Editing IBM PC AT Troubleshooting Guide

[[File:IBM PC AT (photo).jpg|thumb|right|300px|IBM PC AT. Source: Wikimedia Commons.]]
This guide covers diagnostic procedures for the IBM PC AT (5170), including POST audio and numeric error codes, hard drive faults, the well-documented 601 / CMOS battery / CMI HDD issues, and other known AT-specific problems.

== Power-On Audio Beep Codes ==

Beep codes on the 5170 are produced by the BIOS through the PC speaker before video is initialised. Same coding scheme as on the 5150 and 5160.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5170 power-on audio codes'''
! Pattern !! Meaning !! Stage
|-
| (no beep) || Power supply or motherboard not running &mdash; check 5 V/12 V, PSU minimum-load resistor, bank-0 RAM, CPU socket, system clock || Before POST
|-
| 1 short || POST complete, system OK || End of POST
|-
| 2 short || Numeric error code displayed; read the screen for the 1xx&ndash;1Fxx code || Any
|-
| 1 long, 1 short || Motherboard speed test failed (Type 3 BIOS, motherboard speed outside expected window) || POST checkpoint
|-
| 1 long, 2 short || Display adapter failure (MDA/CGA/EGA) || POST
|-
| 1 long, 3 short || Enhanced graphics adapter (EGA) memory failure || POST
|-
| 3 long || 3270 keyboard test failure (variant boards only) || POST
|-
| Continuous beep || Power supply problem, motherboard problem, or stuck speaker || Any
|-
| Repeated short beeps || Power supply problem, often a shorted tantalum on +12 V or +5 V || Any
|}

== Numeric POST Error Codes ==

If the BIOS detects a fault during POST after video has initialised, it displays a numeric code on the screen and (usually) halts or prompts F1 to continue. The codes below are the IBM-specified set for the 5170 with original IBM BIOS; third-party BIOS replacements may use different codes for some classes.

=== 1xx &mdash; System board ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 1xx system board codes'''
! Code !! Meaning
|-
| 101 || Interrupt failure (8259A PIC)
|-
| 102 || Timer failure (8254 PIT channel 0)
|-
| 103 || Timer interrupt failure
|-
| 104 || Protected mode failure (80286 internal)
|-
| 105 || Last 8042 (keyboard controller) command not accepted
|-
| 106 || Converting logic test (post-protected-mode return)
|-
| 107 || Hot NMI test
|-
| 108 || Timer bus test
|-
| 109 || DMA test error (8237A)
|-
| 110 || System board parity error
|-
| 111 / 112 / 113 || Adapter card parity / I/O channel check
|-
| 121 || Unexpected hardware interrupt occurred
|-
| 129 || Cache error (later board steppings with cache support; unsupported CPU upgrade)
|-
| 131 || Cassette wrap test failed (legacy code, no cassette port on the AT)
|-
| 161 || CMOS configuration empty &mdash; '''battery failure''' or freshly fitted board
|-
| 162 || CMOS checksum error (adapter ID mismatch) &mdash; run SETUP
|-
| 163 || Time and date not set &mdash; run SETUP and re-enter
|-
| 164 || Memory size error &mdash; CMOS does not match installed RAM
|-
| 166 || Adapter card system board error
|-
| 181 || Bad CMOS configuration
|-
| 199 || User-indicated configuration not correct
|}

=== 2xx &mdash; Memory ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 2xx memory codes'''
! Code !! Meaning
|-
| 201 || Memory test failed &mdash; the four hex digits to the left of "201" identify the failing byte's segment, the digit pair to the right identifies the failing bit position in that byte
|-
| 202 / 203 || Memory address line failure
|-
| 215 || 64 KB memory module failure on a 16-bit memory expansion card
|}

=== 3xx &mdash; Keyboard ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 3xx keyboard codes'''
! Code !! Meaning
|-
| 301 || Keyboard error (or stuck key) &mdash; the two hex digits before "301" are the scan code of the stuck key
|-
| 302 || Keyboard locked (5170 keylock is in the locked position) or keyboard test failure
|-
| 303 || Keyboard or 8042 keyboard controller error
|-
| 304 || Keyboard or 8042 keyboard controller error &mdash; CMOS does not match keyboard
|}

=== 4xx / 5xx &mdash; Display adapters ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 4xx / 5xx display codes'''
! Code !! Meaning
|-
| 401 || Monochrome display adapter (MDA) failed
|-
| 408 || MDA display attribute failure
|-
| 432 || Parallel printer port test failed (on the MDA "monochrome and printer adapter")
|-
| 501 || Colour Graphics Adapter (CGA) failed
|-
| 508 || CGA display attribute failure
|}

=== 6xx &mdash; Floppy drive / controller ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 6xx floppy codes'''
! Code !! Meaning
|-
| 601 || Diskette or controller adapter error (see "601 quirk" below)
|-
| 602 || Diskette boot record failure
|-
| 606 || Diskette verify error
|-
| 607 || Diskette write-protected
|-
| 608 || Bad diskette command
|-
| 611 || Time out (controller did not respond)
|-
| 612 / 613 || Bad NEC controller chip / DMA error
|}

=== 17xx &mdash; Hard drive ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''5170 17xx hard drive codes'''
! Code !! Meaning
|-
| 1701 || Fixed disk POST error &mdash; controller initialisation failed, or no drive ready
|-
| 1702 || Fixed disk adapter error
|-
| 1703 || Drive error
|-
| 1704 || Adapter or drive error
|-
| 1780 || Fixed disk 0 failure
|-
| 1781 || Fixed disk 1 failure
|}

=== Other ===
* '''7xx''' &mdash; Math coprocessor (80287) error
* '''9xx''' &mdash; Parallel printer adapter error
* '''10xx''' &mdash; Reserved (parallel adapter)
* '''11xx / 12xx''' &mdash; Asynchronous communications (16450 UART) adapter errors
* '''13xx''' &mdash; Game adapter error
* '''14xx''' &mdash; Printer error
* '''15xx''' &mdash; SDLC adapter error
* '''18xx''' &mdash; I/O expansion unit error

== Known Problems ==

The following AT-specific symptoms are documented at minuszerodegrees.net and confirmed by other long-term restorers.

=== 601 error with 06/10/85 or 11/15/85 BIOS ===

With either of the 1985-dated BIOS revisions, removing the IBM Fixed Disk and Diskette Drive Adapter and substituting a third-party replacement '''can''' (not always) result in a 601 error at POST. There is something in those revisions that expects the IBM "combo" card. The error does not stop the boot; an F1 prompt allows boot to continue, although floppy operation may be compromised.

Workarounds:
* Use a replacement controller that the IBM BIOS recognises as the "combo" card: confirmed working replacements include the Western Digital '''WD1003A-WA2''', '''WD1002-WA2''' and '''WD1003-WA2'''.
* Use a patched IBM 5170 BIOS that disables the combo-card check.
* Use a non-IBM BIOS (AMI, Award, Phoenix). This is the cleanest workaround if 1.44 MB diskette boot support is desired.

The 01/10/84 BIOS does '''not''' produce 601 errors when the combo card is missing; this check was added in the second BIOS revision.

=== 161 / 162 / 163 errors &mdash; CMOS battery failure ===

These three errors usually appear together when the external 6 V CMOS battery has run down. The MC146818 loses the CMOS configuration and the date/time. Replace the battery (see [[IBM PC AT Maintenance Guide]]) and re-run SETUP.

If 161/162/163 persist with a fresh battery, suspect a failed MC146818, a dirty or oxidised battery header, or a shorted CMOS line on the motherboard.

=== CMI 5616 / CMI 6426 hard drive failure ===

The 20 MB CMI 5616 drive (early 5170 models) and 30 MB CMI 6426 drive (later models) suffered well-documented reliability problems, leading to a PC Magazine article opening with the line "If you own an IBM PC AT and your hard disk hasn't crashed yet, don't worry &mdash; it probably will." If the drive does not spin up, makes a continuous click, or returns 1701/1780 errors, the drive itself is most likely faulty. Many enthusiasts retire the CMI drives in favour of a Seagate ST-225 (20 MB) or ST-251 (40 MB), or an MFM emulator such as XT-IDE / GoTek.

=== PSU will not start with diskette-only configuration ===

The 192 W AT PSU requires a minimum load on +12 V to start. Diskette-only ATs shipped with a 5 Ω 50 W resistor on the spare hard-drive power connector. If the resistor has been removed or has failed open, the PSU may not start. Verify the resistor is present and resistive (about 5 Ω across the +12 V/GND pins, fan off).

=== Floppy drive incompatibility ===

A 360 KB diskette written in the 1.2 MB drive cannot be reliably re-read in a 360 KB drive. The head track width of the 1.2 MB drive is half that of the 360 KB drive; the wider 360 KB read head picks up the half-track plus the remnants of any earlier track, producing garbled data. Use HD media in HD drives only.

=== Memory size error (164) ===

A 164 error means the BIOS POST detected an installed memory size that does not match the CMOS configuration. Run SETUP and re-enter the actual installed RAM size. If the size cannot be set correctly, suspect a failed RAM chip on the motherboard or memory expansion card &mdash; the 201 code (if present) will identify the failing address.

=== Slot keying and AT bus problems ===

Some XT-era 8-bit cards have a long tab that physically blocks the 16-bit AT extension. Cards designed for the AT are dual-tab: an 8-bit portion plus a shorter 16-bit extension. The slot itself does not damage incompatible cards mechanically, but the card will not seat fully and the system will refuse to boot.

== No-Beep / No-Video Diagnostics ==

If the AT shows no signs of life:
# Verify the PSU fan is turning. If not, check the wall power, the PSU mains lead, and the front-panel power switch.
# With the PSU running, check +5 V at a free drive power connector. If absent, the PSU is faulty or one of its rails is shorted (likely a tantalum capacitor on the motherboard or an ISA card).
# Pull all ISA cards except the video card. Re-test.
# If still dead, swap in a known-good video card.
# If still dead, reseat the CPU, the math coprocessor (if fitted), and the BIOS ROMs.
# If still dead, suspect bank-0 RAM on the motherboard. The AT motherboard requires bank-0 RAM populated to POST.

== Hard Drive Diagnostics ==

* 1701: Run SETUP and confirm the drive type matches the drive's label. The 5170 SETUP table has 15 drive types in the 01/10/84 BIOS, 22 in the 06/10/85 BIOS, and over 40 in the 11/15/85 BIOS.
* 1780/1781: Drive 0 or Drive 1 failure. Verify cable seating (pin 1 marked with a red stripe), drive power, drive ID jumpers (drive 0 vs drive 1), and that the drive is correctly terminated.
* Drive does not spin up: head stiction. Carefully twist the drive (with power off) to free the heads &mdash; this is a stop-gap, replace the drive afterwards.

== Math Coprocessor (80287) ==

The 5170 socket accepts the '''Intel 80287''' running at a divided clock. POST does not test the 80287 unless software explicitly invokes it; a missing or failed 80287 is reported as a 7xx error only by application software. Use IBM AT Advanced Diagnostics to test the 80287.

== ⚠️ 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.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ 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.</ref>
* '''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) &mdash; look for a cracked or discoloured tantalum and lift suspect ones to find the short.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ 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.</ref>

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.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ 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.</ref>

== ⚠️ CMOS / RTC battery ==

This machine keeps its configuration in battery-backed CMOS, and the battery is a common failure. On AT-class boards the clock/CMOS is often a '''Dallas DS1287/DS12887''' module with the cell sealed inside; it lasts about ten years and then dies, giving '''161 / 163''' CMOS and clock errors at POST (and sometimes spurious floppy-drive errors). PS/2 planars use a rechargeable barrel or pack battery that '''leaks''' and corrodes the board. Replace a dead Dallas module (or rework it with an external coin cell), and on a leaking planar battery remove it and clean the corrosion before it eats the traces.<ref name="ibm_batt">[https://www.classic-computers.org.nz/blog/2009-10-10-renovating-a-dallas-battery-chip.htm Fixing a Flat Dallas DS1287 RTC], Classic Computers; and [https://www.ardent-tool.com/misc/Dallas_Rework.html Reworking Dallas RTC Modules], Ardent Tool. Source for the Dallas DS1287/DS12887 internal-battery death (161/163 CMOS errors) and the leaking planar battery.</ref>

== References ==

<references />

== Related Pages ==

* [[IBM PC AT (5170)]]
* [[IBM PC AT Maintenance Guide]]
* [[IBM PC AT Capacitor Replacement Guide]]
* [[IBM PC XT Troubleshooting Guide]] &mdash; preceding model
* [[IBM PC (5150) Troubleshooting Guide]] &mdash; preceding model

== References ==

* [https://minuszerodegrees.net/5170/post_errors/5170_post_codes.htm IBM 5170 &mdash; POST checkpoint codes], minuszerodegrees.net.
* [https://minuszerodegrees.net/5170/bios/5170_bios_revisions.htm IBM 5170 &mdash; BIOS Revisions], minuszerodegrees.net (601 error notes for the 06/10/85 and 11/15/85 BIOS).
* [https://helppc.netcore2k.net/hardware/diagnostic-codes HelpPC &mdash; Diagnostic Codes]. Reference for the 1xx&ndash;18xx numeric error code meanings.
* IBM, ''Personal Computer AT &mdash; Hardware Maintenance and Service'' (IBM service manual; multiple revisions 1984&ndash;1987).
* IBM, ''Personal Computer AT &mdash; Technical Reference'' (1502243, March 1984; 6280070, September 1985).

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