3. The PeriPage protocol (as spoken to a P21)¶

Everything below is byte-exact from y0.h (command builder) and y6.a / y6.f (sequencing). All multi-byte integers are noted individually — the protocol is not consistently big- or little-endian.

3.1 Command families¶

Three prefixes appear on the wire:

3.2 Vendor command reference (10 FF …)¶

Source: y0.h, plus the y6.f.G4(int,int) dispatcher.

Session / job control¶

Print parameters¶

Queries (device answers with an ASCII/UTF-8 string unless noted)¶

Wi-Fi provisioning — 10 FF 31 02¶

The only command that carries free text (y0.h.p, called from y6.a.W0(ssid, pwd)). This is not a text-printing command.

10 FF 31 02  <len16lo> <len16hi=0>
01 <ssidLen> 00  <ssid bytes (UTF-8)>
02 <pwdLen>  00  <password bytes (UTF-8)>
03 01 00 <mode>

ESC/POS subset¶

3.3 Responses¶

The RX path (y0.e.l) decodes 2-byte status frames; anything else is passed up raw as the payload of the current query. Meanings recovered from the y0.c implementations in y6.f$a:

Common ASCII replies: OK (4F 4B) for accept, ER (45 52) for error.

FD 01 / FD 02 are software flow control. The printer can tell the host to abort or resume mid-transfer. A robust client should watch for these during a large raster; see §5.8 for the related hard timeout. Nothing in ref/p21_iobt.py currently acts on them — it only logs whatever arrives.

3.4 Raster encoding¶

Monochrome packing — y0.h.w(bitmap, w, h)¶

bytesPerRow = ceil(width / 8)
for each row y:
    for each pixel x:
        if pixel != 0xFFFFFFFF (not pure white)
           and (R + G + B) / 3 < 190:
               set bit (7 - (x mod 8)) of out[y*bytesPerRow + x/8]

Uncompressed — y0.h.F(bitmap, mode) ← what a plain PPG_P21_ uses¶

1D 76 30 <mode> <bytesPerRow & 0xFF> <bytesPerRow >> 8> <height & 0xFF> <height >> 8>
<raster bytes>

Compressed — y0.h.z(bitmap, mode) ← what a PPG_P21+_ uses¶

byte[] z = Code.code(raster);          // native, libCode.so
byte[] hdr = { 0x1F, 0x00,
               bytesPerRow >> 8, bytesPerRow & 0xFF,   // BIG-endian here!
               height      >> 8, height      & 0xFF,
               (len >> 24), (len >> 16), (len >> 8), len };   // len = z.length - 2
out = hdr ++ z[2 .. end]

libCode.so is statically linked zlib; Java_com_example_sdk_Code_code calls compressBound() then compress() — a standard zlib stream. The Java side then drops the first two bytes (the 78 9C zlib header) and sends the rest, so the wire format is raw DEFLATE followed by the 4-byte Adler-32 trailer. The declared length also excludes those two header bytes.

Note the width/height fields are big-endian here but little-endian in GS v 0.

3.5 Chunking¶

y0.h.G(byte[]) splits every write into 1024-byte chunks with Thread.sleep(1) between them. There is no ACK per chunk.

In practice the 1024 figure is an Android-socket convenience — Android fragments to the RFCOMM MTU underneath. On a stack that does not (macOS writeSync_length_ rejects anything over the MTU), chunk to the negotiated MTU instead; 126 bytes was negotiated with PPG_P21_F530. Keep the 1 ms pacing: it dominates transfer time (9216 raster bytes took 4.2 s at MTU 126).

3.6 Synchronous vs fire-and-forget¶

Verified on hardware — this matters for a re-implementation:

3.7 Image pipeline (photo mode)¶

Text and images take different preprocessing paths in the app. Both end at the same GS v 0 command; what differs is how pixels become 1 bpp.

Auto-exposure — if.b.y() + if.b.i()¶

Grayscale, take the mean, look up a factor f, then apply out = (in/255)^(1/f)*255 (if.b.i does convertTo(1/255), Core.pow(1/f), convertTo(*255)):

Thermal paper has no midtones, so this pulls the histogram toward the middle before dithering, otherwise bright photos dither to near-blank.

Sierra-3 dither — if.b.a()¶

Threshold 127 (not the 190 used for text). Error diffused with divisor 32:

              *    5    3
    2    4    5    4    2
         2    3    2

That is the standard Sierra-3 ("Sierra filter") kernel. Note Java truncates integer division toward zero, so terms are int(err * w / 32), not a floor -- this differs for negative error, which occurs on every white pixel (err = value - 255).

Output is pure 0 or 255, so the later <190 test in y0.h.w() is a no-op for images; it only packs bits.

Why it matters¶

Same 192x192 source icon scaled to 384 wide:

photo mode (dither)     34.6% black    recognisable, tonal
text mode (threshold)   80.6% black    mid-grey background crushed to solid