Friday, June 27, 2008

CHED - Introduction

Project CHED (Combined Hardware Emulated Drives)

Total floppy and hard drive replacement for the Apple II that uses disk images instead of floppy media and contains a user interface.

I started this project with the aim of modifying the floppy disk support on the Apple IIGS in a way that does the following :-

1. Removes the need for floppy disk media. The media is becoming harder to come by and the chance of failure for existing media is always increasing.
2. Provides an easy way to transfer disk images to and from the unit.
3. Stores the images in a stand alone unit.
4. Contains a user interface for selecting disk images.
5. Reduces the noise existing floppy drives produce. This was a priority in my household when I started this project but not so much any more.

There were two main ideas which I considered developing. They are as follows :

1. A disk drive that communicates over the disk port or disk controller card. IDE controller and a 2.5" hard disk drive. A hard drive is preferred over Compact Flash / Secure Digital due to the extensive read/write usage. For ease of transfer, images are stored using FAT and the unit contains a USB to ATA/ATAPI bridge. User interface is implemented using keys and an LCD module.

2. A peripheral card that fits into the Apple II slot. Similar to option 1 in that it contains an IDE controller / 2.5" hard drive / FAT / USB to ATA/ATAPI bridge. The user interface can be hacked into the IIGS case or it can be driven by software.
Option 1 felt like it would be easier and simpler to build.

System overview

The first implementation will involve using the Apple 3.5" floppy disk drive as the enclosure. The front will be modified to fit the LCD and keys. The back will be modified to take the USB cable and a power source. The disk usage indication led and the eject button will be reused.

To start with, CHED will emulate two Apple 3.5" disk drives and two Apple 5.25" disk drives. Smartport hard drive emulation can be added later.

CHED will contain two modes.
Mode 1 will enable it to act as a disk image controller.
Mode 2 will enable the IDE hard drive to be visible by a PC as an externally connected USB drive.

An Atmel ATMega128 microcontroller will be used to handle the user interface, Apple II disk interface communications and IDE/ATA communications.

It will contain options depending on the platform used ie IIGS / IIc / IIc+ / IIe (via an Apple 5.25" or a Superdrive controller)

Future ideas.
Emulate a smartport hard drive.
Integrate a USB peripheral controller onto the circuit board and remove the relay.
Extend the daisychain interface so that other drives can be added downstream of the unit.
Implement using a Disk II interface.
Implement option 2 - Internal card via the expansion slot.

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