Gameboy Printer

The Gameboy Printer is a portable thermal printer made by SII for Nintendo, which a few games used to print out bonus artwork, certificates, pictures (Gameboy Camera)...

It can use standard 38mm paper and interfaces with the Gameboy through the Link port.

It is operated by an embedded 8bit microcontroller which has its own 8KiB of RAM to buffer incoming graphics data. Those 8KiB allow a maximum bitmap area of 160*200 (8192/160*4) pixels between prints.

Communication

The Gameboy Printer doesn't use the full-duplex capability of the Link port. It accepts variable length data packets and then answers back its status after two $00 writes.

The packets all follow this format:

Description	Magic bytes		Command	Compression flag	Length of data		Command-specific data	Checksum		Alive indicator	Status
Size	2 bytes		1 byte	1 byte	2 bytes		Variable	2 bytes		1 byte	1 byte
GB to Printer	$88	$33	See below	0/1	LSB	MSB	See below	LSB	MSB	$00	$00
Printer to GB	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x81	See below

Warning: There is a 250ms timeout. If no byte is received during this period, the link and graphics buffers are reset.

The checksum is simply a sum of every byte sent except the magic bytes and obviously, the checksum itself.

Detection

Send these 9 bytes: $88,$33,$0F,$00,$00,$00,$0F,$00 (Command $0F, no data).

Send $00 and read input, if the byte is $81, then the printer is there. Send a last $00, just for good measure. Input can be ignored.

Command 1: Initialize

This clears the printer's buffer RAM.

No data required. The normal status replied should be $00.

Command 2: Start printing

Data length: 4 bytes

• Byte 1: Number of sheets to print (0-255). 0 means line feed only.
• Byte 2: Margins, high nibble is the feed before printing, low nibble is after printing. GB Camera sends $13 by default.
• Byte 3: Palette, typically $E4 (0b11100100)
• Byte 4: 7 bits exposure value, sets the burning time for the print head. GB Camera sends $40 by default. Official manual mentions -25% darkness for $00 and +25% for $7F.

Command 4: Fill buffer

Data length: max. $280 (160*16 pixels in 2BPP) To transfer more than $280 bytes, multiple "command 4 packets" have to be sent.

The graphics are organized in the normal tile format (16 bytes per tile), and the tiles are sent in the same order they occur on your tilemap (do keep in mind though that the printer does *not* have 32x32 tiles space for a map, but only 20x18).

Command $F: Read status

No data required, this is a "nop" command used only to read the Status byte.

Status byte

A nonzero value for the higher nibble indicates something went wrong.

Bit 7	Low Battery	Set when the voltage is below threshold
Bit 6	Other error
Bit 5	Paper jam	Set when the encoder gives no pulses when the motor is powered
Bit 4	Unprocessed data	Set when there's unprocessed data in memory - AKA ready to print
Bit 3	Ready to print	Set when at least $280 bytes if graphics were received
Bit 2	Image data full
Bit 1	Currently printing	Set as long as the printer's burnin' paper
Bit 0	Checksum error	Set when the calculated checksum doesn't match the received one

Example

• Send command 1, the answer should be $81, $00
• Send command 4 with $280 of your graphics, the answer should still be $81, $00
• Ask for status with command $F, the answer should now be $81, $08 (ready to print)
• Send an empty command 4 packet, the answer should still be $81, $08
• Send command 2 with your arguments (margins, palette, exposure), the answer should still be $81, $08
• Ask for status with command $F until it changes to $81, $06 (printing !)
• Ask for status with command $F until it changes to $81, $04 (printing done)
• Optionnally send 16 zero bytes to clear the printer's receive buffer (GB Camera does it)

Compression

Some sort of RLE ? The GB Camera doesn't use it.

(Details and pictures, need to be copied here)