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IBM 3270 PC Troubleshooting Guide

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This guide documents fault diagnosis for the IBM 3270 PC (machine type 5271). POST is the standard IBM PC XT (5160) sequence in the 1xx–19xx range plus 28xx codes for the 3278/79 Emulation Adapter and 32xx codes for the 3270 PC Display Adapter / APA / PSS subsystem.

Reference Documents

  • IBM SA38-0037-00 — Personal Computer Family Service Information Manual (July 1989), Chapter 10.[1]
  • IBM G320-9539-0 — 3270 Personal Computer Hints and Tips (June 1986).
  • IBM 1502336 — PC 3278/79 Emulation Adapter Technical Reference (October 1983).
  • HelpPC diagnostic codes reference covers 28xx and 32xx ranges.[2]

Initial Diagnosis Workflow

A working 3270 PC boots:

  1. Standard IBM PC XT POST (no error message, single short beep).
  2. Keyboard adapter card initialises and exposes its video BIOS.
  3. 3270 PC Display Adapter video appears.
  4. IBM PC DOS 2.0 or 2.1 boots.
  5. AUTOEXEC.BAT loads the 3270 PC Control Program.
  6. Control Program initialises:
3278/79 Emulation Adapter — connects to 3174/3274 cluster controller.
APA / PSS cards (where fitted).
Multi-session windowed display on the 5272 monitor.
  1. Control Program presents the user with 4 mainframe sessions + 1 DOS task + 2 notepads in a window layout.

If any of these does not happen, stop and diagnose at that stage.

Stage 1 — No Display at All

  • Keyboard adapter card unseated — this is the most common cause of "3270 PC won't display anything" because the keyboard adapter card holds the video BIOS for the Display Adapter. Reseat it.[3]
  • PSU rails — probe at the planar power connector.
  • Standard XT chassis diagnosis applies (1xx planar code if planar fault).

Stage 2 — POST 302 "Keyboard Error"

The 3270 PC POST requires the 122-key 5271 Keyboard Element (or, on P-models, the 101-key Enhanced AT-style). If a plain XT 83-key keyboard is connected via the keyboard adapter dongle, POST will display 302 and prompt to press F1 to continue.

This is normal behaviour, not a fault — but if you want to silence the 302, use a 122-key keyboard.

POST Error Code Ranges

The 3270 PC inherits the standard IBM PC XT POST codes (1xx–19xx) and adds two ranges specific to the 3270 hardware:

28xx — 3278/79 Host Connect Adapter

3270 PC 28xx POST codes (3278/79 Emulation Adapter)
Code Meaning
2801 Host Connect card initialisation failed
2802 Host Connect card power-on diagnostic failed
2803 Buffer test failed
2804 ROM checksum bad
2805 I/O register test failed
2806 Status-bit test failed
2807 Interrupt test failed
2808–2818 Various sub-tests within the adapter

32xx — Display Adapter Subsystem

3270 PC 32xx POST codes (Display Adapter / PSS / APA)
Code range Subsystem Meaning
3201–3250 3270 PC Display Adapter Sub-tests of the SCN2672 PVTC and framebuffer
3201 Graphics card fault Display Adapter not detected or not responding
3202 Programmed Symbols card fault PSS card (where fitted) failed
3203 APA card fault APA card (where fitted) failed
3261–3279 PSS card Sub-tests for Programmed Symbol Set RAM and font load
3280–3289 APA card Sub-tests for All Points Addressable graphics

3xx — Keyboard

  • 301 — keyboard cable disconnected or stuck key.
  • 302 — wrong keyboard (non-3270 keyboard connected via the keyboard adapter dongle). Press F1 to continue, or fit a 122-key 5271 Keyboard Element.

Display Diagnosis

  • No display, no beeps — keyboard adapter card unseated (this card holds the video BIOS), or PSU dead.
  • Single beep, no display — Display Adapter card not detected. POST will produce a 3201 code on the speaker or alternate display device.
  • Garbled colour display — IBM 5272 timing mismatch. Verify pin 4 of the DE-9 is correctly asserted; verify hsync 24 kHz, vsync 63 Hz.
  • Display shows only B&W on a colour monitor — pin 4 not asserted; 5272 is in mono mode.
  • Display works in DOS but not in 3270 sessions — Control Program not loaded, or Display Adapter / Keyboard Adapter card combination is wrong.

Host Connection Diagnosis

The 3270 PC connects to an IBM 3174 or 3274 cluster controller via BNC twinax. Common faults:

  • 2801 / 2802 at POST — 3278/79 Emulation Adapter card. Reseat or replace.
  • POST passes but Control Program reports "host not responding" — cable, terminator, or controller-side configuration.
  • Host responds but the workstation cannot log on — controller-side LU not defined as a 3278 / 3279, or VTAM application not bound to the LU.
  • Intermittent host disconnection — coax cable termination, or interference. Verify the cable run is within the 3270 family distance limits (typically 5000 ft for RG-62).

Control Program Diagnosis

The 3270 PC Control Program is unusual software for its era. Common failures:

  • Control Program will not load — verify Control Program version matches the model. v3.0 is required for Models 31/51/71 and P-models.
  • "Memory hog" issue — Control Program consumes ~200 KB resident. PC Magazine (Jan 1985) explicitly notes that this leaves limited free RAM for the single DOS session. If you need more RAM for a DOS session, reduce the number of mainframe sessions configured.
  • DOS application incompatibility — QuickBasic and some other DOS applications are documented as incompatible with the Control Program's video / keyboard hooks. Run the DOS application directly under PC DOS without loading the Control Program.
  • Mainframe sessions limited to 24 lines instead of 32 — this is a Control Program limitation, not a fault. The IBM 3279 native terminal supported 32-line mode; the 3270 PC Control Program does not.

APA / PSS Diagnosis

  • APA card present but graphics applications crash — verify APA card is at the expected I/O address (typically a different address from the Display Adapter); reseat.
  • PSS-loaded font not appearing — PSS RAM not loaded by Control Program. Run the appropriate font-load utility.
  • 3270 graphics application reports "no PSS card" — PSS card not detected; reseat or replace.

Diagnostic Diskettes

The 3270 PC ships with diagnostic diskettes specific to the configuration:

  • INDLTDUK.DGS — display subsystem diagnostic.
  • INDSFOMB.COM — 3270 client driver diagnostic.

These are run from DOS, not from the Control Program.

Common Field Symptoms

  • No display at all — keyboard adapter card unseated (very common; this card holds the Display Adapter's video BIOS).
  • POST 302 — keyboard is not a 122-key 5271 Keyboard Element; press F1 to continue.
  • POST 2801 — 3278/79 Emulation Adapter card failure. Reseat or replace.
  • POST 3201 — Display Adapter card failure.
  • Boots OK but no host connection — BNC cable, terminator, or controller-side LU configuration.
  • DOS sessions run too slowly — Control Program's ~200 KB resident overhead leaves limited RAM. Reduce active sessions or upgrade to an XMA-equipped Model 31/51/71.
  • Random reboots when warm — PSU caps aged. Recap; see IBM 3270 PC Capacitor Replacement Guide.
  • Smell of fish from PSU — RIFA X2 mains-suppression cap venting. Power off immediately.
  • 3295 plasma display shows pixel-rot patterns — plasma panel end-of-life; replacement requires a donor.

Diagnostic Workflow Summary

  1. Power on; observe POST.
  2. If no display, reseat keyboard adapter card first (it holds the video BIOS).
  3. If 1xx–19xx code, treat as standard XT chassis fault.
  4. If 28xx code, treat as 3278/79 Emulation Adapter fault.
  5. If 32xx code, treat as Display Adapter / APA / PSS fault.
  6. If POST passes but Control Program fails, verify CP version against model.
  7. If Control Program loads but host connection fails, check the BNC cable and the 3174/3274 controller.

⚠️ Power-supply RIFA capacitor and tantalum shorts

Two age-related failures are near-universal on this era of IBM hardware:

  • RIFA mains-filter capacitors in the power supply are metallised-paper parts that crack and fail short with age, producing acrid smoke shortly after power-on. Replace them pre-emptively with modern X2-class parts.[4]
  • Tantalum capacitors on the planar (system board) and on ISA cards fail short with age. A shorted tantalum will prevent the power supply from starting (dead machine, PSU protection latched) — look for a cracked or discoloured tantalum and lift suspect ones to find the short.[4]

IBM PC/XT switching supplies also need a minimum load to start, so a bare supply on the bench may not run without a dummy load.[4]

POST beep and error codes

The IBM Power-On Self Test signals faults by beeps and, where a display works, by a numeric code:

 
IBM 3270 PC. Source: Wikimedia Commons.
IBM POST beep codes
Beeps Meaning
1 short Normal POST — system OK
2 short POST error (numeric code shown on screen)
No beep Power supply or system-board fault
Continuous / repeating short Power supply or system board
1 long, 1 short System-board fault
1 long, 2 short Display-adapter fault (MDA/CGA)
1 long, 3 short Display-adapter fault (EGA/later)

Common numeric codes include 161/163 (dead CMOS battery/clock), 201 (memory), 301 (keyboard) and 1701 (hard disc). A code ending in the family prefix identifies the failing subsystem.[5]

References

  1. ↑ http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf
  2. ↑ https://www.stanislavs.org/helppc/diagnostic_codes.html
  3. ↑ http://www.seasip.info/VintagePC/5271.html
  4. ↑ 4.0 4.1 4.2 minuszerodegrees.net — IBM failure symptoms; Repairing and Restoring an IBM XT; and Adam's Vintage Computer Restorations. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.
  5. ↑ Standard Original IBM POST Error Codes; and IBM — POST beep errors. Source for the IBM POST beep and numeric error codes.

References


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