Wiznet makers

* All images in this article were generated by Gemini.

ISACard – SD Storage & WIZnet W5500 Network Interface for the PC ISA Bus

1. Overview

The ISACard is an open-source ISA expansion card designed for 1980s–1990s IBM PC compatible systems (XT/AT class).
It enables retro PCs to use modern SD storage and Ethernet networking (via WIZnet W5500) on a single card.

Originally inspired by the Apple II SDCard interface project, the ISACard adapts and extends the concept for PC architecture—providing two major enhancements to legacy systems: large-capacity storage and network connectivity.

2. Technical Architecture & Specifications

Main MCU

• Microchip ATmega328P (Arduino-compatible)
• Emulates an 8255 PPI (Programmable Peripheral Interface) in software
  → Bridges the PC ISA bus with SD storage and network functions.

Storage Interface

• Two SD card slots (on ISACard + ExtCard)
• Supports Raw Block Device mode
• Can mount FAT16/32 filesystem images (.IMG) as virtual disks

BIOS & Firmware

• Modified XT-IDE Universal BIOS (up to 32 KB ROM)
• Detects SD cards as HDDs during boot
• Handles CHS/LBA translation
• MS-DOS utility (SetBlkDv)
• Manages block device mapping configurations

Network Interface

• 10-pin SPI header for connecting a WIZnet W5500 Ethernet module
• Utilizes W5500’s hardwired TCP/IP stack for stable and efficient network communication

3. Why WIZnet W5500? Technical Rationale & Insights

Why W5500?

The ATmega328P features:

• 2 KB SRAM
• 32 KB Flash

Such limited resources make software-based TCP/IP stacks (e.g., LwIP) practically impossible.

In contrast, the WIZnet W5500 includes a fully hardware-implemented TCP/IP stack (Hardwired TCP/IP) and integrated TX/RX buffers.
This allows:

• Minimal MCU overhead
• Reliable Ethernet connectivity even on very low-end systems

Therefore, the W5500 is the most realistic and optimal solution for networking on a retro PC interface using an 8-bit MCU.

How It Works

1. The ATmega328P receives requests from the ISA bus
2. When network access is required, the MCU sends commands over SPI to the W5500
3. The W5500 handles TCP/IP processing and communicates with the external network
4. Returned data is passed back through the ISA bus to DOS applications

Possible Use Cases

• FTP file transfer → move files to/from SD storage
• NTP time synchronization
• Telnet access to BBS services
• Simple TCP/UDP-based IoT experiments on retro hardware

4. Key Features Summary

5. System Architecture

This diagram illustrates an ISA expansion card that enables a vintage IBM PC/XT to use modern SD cards and connect to the internet via an Ethernet (W5500) module. It shows how the Arduino acts as a bridge, handling complex network protocols and storage systems so the classic computer can communicate with contemporary networks.

6. Conclusion

The ISACard provides an elegant open-source solution to two major limitations of retro PCs:

1. Aging, unreliable storage devices
2. Lack of native network connectivity

By integrating SD storage and WIZnet W5500 Ethernet on a single ISA card, the project demonstrates how modern components can meaningfully extend the usability of vintage PCs.

The use of the W5500, in particular, showcases an excellent example of achieving stable TCP/IP networking with minimal MCU resources—making the ISACard highly valuable to the retro computing and maker communities.