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07 — Driver Compatibility Contract

Part of DiKon — PCL3 raster firmware for the Kodak Diconix 150, by RND2 Labs / TheRND2 (thernd2.com/diconix) — see the repository README. Firmware: M7 v5 (d150_m7v5.bin MD5 fe9215abec1f8c4f4f407867302fa898; burn image d150_m7v5_W27C512.bin MD5 2f2a10f449f522df2500effe616f5f67). Project: TheRND2 / RND2 Labs, thernd2.com/diconix.

This is the contract for pointing ANY OS / driver at the modified Diconix 150. The printer is nominally a PCL3-class inkjet; M7 implements a hardened PCL raster parser on top of the stock Epson-compatible text engine. What matters is the raster encoding a driver emits, not the PCL level on the box.

The four knobs (set these and it works)

  1. Compression mode 0 (raw) or 2 (PCL TIFF/packbits) — ESC*b0M / ESC*b2M. Not 1 or 3.
  2. 300 dpi, single-plane monochrome.
  3. Rows < 2000 bytes each (ESC*b<n>W, n ≤ 2000).
  4. End the job with ESC*rB — the end-of-job flush that prints the last partial band.

WORKS (renders ink, correct layout)

Any driver whose output satisfies ALL of:

  • PCL raster framing ESC*r1A (or *r#A) … rows … ESC*rB end. The job must end with ESC*rB.
  • Compression mode 0 or 2 (ESC*b0M / ESC*b2M).
  • Row data ESC*b<n>W, n ≤ 2000 bytes.
  • Blank-line runs ESC*b<n>Y — advance vertical position correctly (v5).
  • 300 dpi single-plane mono (native dot pitch after the fixed 300-dpi→carriage mapping; rows wider than 2304 dots are right-clipped at the carriage).
  • Text, if any, via the stock Epson escape/text path (PCL raster jobs normally contain none).

Verified WORKS:

  • Gutenprint pcl-500 (HP DeskJet 500) at 300 dpi (and Draft/150 dpi*), incl. the rastertogutenprint CUPS filter — the engine used by NokoPrint (Android), Linux CUPS, and macOS Gutenprint. (*150 dpi prints at half scale — see IGNORED.)
  • Windows DJ500 driver (Win11, regression-tested every firmware rev).
  • Generic PCL4 mono 300 dpi drivers (mode 0/2, no delta) — incl. NokoPrint's "Generic PCL4".

SAFE-BUT-BLANK (consumed cleanly, prints nothing, never garbage, never wedges)

  • Compression modes 1 and 3 (delta row) and any other *b#M value: rows consumed count-based and printed blank; vertical spacing stays correct; a later *b0M/*b2M recovers.
  • Color-plane data ESC*b<n>V (uppercase V): discarded byte-exact. If the driver sends K as the final W plane, K still prints.

IGNORED (consumed, no effect — pre-render content into raster rows)

  • ESC*p#X/#Y cursor positioning · ESC*t#R resolutions ≠ 300 (dot pitch is fixed at the 300-dpi mapping, so 150 dpi prints at half size — legible, wrong scale; use 300).
  • ESC&l… page format, ESC&a… positioning, ESC&k…, ESC(… fonts/symbol sets: swallowed.
  • ESC*r#S/#T width/height, *r#Q/*o#Q/*o#D quality/shingling/depletion.
  • Any ESC*<group> with a lowercase group letter: parameters parsed and dropped; a final W/V payload consumed byte-exact (v5) — nothing leaks.

AVOID / NOTES

  • Color DeskJet 500C/550C ("cDeskJet") drivers: compact plane encoding ESC*b#v<data> (lowercase v carrying binary) is indistinguishable from a parameter to any grammar parser; the binary floods the text path as garbage glyphs and can stall the job. Use mono DJ500 or generic PCL4.
  • PCL5+ (macros, fonts, HP-GL/2 ESC%#B): untested; HP-GL/2 contains non-escape command text that would print as glyphs. Don't send PCL5 streams.
  • First-byte-loss race: if the host fires the instant it opens the port while the printer is mid-init, the first byte can be lost (the leading ESC drops → its partner prints as one stray glyph, first band may misparse, then self-heals at the next ESC). Release the job only when the printer is idle/ready. This is operational, not a firmware bug.
  • Always pick 300×300 dpi, mono/grayscale. Paper eject relies on the raster band cadence (ESC J units); the &l0H eject is ignored, but the *rB flush already feeds past the last band.

Recommended known-good setups: Windows → HP DeskJet 500 driver. Linux/CUPS & macOS → Gutenprint pcl-500 @300 dpi. Android → NokoPrint "HP DeskJet 500 (Gutenprint)" (after v5) or "Generic PCL4" (mono, 300 dpi, no delta). Anything else → any generic PCL 3/4 mono 300 dpi raster driver.

Why "generic PCL4" works on a nominally PCL3 printer

M7 implements the raster-graphics core that PCL3 and PCL4 share — *rA/*rB framing, *b#M mode select (0/2), *b#W rows, *b#Y blank lines — and safely ignores everything else. What a PCL "level" adds is mostly fonts, macros and positioning that a raster driver never uses: the driver pre-renders the whole page and ships it as compressed raster rows. So compatibility is decided by the raster encoding (compression 0/2, row width ≤2000, mono, 300 dpi), not the PCL label. A generic "PCL4 mono" driver emitting mode-0/2 raster is, at the byte level this printer sees, indistinguishable from the DJ500's own PCL3 output.

Provenance

Real rastertogutenprint.5.3 (Gutenprint 5.3.3) pcl-500 streams at 300/150/Draft dpi, replayed instruction-level through the audited firmware rig: 100% consumption, zero text-path leaks, zero invariant violations; renders match ghostscript's own raster of the same page. Gutenprint print-pcl.c audit: DJ500 = PCL_PRINTER_DJ|TIFF|BLANKLINE, mono; always \033*b2M + pcl_mode2() rows + \033*b%dY blanks + \033*rB\033&l0H\033E; at ≥300 dpi additionally \033*r2Q \033*o2Q \033*o5D (the v4b garbage source, fixed in v5). No delta-row encoder exists in the driver. Windows DJ500 captures + the full adversarial corpus regressed byte-identical to the hardware-proven v4b behavior.

Firmware history: v4b printed Gutenprint/NokoPrint-DJ500 jobs with 4 garbage chars per page (2Q5D) and collapsed blank gaps; v5 fixes both and adds binary-payload discard for unknown groups. v4b remains the fallback chip.