Editing IBM PC AT Troubleshooting Guide (section)

== 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.