasip.info/VintagePC/5271.html
  7. ↑ http://bitsavers.trailing-edge.com/pdf/ibm/pc/cards/1502336_PC_3278_79_Emulation_Adapter_Technical_Adapter_Oct83.pdf
  8. ↑ http://www.bitsavers.org/pdf/ibm/pc/3270/84X0982_3270_Workstation_Program_V1.0_Users_Guide_and_Reference_Apr87.pdf
  9. ↑ http://www.seasip.info/VintagePC/5271.html
  10. ↑ https://sharktastica.co.uk/wiki/ibm-model-f
  11. ↑ https://deskthority.net/wiki/IBM_Model_F_122-key
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