IBM 3270 PC Troubleshooting Guide - Revision history

Josh: Add representative photo (Wikimedia Commons, attributed on file page)

2026-07-16T12:00:28Z

Add representative photo (Wikimedia Commons, attributed on file page)

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Josh: Expand troubleshooting: RIFA PSU cap + tantalum shorts (+ POST beep codes); cited (minuszerodegrees/classic-computers)

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Expand troubleshooting: RIFA PSU cap + tantalum shorts (+ POST beep codes); cited (minuszerodegrees/classic-computers)

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 # If POST passes but Control Program fails, verify CP version against model.
 # If Control Program loads but host connection fails, check the BNC cable and the 3174/3274 controller.
 == Related Pages ==

Josh: Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

2026-05-23T21:32:12Z

Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

New page

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.<ref>http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf</ref>
* '''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.<ref>https://www.stanislavs.org/helppc/diagnostic_codes.html</ref>

== Initial Diagnosis Workflow ==

A working 3270 PC boots:

# Standard IBM PC XT POST (no error message, single short beep).
# Keyboard adapter card initialises and exposes its video BIOS.
# 3270 PC Display Adapter video appears.
# IBM PC DOS 2.0 or 2.1 boots.
# AUTOEXEC.BAT loads the 3270 PC Control Program.
# 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.
# 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.<ref>http://www.seasip.info/VintagePC/5271.html</ref>
* 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 ===

{| class="wikitable styled-table" style="width:100%;"
|+'''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 ===

{| class="wikitable styled-table" style="width:100%;"
|+'''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 ==

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

== Related Pages ==

* [[IBM 3270 PC]]
* [[IBM 3270 PC Maintenance Guide]]
* [[IBM 3270 PC Capacitor Replacement Guide]]
* [[IBM PC XT (5160) Troubleshooting Guide]] — chassis-level codes
* [[IBM 3270 PC/G]] — graphics variant (similar POST and Control Program codes)
* [[IBM 3270 PC/GX]] — extended-graphics variant

== References ==

* [http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf IBM SA38-0037-00 (July 1989)], Chapter 10.
* [http://bitsavers.trailing-edge.com/pdf/ibm/pc/cards/1502336_PC_3278_79_Emulation_Adapter_Technical_Adapter_Oct83.pdf IBM 1502336 — PC 3278/79 Emulation Adapter Technical Reference (October 1983)]. 28xx code definitions.
* [https://www.stanislavs.org/helppc/diagnostic_codes.html HelpPC — IBM PC diagnostic codes].
* [http://www.seasip.info/VintagePC/5271.html John Elliott, seasip.info — IBM 5271 (3270 PC)].

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