Game Boy Printer

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

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

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

Communication

The Game Boy 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:

ContentSize (bytes)GB -> PrinterPrinter -> GB
Magic bytes2$88, $33$00
Command1See below$00
Compression flag10/1$00
Length of data2LSB, MSB$00
Command-specific dataVariableSee below$00
Checksum2LSB, MSB$00
Alive indicator1$00
Status1See below$00

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.

Commands

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 (%11100100)
  • 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).

An empty data packet must be sent before sending command 2 to print the data, otherwise the print command will be ignored.

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 #NameDescription
7Low BatterySet when the voltage is below threshold
6Other error
5Paper jamSet when the encoder gives no pulses when the motor is powered
4Packet error
3Unprocessed dataSet when there’s unprocessed data in memory - AKA ready to print
2Image data full
1Currently printingSet as long as the printer’s burnin’ paper
0Checksum errorSet 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)
  • Optionally send 16 zero bytes to clear the printer’s receive buffer (GB Camera does it)

Tips

  • The printer has a timeout of 100ms for packets. If no packet is received within that time, the printer will return to an initialized state (meaning the link and graphics buffers are reset).
  • There appears to be an undocumented timeout for the bytes of a packet. It’s best to send a packet completely or with very little delay between the individual bytes, otherwise the packet may not be accepted.
  • To print things larger than 20x18 (like GB Camera images with big borders), multiple data packets with a following print command need to be sent. The print command should be set to no linefeed (neither before nor after) to allow for continuous printing.

Compression

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

(Details and pictures, need to be copied here)