IBM FDD Adapter
| IBM 5-1/4" Diskette Drive Adapter | |
|---|---|
 |
|
| IBM 5-1/4-inch Diskette Drive Adapter | |
| Manufacturer | IBM |
| Type | 8-bit expansion-slot diskette controller |
| Release date | 1981 (with the original IBM PC) |
| Interface | 8-bit IBM PC I/O channel |
| Compatible | IBM PC (5150), IBM PC XT (5160) |
| Model | IBM publication 6361505 |
| Connectivity | 34-pin internal edge connector P2 (two drives); 37-pin D-shell J1 on the rear bracket (two more) |
The IBM 5-1/4" Diskette Drive Adapter is the diskette controller card for the IBM PC (5150) and IBM PC XT (5160). Neither machine has a diskette controller on the motherboard, so a PC with diskette drives uses one expansion slot for this card, and only one can be fitted.[1]
The adapter supports four drives. Two connect inside the system unit on a daisy-chained flat cable, and the connector through the rear panel carries the signals for two external drives.[2]
Hardware
[edit | edit source]IBM's description:[2]
- NEC µPD765 or equivalent floppy disk controller, so drive parameters are programmable.
- Designed for double-density, MFM-coded drives, with write precompensation and an analogue phase-locked loop for clock and data recovery.
- Supports the drive's write-protect feature.
- Buffered on the I/O bus. Uses the system board's DMA for data transfers and interrupt IRQ6 to signal completion and status.
- Registers: digital output register at hex 3F2 (motor enables, drive selects, controller reset, and enabling of interrupt and DMA requests); µPD765 main status register at 3F4 and data register at 3F5.
The later version of the card has a jumper on P3 that is for factory use only and connects the adjacent 16 MHz oscillator; leave it fitted. The early card has no jumper there.[1] The external connector J1 does not supply power to external drives.[3]
Cable and termination
[edit | edit source]The internal connector is a keyed 34-pin edge connector. Odd-numbered lands (1 to 33) are on the back of the board and are all ground; even-numbered lands are on the component side. All signals are TTL.[2]
| Land | Signal | Land | Signal |
|---|---|---|---|
| 8 | Index | 22 | Write Data |
| 10 | Motor Enable A | 24 | Write Enable |
| 12 | Drive Select B | 26 | Track 0 |
| 14 | Drive Select A | 28 | Write Protect |
| 16 | Motor Enable B | 30 | Read Data |
| 18 | Direction (stepper motor) | 32 | Select Head 1 |
| 20 | Step Pulse | 2, 4, 6, 34 | Unused |
The IBM controller expects the floppy-type twisted cable, which swaps both the motor-enable and the drive-select lines between the two drive connectors. A hard-drive type twisted cable has its twist in a different place and will not work.[4] With the IBM cable, both drives are jumpered for the second drive select (DS1 on a drive numbered DS0 to DS3). The drive on the connector beyond the twist, at the end of the cable, becomes A:; the drive on the middle connector becomes B:.[3]
IBM's interface specification says the adapter's outputs are open-collector, the drives must provide termination networks to Vcc (except on motor enable, which has a 2,000 Ω resistor to Vcc on the adapter), and each adapter input is terminated with a 150 Ω resistor to Vcc.[2] In practice the 5.25-inch drive at the end of the cable keeps its terminator and a 5.25-inch drive in the middle position has it removed or disabled. A 3.5-inch drive can go in either position; if it is at the end, the 5.25-inch drive in the middle stays unterminated.[3]
External drives on J1 are cabled and terminated the same way. With DOS 4 or earlier, the first external 360K drive appears as C: and the second as D:.[3] Set the floppy drive count switches on the motherboard to match the number of drives.[3]
Drive and diskette support
[edit | edit source]Support with the IBM controller and the 5150 BIOS:[4][1]
| Drive | Controller | 5150 BIOS (all three revisions) | DOS |
|---|---|---|---|
| 5.25-inch 160/180 KB single-sided | Yes | Full support | 1.0 for 160 KB; 2.0 for 180 KB |
| 5.25-inch 320/360 KB double-sided | Yes | Full support | 1.1 for 320 KB; 2.0 for 360 KB |
| 5.25-inch 1.2 MB | No | No | 3.0 |
| 3.5-inch 720 KB | Yes | Partial: read and write 720 KB diskettes but not format them | 3.2 |
| 3.5-inch 1.44 MB | No (no 500 kbit/s data rate) | No | 3.3 |
A 1.44 MB drive on this card can read and write 720 KB diskettes, but not format them without extra software.[4]
Faults
[edit | edit source]Before suspecting the card or a drive, check the cabling points above: the floppy-type twisted cable (not a straight or hard-drive cable), termination only at the end of the cable, both drives jumpered for the second drive select, and the motherboard drive-count switches.[4][3] For the drives themselves see Tandon TM100-1, Tandon TM100-2 and Floppy Drive Repair.
References
[edit | edit source]- ↑ 1.0 1.1 1.2 IBM 5150/5160: Some IBM Expansion Cards and Options, minuszerodegrees.net, "IBM 5.25" Diskette Drive Adapter" section.
- ↑ 2.0 2.1 2.2 2.3 IBM, IBM 5-1/4" Diskette Drive Adapter (6361505), Options and Adapters, Personal Computer Hardware Reference Library: description (p. 1), programming considerations and I/O addresses (pp. 3 to 17), drive interface signal levels and termination (pp. 20 to 22), and connector specifications (p. 23 onwards). Hosted here as File:IBM Options and Adapters - IBM 5.25-inch Diskette Drive Adapter.pdf and described at IBM Options and Adapters - IBM 5.25-inch Diskette Drive Adapter.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 IBM 5.25" Diskette Drive Adapter: Cabling Information, minuszerodegrees.net, with its cabling and termination diagrams.
- ↑ 4.0 4.1 4.2 4.3 IBM 5150: Floppy Drive Support, minuszerodegrees.net: cable and termination, and support for each drive type by the IBM controller and the three 5150 BIOS revisions.
Related pages
[edit | edit source]- IBM PC (5150) · IBM PC XT (5160)
- Tandon TM100-1 · Tandon TM100-2
- IBM Options and Adapters - IBM 5.25-inch Diskette Drive Adapter
- Floppy Drive Repair · IBM PC (5150) Troubleshooting Guide