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Communication protocol

All Coolscans use SCSI internally - even the Firewire/USB ones. The scanners operate using SGC-2 (SCSI Graphics Commands) with some proprietary extensions in the form of extra opcodes. Those opcodes can be inferred from the firmware code (SCSI dispatch tables) and/or SANE coolscan3.c driver (which contains some important nuggets of knowledge, especially about USB communication protocol).

SCSI scanners

The flavor of SCSI is SCSI-2. The parallel bus is 8-bit and runs at 10MHz, giving an effective transfer rate of 10MB/s or 80Mbit/s. The bus is single-ended and operates at TTL voltage levels, making interfacing from a simple microcontroller quite possible.

See the Wikipedia article for a timeline of Parallel SCSI evolution.

FireWire scanners

The protocol used is SBP-2. This is a direct SCSI transport for Firewire devices, meaning very little to no extra translation is needed.

USB scanners

The USB scanners use extra framing commands to communicate SCSI phase and sense data over USB. They use a simple bulk interface consisting of one Bulk-Out endpoint and one Bulk-In endpoint. This makes them suitable to be used with Windows' usbscan.sys driver, since the following is enough to perform all communication with the scanner - no extra syscalls are needed:

CreateFile('\\\\.\\usbscan0', OPEN_EXISTING...);
WriteFile(hScanner, data, len);
ReadFile(hScanner, data, len);

Based on reading the SANE source code (coolscan3.c) and sniffing Vuescan's communication, the following protocol has been established:

  1. Send the SCSI command block.
  2. Query the current SCSI phase by sending 0xD0.
  3. Receive the status byte that indicates the current phase.
  4. Perform read/write depending on the phase.
  5. Once phase indicates the command has completed, read the SENSE data by sending 0x06 and reading 8 bytes.

The intricacies of this protocol are not yet known to me but the above information is enough for performing basic communication with success.