Editing IBM Fixed Disk Adapter

{{Infobox computer peripheral
| name          = IBM Fixed Disk Adapter
| manufacturer  = IBM (manufactured under contract by Xebec)
| type          = 8-bit ISA MFM hard disk controller card
| release_date  = March 1983 (with [[IBM PC XT (5160)|PC XT]])
| interface     = 8-bit ISA; ST-506 / ST-412 to drive
| compatible    = [[IBM PC XT (5160)]]; [[IBM PC (5150)]] with 130 W Type 2 PSU
| connectivity  = 34-pin control ribbon (daisy-chained); 20-pin data ribbon (one per drive)
}}

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 [[IBM PC (5150)|5150]]s fitted with the 130 W Type 2 power supply. It supports MFM-encoded 5.25" full-height hard disk drives — typically a 10 MB Seagate ST-412 or a 20 MB ST-225 — through a separate data ribbon and a per-drive control ribbon.

According to minuszerodegrees.net, there were three documented variants of the card. The earliest variant dates to circa 1983; another variant dates to circa 1984; a third variant from circa 1986 added support for 20 MB drives fitted to later 5160s.

== Hardware ==

* '''Xebec OEM design''' — branded "IBM" but manufactured under contract by Xebec.
* '''ST-506 / ST-412 interface''' — MFM encoding at 5 Mbit/s.
* '''BIOS expansion ROM at address C8000''' — contains a drive table for IBM-supported drive types.
* '''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 a BIOS expansion ROM at C8000. The 5150's POST scans for valid BIOS ROMs and runs any whose first three bytes match the AA 55 ''length'' signature. If the C8000 ROM is corrupted (except for its first two bytes), the [[IBM PC (5150)#Memory and ROM|10/27/82 BIOS]] beeps the "1 long + 2 short" 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.

== 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. Modern replacements (XT-IDE Universal BIOS, ZuluSCSI / MFM emulator) provide alternatives that bypass the original BIOS limitations.

== References ==

* [https://minuszerodegrees.net/ibm_xebec/ibm_xebec.htm IBM/Xebec Fixed Disk Adapter], minuszerodegrees.net.
* [https://archive.org/details/ibm-fixed-drive-adapter-xebec-mfm-bios-rom-source IBM Personal Computer 20MB Fixed Disk Drive Adapter + ROM BIOS Source], Internet Archive.

== Related Pages ==

* [[IBM PC (5150)]]
* [[IBM PC XT (5160)]]
* [[IBM PC (5150) Troubleshooting Guide]]

[[Category:IBM]]