ZX-Spectrum-plus2A-fdd-interface-light

Native μPD765-based Floppy Disk Drive interface for ZX Spectrum +2A/+2B. Provides 100% hardware compatibility with the built-in +3DOS.

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This is an open-source hardware project featuring a Floppy Disk Drive (FDD) interface tailored for ZX Spectrum +2A and +2B computers (the classic black-case models).

The circuit is built around the classic μPD765A (or Intel 8272A) floppy disk controller chip. This is the exact same FDC silicon used in the original flagship ZX Spectrum +3 machine.

By executing a full hardware replication of the +3 disk subsystem architecture, this interface provides 100% native compatibility with the built-in +3DOS operating system. You do not need any custom EPROM firmware, complex hardware bank patches, or soft emulators. Simply plug the interface into the Expansion Bus, connect a drive, and instantly use standard +3 BASIC disk commands (LOAD "a:", FORMAT, etc.) straight out of the box.

Why is this project relevant?

The Sinclair ZX Spectrum +2A/+2B models shipped with a built-in cassette datacorder, yet they inherited the updated mainboard architecture and ROM structure from the disk-enabled +3 flagship. This interface bridges that historical gap, allowing you to easily upgrade your +2A/2B system by connecting either a real mechanical 3.5" floppy drive or a modern Gotek hardware emulator running FlashFloppy firmware.

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🎛 PCB Form-Factor Revisions

The project has been engineered in two distinct layout variations to suit your component sourcing preferences and soldering skills:

1. DIP Version (Classic Through-Hole)

• All glue logic, buffers, and support chips utilize standard through-hole DIP packages.
• Highly recommended for beginners and easy home soldering with basic tools.
• Includes an expanded matrix of configuration hardware jumpers for solderless drive adjustment.
  • Schematics | Placement | Gerber

2. SOP Version (Compact Surface-Mount)

• All auxiliary logic gates are converted to surface-mount components (SOP/SOIC packages).
• Provides a dense, modern, and highly compact PCB footprint. Requires basic SMD soldering proficiency.
  • Schematics | Placement | Gerber

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🔌 Hardware Jumper Settings (DIP Version Only)

The DIP-variant board includes dedicated hardware jumper matrices. These allow you to adapt the interface to various floppy drives without physically modifying or altering the case of the disk drive itself:

• READY Signal Selection: Manually configures the logic behavior of pin 34 on the floppy ribbon cable. You can hardwire a permanent readiness status if utilizing a standard PC-type drive that lacks Shugart-standard signaling. (Note: This behavior can also be adapted on the SOP revision via custom wiring).
• Drive Swapping (Swap A/B): Provides an instant hardware line inversion for drive ordering. You can designate any physical drive or Gotek emulator as logical drive A: or B: instantly, without needing to desolder or alter selection SMD resistors on the drive's internal PCB.
• Side Select Lock: Completely locks down the head side selection logic line. This feature is highly useful for specific hardware tests, processing vintage single-sided floppy disks, or debugging complex program disk images.

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🔧 3.5" Floppy Drive Modifications (Shugart / READY Adaptation)

Standard PC floppy disk drives (manufactured by Sony, Samsung, Mitsumi, Panasonic, etc.) are factory-configured according to IBM PC standards. To operate correctly with the μPD765 controller on a ZX Spectrum, two conditions must be fulfilled:

1. Drive Selection as Drive A: The system expects the primary boot device at logical address DS0, whereas standard PC drives are permanently hardwired to DS1.
2. READY Signal on Pin 34: Standard PC drives output a Disk Change signal on pin 34. The Spectrum, however, strictly requires a true hardware READY signal.

🔥 Crucial Note for DIP-Board Builders

If you built the DIP Version of this interface interface, you do not need to cut PCB traces or solder bodge wires inside your physical floppy drive to achieve the READY signal or swap the A/B address. Simply configure the dedicated onboard jumpers on the interface PCB to match your unmodified PC drive.

🛠 Instructions for SOP-Board Builders (Drive Internal Hack)

If you opted for the compact SOP Version of this interface, the signal adaptation must be carried out manually on the internal circuit board of your physical 3.5" drive. Here are guides for the two most common vintage models:

1. Sony MPF920

• Address Selection (DS0): Locate the SMD contact solder pads labeled JC30 / JC31 (or DS0 / DS1) sitting close to the main data connector. Carefully desolder and move the SMD bridge resistor (or solder blob) from position DS1 straight over to position DS0.
• READY Signal Routing:
  1. Trace the circuit line leading to pin 34 of the flat cable connector. Carefully slice it using a sharp hobby knife to completely disconnect the factory Disk Change signal.
  2. Locate pin 5 of the main internal FDC microchip controller (or find a test point explicitly stamped RDY).
  3. Solder a thin modding wire from this RDY point directly to pin 34 of the flat interface connector.

2. Samsung SFD-321B

• Address Selection (DS0): Locate the solder bridge pads labeled DC0 / DC1 (or S0 / S1). Safely shift the solder blob or resistor bridge from position 1 directly over to position 0.
• READY Signal Routing: The internal board features pre-routed solder pads explicitly labeled DC (Disk Change) and RDY (Ready) near the 34-pin connector. By default, they are shorted in PC mode. Simply remove the solder blob from the DC jumper path and use it to short the RDY pads together. No trace cutting is required on this specific drive model.

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⚙ Alternative: Gotek Hardware Emulator Setup

If you plan to connect a popular Gotek hardware emulator instead of a mechanical floppy drive, no internal soldering or case modification is required. Simply flash the Gotek device with the open-source FlashFloppy firmware and add the following configuration strings inside your root FF.CFG text file on the USB flash drive:

interface = shugart
host = dec-shugart
pin34 = rdy