IBM Fixed Disk Adapter
The IBM Fixed Disk Adapter (often called the IBM XT MFM controller) is the 8-bit ISA hard disk controller IBM shipped with the IBM PC XT (5160) and offered as an upgrade for late-production 5150s fitted with the 130 W Type 2 power supply. It supports one or two MFM-encoded 5.25" full-height hard disk drives — typically a 10 MB or 20 MB Seagate ST-225 — through a separate data ribbon and a per-drive control ribbon.
Hardware
- Xebec OEM design — branded "IBM" but manufactured under contract by Xebec.
- ST-506 / ST-412 interface — MFM encoding at 5 Mbit/s.
- Drive types — IBM-supplied BIOS expansion ROM at address C8000 contains a 4-entry drive table. The default type 1 is the 10 MB ST-412; type 2 is the 20 MB ST-225. Tables for other drives (e.g. Tandon TM-262, NEC D5126) are not present in the IBM ROM.
- Two ribbon connectors — 34-pin control (daisy-chained to both drives) and 20-pin data (one cable per drive).
BIOS Expansion ROM
The card carries an 8 KB BIOS expansion ROM at C8000. The 5150's POST scans for valid BIOS ROMs at 2 KB intervals from C0000 to F4000 and runs the initialisation routine in any ROM whose first three bytes match the AA 55 length signature. If the C8000 ROM is corrupted (except for its first two bytes), the 10/27/82 BIOS beeps the 1 long + 2 short "video failure" pattern instead of the expected "C800 ROM" on-screen message — this is a documented BIOS bug.
Common Faults
- "1701" error at POST — the controller cannot communicate with the drive. Suspect: ribbon cable orientation, dead drive, failed controller logic.
- C8000 ROM corruption — see above. Replace the expansion ROM.
- Drive spins but never seeks — stepper motor stuck (lubricate), or stepper driver failed on the drive.
- Random read errors — heads need alignment, or the MFM data separator on the controller has drifted. Recapping the analog board often helps.
Drive Compatibility
The IBM Fixed Disk Adapter will work with most ST-506/412 MFM drives if you have a drive table that matches, but the stock IBM ROM only supports a handful of drive types. The third-party XT-IDE Universal BIOS and modern replacements like the ZuluSCSI / MFM emulator provide modern alternatives that bypass the original BIOS limitations.