Pinout diagram
Used in Retro Machine Hubs
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Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | /CS | Control | Input | TTL, active low | Chip Select input. |
| 2 | R/W | Control | Input | TTL | Read/Write input (1 = Read FDC register, 0 = Write FDC register). |
| 3 | A0 | Address Bus | Input | TTL | Register Select bit 0 (00=Command/Status, 01=Track, 10=Sector, 11=Data). |
| 4 | A1 | Address Bus | Input | TTL | Register Select bit 1. |
| 5 | D0 | Data Bus | Bidirectional | TTL | CPU Data bus bit 0. |
| 6 | D1 | Data Bus | Bidirectional | TTL | CPU Data bus bit 1. |
| 7 | D2 | Data Bus | Bidirectional | TTL | CPU Data bus bit 2. |
| 8 | D3 | Data Bus | Bidirectional | TTL | CPU Data bus bit 3. |
| 9 | D4 | Data Bus | Bidirectional | TTL | CPU Data bus bit 4. |
| 10 | D5 | Data Bus | Bidirectional | TTL | CPU Data bus bit 5. |
| 11 | D6 | Data Bus | Bidirectional | TTL | CPU Data bus bit 6. |
| 12 | D7 | Data Bus | Bidirectional | TTL | CPU Data bus bit 7. |
| 13 | /MR (/RESET) | Control | Input | TTL, active low | Master Reset input. |
| 14 | GND | Ground | Power | 0 V | System ground (0 V). |
| 15 | VCC (+5V) | Power | Power | +5 V | +5 V single-rail supply. |
| 16 | STEP | I/O | Output | TTL | Stepper motor pulse output (active high inside FDC, inverted by open-collector buffer to Shugart Pin 20 /STEP). |
| 17 | DIRC | I/O | Output | TTL | Step Direction output (1 = Step In toward Track 79, 0 = Step Out toward Track 00). |
| 18 | CLK | Clock / Timing | Input | TTL 8.0 MHz | 8.000 MHz master clock input (controls all MFM bit cell and motor timeout timing!). |
| 19 | /RD | I/O | Input | TTL, active low | Raw Read Data MFM pulse input from floppy drive Pin 30. |
| 20 | MO | I/O | Output | TTL | Motor On output (automatically turns spindle motor on before commands and times out after 10 index revolutions!). |
| 21 | WG | I/O | Output | TTL | Write Gate output (enables drive erase/write head). |
| 22 | WD | I/O | Output | TTL | Precompensated MFM Write Data pulse stream output to drive Pin 22. |
| 23 | /TR00 | I/O | Input | TTL, active low | Track 00 optical/microswitch sensor input from drive Pin 26. |
| 24 | /IP | I/O | Input | TTL, active low | Index Pulse input from drive Pin 8 (300 RPM = 1 pulse every 200 ms). |
| 25 | /WPRT | I/O | Input | TTL, active low | Write Protect tab sensor input from drive Pin 28. |
| 26 | /DDEN | Control | Input | TTL, active low | Double-Density Enable input (Low = 250 kbps MFM Double Density; High = 125 kbps FM Single Density!). |
| 27 | DRQ | Control | Output | TTL | Data Request output (triggers DMA on Atari ST, or NMI on BBC Micro / ZX Spectrum Disciple). |
| 28 | INTRQ | Control | Output | TTL | Interrupt Request output (asserts when Seek, Read, or Write command completes). |
Common uses and repair diagnostics
The 28-pin Western Digital WD1770 and WD1772 replaced the bulky 40-pin FD1793 / Intel 8271 by integrating both the digital PLL data separator AND the write precompensation circuitry inside a single +5 V 28-pin DIP requiring only an external 8.000 MHz clock! The WD1770 and WD1772 have the EXACT SAME 28-pin pinout and register set — the ONLY difference between them is the faster head-stepping rate table burned into the WD1772 (2, 3, 5, 6 ms for fast 3.5-inch drives in the Atari ST and Commodore 1581) versus the WD1770 (6, 12, 20, 30 ms for 5.25-inch drives in the BBC Micro/Master, Acorn Electron Plus 3, and Commodore 1571).
Used in the Atari 520ST/1040STF/STE (WD1772), Acorn BBC Micro 1770 DFS upgrade & BBC Master 128 (WD1770), Acorn Electron Plus 3 (WD1770), Commodore 1570/1571 (WD1770) & 1581 (WD1772/1770), SAM Coupé (VL1772), and Miles Gordon Technology DISCiPLE / +D interfaces for the ZX Spectrum.
- remember on the Atari ST: the YM2149 sound chip's Port A selects Drive A (/DS0), Drive B (/DS1), and Side 0/1, while the WD1772 handles Step, Direction, Motor On, and MFM data!
- If the floppy head bangs against the outer stop during Restore/Boot, check Pin 23 (/TR00) — a dusty Track 00 optical sensor on the drive prevents Pin 23 from pulling low.
- On the BBC Micro: DFS (ADFS) uses /DDEN (Pin 26) to switch between 8271-compatible Single-Density FM (High) and Double-Density ADFS MFM (Low).