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External Floppy Adapter

Contents


IBM P70 External Floppy Adapter
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

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

Removing the casing
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.

  1. Remove the SCSI connector by desoldering it from the adapter.
  2. Wire the HDI-30 connector to a 34-pin floppy connector using the signal mappings below.

Post-wiring
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

Floppy Connector Pinout


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

HDI-30 Pinout


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

Reading from Drive B
Drive B in action

Male HDI-30
HDI-30 connector