Printhead data protocol¶

The encoder packs each printed column into the format described here. The firmware stores and sends it to the head's shift registers. This interface was reverse-engineered.

Both supported heads use this format. Their per-bus tables assign different physical nozzles to the same data positions.

The bus is the unit¶

Each data pin carries one bus: 180 nozzle positions with a 2-bit code each, followed by a 32-bit window-enable map. That is 392 bits, sent at two bits per clock.

The two bits of a code are not adjacent. A bus is two blocks of 180:

 index   0 .......... 179 | 180 .......... 359 | 360 ...... 391
         first bit of the | fire bit of the    | window-enable
         code, positions  | code, positions    | map
         0..179           | 0..179             | (32 bits)

Bus position p has its code's first bit at index p and its fire bit at 180 + p. Only codes 00 and 01 are emitted today, so the first block is always zero.

Code	Meaning today	Reserved for
`00`	do not fire
`01`	fire
`10`, `11`	(unused)

The window-enable map determines when each nozzle code receives the drive waveform.

What rides a bus is the head-specific part¶

A table of 180 entries per bus maps data positions to physical nozzles.

On c6n90, each bus interleaves two 90-nozzle columns. Three buses cover all six columns: A carries slots 0 and 1, B carries slots 2 and 3, and C carries slots 4 and 5.

On c4n180, each bus serves one 180-nozzle column with a direct mapping, p → nozzle p. Each 180-bit block divides into three runs of 60. Only two buses are wired; the third sends zeros.

The tables differ; the packing code does not (paintress-rip-encoder/encoder/head_packer.py).

The window-enable map¶

The last 32 bits of each bus select the firing windows for each nozzle code. This map must be measured on the physical head; geometry alone does not determine it. The supported heads use different maps:

Head	Window map	Meaning
`c6n90`	`00000000000000100000000000000000`	bit 14
`c4n180`	`00000000000000000000000000000010`	bit 30, window 7, code `01`

Both maps enable only code 01, giving one drop size: 01 fires the waveform and 00 does not fire.

Clocking and the 147-byte line¶

The buses shift simultaneously over a 3-line DDR data bus, so every clock carries 6 bits, three pins on the rising edge and the same three on the falling edge. A 392-bit bus takes 196 clocks, and one complete column is:

196 clocks × 6 bits = 1176 bits = 147 bytes

Unused buses send zeros. This lets c4n180 and c6n90 share the 147-byte frame and the same firmware binary.

Each 147-byte column is one USB DATA frame and one job-file line (bytes_per_line). The firmware expands it into 49 32-bit words with 24 payload bits per word, then stores those 196 bytes in PSRAM for PIO/DMA. Packing saves 25% on USB transfer size, and expanding on receipt avoids repacking during firing.

Sequencing¶

For each column, the data bus loads the nozzle data, the window selector latches it and sequences the firing windows, and the DAC sends the waveform to enabled nozzles. Loading the next column's data overlaps the current firing interval.

Line size, clocks, edges and packing are included in the frame fingerprint exchanged at IDENTIFY. Changing them requires a firmware reflash. Nozzle count and channels belong to the separate head fingerprint, checked by the daemon. See File formats.