StephensDev
External Floppy Adapter
Contents
- External Floppy Adapter
- Problem
- Parts Sourcing
- Build Instructions
- 34-Pin Floppy Connector Pinout
- HDI-30 Connector (Female) Pinout
- Signal Mapping (HDI-30 to Floppy)
- Configuration
- Final Results
- Summary
- Images
IBM P70 HDI-30 to 34-Pin Floppy Adapter
Problem
Many IBM P70 units, including mine, suffer from floppy drive failures. Initially, I replaced the floppy drive with a Gotek emulator to keep the system functional. However, I wanted the ability to use both physical floppy disks and emulated drives, which a single floppy device couldn’t provide.
After researching, I found that the IBM P70’s external storage device port (HDI-30), used by the IBM PS/2 5.25" external diskette drive (model 4869-001), supports 360KB floppy disks. According to the IBM P70 technical reference, this port carries all the standard floppy drive signals.
This inspired me to create an adapter that converts the HDI-30 connector to a standard 34-pin floppy interface, allowing me to connect an external floppy drive easily.
Parts Sourcing
- HDI-30 Connector
- Standard 34-pin Floppy Connector (or DIY with prototype board and header pins)
The hardest part is sourcing the HDI-30 connector since it’s rare and obsolete. However, Apple used HDI-30 for some SCSI laptop connections, so I bought an Apple HDI-30 to SCSI adapter for about $15 on eBay.
Build Instructions
Opening the Apple HDI-30 to SCSI Adapter
I carefully pried open the case by leveraging the metal Centronics connector against the plastic housing with a flathead screwdriver.
- Remove the SCSI connector by desoldering it from the adapter.
- Wire the HDI-30 connector to a 34-pin floppy connector using the signal mappings below.
After wiring the floppy connector
34-Pin Floppy Connector Pinout
| Pin | I/O | Signal | Pin | I/O | Signal |
|---|---|---|---|---|---|
| 1 | Input | 2nd Drive Installed | 2 | Output | High Density Select |
| 3 | Output | +5 Vdc | 4 | N/A | Reserved |
| 5 | N/A | Ground | 6 | Output | +12 Vdc |
| 7 | N/A | Ground | 8 | Input | Index |
| 9 | N/A | Ground | 10 | N/A | Reserved |
| 11 | N/A | Ground | 12 | Output | Drive Select |
| 13 | N/A | Ground | 14 | N/A | Reserved |
| 15 | N/A | Ground | 16 | Output | Motor Enable |
| 17 | N/A | Ground | 18 | Output | Direction |
| 19 | N/A | Ground | 20 | Output | Step |
| 21 | N/A | Ground | 22 | Output | Write Data |
| 23 | N/A | Ground | 24 | Output | Write Enable |
| 25 | N/A | Ground | 26 | Input | Track 0 |
| 27 | N/A | Ground | 28 | Input | Write Protect |
| 29 | N/A | Ground | 30 | Input | Read Data |
| 31 | N/A | Ground | 32 | Output | Side 1 Select |
| 33 | N/A | Ground | 34 | Input | Diskette Change |
HDI-30 Connector (Female) Pinout
| Pin | I/O | Signal | Pin | I/O | Signal |
|---|---|---|---|---|---|
| 1 | N/A | Ground | 2 | Input | 2nd Drive Installed |
| 3 | N/A | Ground | 4 | Output | High Density Select |
| 5 | N/A | Ground | 6 | N/A | Reserved |
| 7 | N/A | Ground | 8 | Input | Index |
| 9 | Output | Motor Enable | 10 | Output | Drive Select |
| 11 | N/A | Reserved | 12 | N/A | Reserved |
| 13 | N/A | Ground | 14 | Output | Direction |
| 15 | N/A | Ground | 16 | Output | Step |
| 17 | N/A | Ground | 18 | Output | Write Data |
| 19 | N/A | Ground | 20 | Output | Write Enable |
| 21 | N/A | Ground | 22 | Input | Track 0 |
| 23 | N/A | Ground | 24 | Input | Write Protect |
| 25 | N/A | Ground | 26 | Input | Read Data |
| 27 | N/A | Ground | 28 | Output | Side 1 Select |
| 29 | N/A | Ground | 30 | Input | Diskette Change |
Signal Mapping (HDI-30 to Floppy)
| HDI-30 Pin | I/O | Signal | Floppy Pin |
|---|---|---|---|
| 2 | Input | 2nd Drive Installed | 1 |
| 4 | Output | High Density Select | 2 |
| 8 | Input | Index | 8 |
| 10 | Output | Drive Select | 12 |
| 14 | Output | Direction | 18 |
| 16 | Output | Step | 20 |
| 18 | Output | Write Data | 22 |
| 20 | Output | Write Enable | 24 |
| 22 | Input | Track 0 | 26 |
| 24 | Input | Write Protect | 28 |
| 26 | Input | Read Data | 30 |
| 28 | Output | Side 1 Select | 32 |
| 30 | Input | Diskette Change | 34 |
| 9 | Output | Motor Enable | 16 |
Configuration
This adapter supports up to two external drives. If you use a second external floppy drive, be sure to set the drive select jumpers appropriately.
Final Results
| Drive | Type | Jumper Setting |
|---|---|---|
| A | Internal Drive (Floppy) | 00 |
| B | External Drive | 01 |
Summary
This simple HDI-30 to 34-pin floppy adapter lets the IBM P70 connect to an external 5.25" floppy drive through the HDI-30 port, significantly increasing floppy disk flexibility.
Images
Drive B in action
HDI-30 connector