25 PIN D-SUB FEMALE at the Computer
The MSX specification never defined a standard external floppy connector; each manufacturer chose its own interface. This 25-pin D-sub female socket was used by SpectraVideo on its SVI-series MSX computers (mid-1980s) and carries a subset of Shugart-derived double-density MFM floppy signals together with +5 V and +12 V power on the same connector. Internally, most MSX machines used a standard Shugart-compatible 34-pin flat-cable header; the SpectraVideo external variant adapts those signals to the 25-pin D-sub, omitting the Index-hole signal path used on Shugart pin 8 and combining multiple power pins. Direction conventions are Computer-relative-to-Drive throughout.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | +12V | OUT | +12 VDC |
| 2 | +5V | OUT | +5 VDC |
| 3 | +5V | OUT | +5 VDC |
| 4 | /INDEX | IN | Sector hole passed sensor. |
| 5 | /DSEL1 | OUT | Drive Select 1 |
| 6 | DIR | OUT | Direction (0=In, 1=Dir) |
| 7 | /STEP | OUT | Moves head 1 step in DIR direction. |
| 8 | WRITEDATA | OUT | Write Data |
| 9 | /WRITEGATE | OUT | Write Gate |
| 10 | /TRACK00 | IN | Head is over Track 00 (outermost track) |
| 11 | /WRITEPROTECT | IN | Write protected disk (0=Write protected) |
| 12 | READDATA | IN | Data read from diskette. |
| 13 | /SIDESELECT | OUT | Side Select (0=Side 1, 1=Side 0) |
| 14 | +12V | OUT | +12 VDC |
| 15 | +12V | OUT | +12 VDC |
| 16 | +5V | OUT | +5 VDC |
| 17 | /DSEL1 | OUT | Select Drive 0 |
| 18 | /MOTOR | OUT | Motor On |
| 19 | READY | IN | Ready |
| 20 | GND | - | Ground |
| 21 | GND | - | Ground |
| 22 | GND | - | Ground |
| 23 | GND | - | Ground |
| 24 | GND | - | Ground |
| 25 | GND | - | Ground |
Signals
- /INDEX — once-per-revolution pulse from the index hole sensor; active low input from drive.
- /DSEL1 — selects drive 1 (second external drive); active low output from computer.
- DIR — step direction control; low = step inward (towards centre), high = step outward.
- /STEP — each active-low pulse moves the read/write head one track in the direction set by DIR.
- WRITEDATA — MFM-encoded write data stream from the computer to the drive.
- /WRITEGATE — enables writing to the disk when asserted low; must be asserted before WRITEDATA is valid.
- /TRACK00 — asserted low by the drive when the head is positioned over track 0 (outermost track).
- /WRITEPROTECT — asserted low by the drive when the inserted disk is write-protected.
- READDATA — MFM-encoded data stream read from the disk surface; active low input to computer.
- /SIDESELECT — selects the recording surface; low = side 1, high = side 0.
- /DSEL0 — selects drive 0 (first external drive); active low output from computer.
- /MOTOR — asserts drive spindle motor on when low.
- READY — asserted by the drive when the motor is up to speed and a disk is present.
- +5V / +12V — logic and spindle/head motor power supplied by the computer to the drive; multiple pins are paralleled to reduce connector resistance.
- GND — signal and power return; pins 20–25 are all ground.
Notes
- Direction is Computer relative Diskdrive.
- The MSX external floppy interface is not part of the MSX1 or MSX2 specification; this 25-pin D-sub variant is specific to SpectraVideo (SVI-series) machines. Other MSX manufacturers used entirely different connectors.
- Philips VG8235 MSX computers used a standard 34-pin internal-style header; the MSX Turbo R used a 24-pin flat cable with power on pins 1–2 and 4–5.
- Signals are Shugart-bus derived and use MFM (Modified Frequency Modulation) double-density encoding at 250 kbits/s for 300 RPM drives.
- Power (+5 V and +12 V) is supplied directly through the D-sub connector; no separate power cable is used.
- Pin 17 name corrected from /DSEL1 to /DSEL0 (Drive Select 0); the description field was already correct.