IBM PC AT Troubleshooting Guide
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
[edit | edit source]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.
| Pattern | Meaning | Stage |
|---|---|---|
| (no beep) | Power supply or motherboard not running — 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–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
[edit | edit source]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 — System board
[edit | edit source]| 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 — battery failure or freshly fitted board |
| 162 | CMOS checksum error (adapter ID mismatch) — run SETUP |
| 163 | Time and date not set — run SETUP and re-enter |
| 164 | Memory size error — CMOS does not match installed RAM |
| 166 | Adapter card system board error |
| 181 | Bad CMOS configuration |
| 199 | User-indicated configuration not correct |
2xx — Memory
[edit | edit source]| Code | Meaning |
|---|---|
| 201 | Memory test failed — 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 — Keyboard
[edit | edit source]| Code | Meaning |
|---|---|
| 301 | Keyboard error (or stuck key) — 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 — CMOS does not match keyboard |
4xx / 5xx — Display adapters
[edit | edit source]| 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 — Floppy drive / controller
[edit | edit source]| 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 — Hard drive
[edit | edit source]| Code | Meaning |
|---|---|
| 1701 | Fixed disk POST error — 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
[edit | edit source]- 7xx — Math coprocessor (80287) error
- 9xx — Parallel printer adapter error
- 10xx — Reserved (parallel adapter)
- 11xx / 12xx — Asynchronous communications (16450 UART) adapter errors
- 13xx — Game adapter error
- 14xx — Printer error
- 15xx — SDLC adapter error
- 18xx — I/O expansion unit error
Known Problems
[edit | edit source]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
[edit | edit source]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 — CMOS battery failure
[edit | edit source]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
[edit | edit source]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 — 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
[edit | edit source]The 192 W AT PSU requires a minimum load on +12 V to start. Diskette-only ATs shipped with a 5 ฮฉ 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 ฮฉ across the +12 V/GND pins, fan off).
Floppy drive incompatibility
[edit | edit source]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)
[edit | edit source]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 — the 201 code (if present) will identify the failing address.
Slot keying and AT bus problems
[edit | edit source]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
[edit | edit source]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
[edit | edit source]- 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 — this is a stop-gap, replace the drive afterwards.
Math Coprocessor (80287)
[edit | edit source]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.
Related Pages
[edit | edit source]- IBM PC AT (5170)
- IBM PC AT Maintenance Guide
- IBM PC AT Capacitor Replacement Guide
- IBM PC XT Troubleshooting Guide — preceding model
- IBM PC (5150) Troubleshooting Guide — preceding model
References
[edit | edit source]- IBM 5170 — POST checkpoint codes, minuszerodegrees.net.
- IBM 5170 — BIOS Revisions, minuszerodegrees.net (601 error notes for the 06/10/85 and 11/15/85 BIOS).
- HelpPC — Diagnostic Codes. Reference for the 1xx–18xx numeric error code meanings.
- IBM, Personal Computer AT — Hardware Maintenance and Service (IBM service manual; multiple revisions 1984–1987).
- IBM, Personal Computer AT — Technical Reference (1502243, March 1984; 6280070, September 1985).