VG93-MB8877-lgt8f328p-emulator

Hardware layout for Evgeny Ugryumov's WD1793/MB8877/KR1818VG93 floppy disk controller emulator based on the LGT8F328P MCU.

English | Русский

This project features a hardware emulator for the Soviet KR1818VG93 (КР1818ВГ93) floppy disk controller chip and its international equivalents, the Western Digital WD1793 and Fujitsu MB8877. Powered by the fast and affordable LGT8F328P microcontroller, this adapter board acts as a direct drop-in replacement. It is designed to replace scarce, expensive, or faulty vintage FDC chips in retro-computers like the ZX Spectrum and other compatible 8-bit systems.

• Firmware Development: Evgeny Ugryumov (ZX-FLOPPY group).
• PCB Layout & Hardware Design: Alex-2-Graf.

The adapter board is routed specifically to plug directly into a standard 40-pin IC socket on the host computer board, requiring no permanent modifications to the vintage hardware.

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⚡ Key Features of LGT8F328P

• 32 MHz Clock Speed: Provides high-precision bus timing synchronization to accurately match vintage hardware cycles.
• High Performance Core: Executes most machine instructions in a single clock cycle.
• Native 5V Logic: Provides full signal level compatibility with legacy 8-bit computers without requiring logic level shifters or resistors.
• Cost-Effective: The LGT8F328P microcontroller is significantly more affordable and accessible than surviving original WD1793/VG93 silicon chips.

🛠️ Supported Functionality

• Emulation of core FDC commands for sector reading and writing.
• Accurate tracking of stepper motor control signals (STEP, DIR).
• Precise hardware emulation of standard drive feedback signals: READY, INDEX, and TRACK 00.

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🔌 Hardware Revisions

The emulator has evolved through three distinct board layouts:

Revision 1 (Not recommended for new builds)

The initial prototype version. Included for historical log purposes.

• Schematics | Placement | Gerber

Revision 2 (Diode-based version)

An improved layout utilizing discrete steering diodes to condition signals.

• Schematics | Placement | Gerber

Revision 3 (74AHC1G08 logic version - Recommended)

The latest and most stable layout utilizing a tiny 74AHC1G08 AND-gate chip. Since this logic IC comes in two common SMD packages, two distinct PCB options are available:

1. Ultra-compact layout using the SC-70 IC package package:
   • Schematics | Placement | Gerber
2. Standard layout using the SOT-23-5 IC package package:
   • Schematics | Placement | Gerber

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💾 Flashing Instructions

To flash the compiled firmware onto the onboard LGT8F328P microcontroller, a specialized ISP programmer is required.

You can assemble your own programmer using one of these DIY options:

1. An Arduino-based programmer using LarduinoISP.
2. An RP2040-based programmer using LarduinoISP.

Once your hardware programmer is verified, flash the precompiled binary .hex files using the AVRDUDESS GUI software tool.

• Firmware Binaries: Available inside the /Firmware directory.
• Latest Releases: Updated software drops can be monitored directly in the ZX-FLOPPY community group.
• Source Material: Complete design files can be inspected in the /Sources folder.

⚠️ Interface Modification (Nemo-BDI Correction)

For Beta Disk Interfaces utilizing the Nemo-BDI standard architecture, an external hardware patch is required to correct the chip select (CS) signal timing. This modification requires a standard 555LA3 / 74LS00 discrete NAND logic gate chip.

• Modification Schematic

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👥 Credits & Acknowledgments

• Evgeny Ugryumov — Special thanks for his incredible work developing the firmware core inside the ZX-FLOPPY project.
• Alexander (UR4QBP) — For running extensive hardware testing, experiments, and optimizations.
• HRDY (Dmitry) — For developing the excellent RP2040-based hardware programmer design.

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📜 License

This project is distributed under the open-source MIT License. See the LICENSE file for more details.