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# tashtwenty
# TashTwenty
An interface bridging the DCD protocol used by Apple's Hard Disk 20 to a MMC card, implemented using a PIC16F1825.
## Elevator Pitch
# MMC Card Format
It's a DCD (the interface used by the Hard Disk 20 to plug in through the disk port) interface, contained entirely within a PIC16F1825
(14 pins, ~$1.70) microcontroller. It bit-bangs the low-level IWM protocol using the read/write and phase lines on the "DB"-19 disk
port, no programmable logic devices of any kind required.
It interfaces to an MMC card and can emulate up to four DCD devices. It also has an enable output which can be used to
chain more DCD devices (though Mac OS only supports a total of four) and a floppy disk drive.
(Despite the name, there is nothing limiting it to 20 megabytes.)
## Project Status
Functional, though not rigorously tested.
## Caveats
Due to the dearth of documentation on the DCD protocol (see below), the protocol only implements the commands whose formats are known,
namely read, write, and device identification. Responses to other commands are faked. Fortunately, this seems to be enough for the
device to function properly, including formatting.
## Technical Details
### MMC Card Format
The MMC card must have an MBR (Master Boot Record) aka DOS-type partition table with up to four primary partitions of type 0xAF (HFS). Partitions of other types will be ignored. Extended partitions are not supported.
# DCD (Directly Connected Disks) Protocol
## Details Missing or Inaccurate in the [May 1985 DCD Document](http://bitsavers.trailing-edge.com/pdf/apple/disk/hd20/Directly_Connected_Disks_Specification_1.2a_May85.pdf)
### DCD (Directly Connected Disks) Protocol
#### Details Missing or Inaccurate in the [May 1985 DCD Document](http://bitsavers.trailing-edge.com/pdf/apple/disk/hd20/Directly_Connected_Disks_Specification_1.2a_May85.pdf)
* The sync byte, in either direction, is always 0xAA, 0x96 is not used.
* When Mac is transmitting a command, the sync byte is followed by two more raw IWM bytes before the 7-to-8 groups begin. DCD transmits only a sync byte and does not transmit these extra bytes.
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* The Filler field is replaced by a 16-byte Pascal string (first byte is length) that determines what appears in the "Where:" field of the Get Info dialog box.
* The checksum byte is chosen such that all data bytes in all 7-to-8 groups (not including the sync byte or the command/response length IWM bytes) sum to 0 modulo 256.
## Details Out of Scope for DCD Documentation But Useful to Know
#### Details Out of Scope for DCD Documentation But Useful to Know
* The IWM transmits and receives MSB first and the MSB is always set; the chip uses this for timing.
* The IWM transmits at its "fast" speed, 47/96 MHz, or approximately 489.58 Kbps, data cell width 2.043 us.
* The IWM's output (on WR pin) is in NRZI format (inversion == 1, no inversion == 0).
* The IWM's input (on RD pin) detects only falling edges.
## Beyond
#### Beyond
Other details about the protocol at the signal level and the read, write, and controller status commands are accurately reported by the [May 1985 document](http://bitsavers.trailing-edge.com/pdf/apple/disk/hd20/Directly_Connected_Disks_Specification_1.2a_May85.pdf); other commands are unknown to me, but hopefully this information will be useful to anyone in the future who wishes to implement DCD.