Imported wiki with information about protocol

Credits: kjy00302

docs/protocol.md

@@ -0,0 +1,68 @@
+---
+hide:
+  - navigation
+---
+
+## Packet structure
+
+Niimbot uses simple TLV packet with preamble, postamble and checksum.
+
+Every field of packet uses big endian.
+
+    |5555|c2|01|02|c1|aaaa|
+     │    │  │  │  │  └ Postamble (constant)
+     │    │  │  │  └─── Checksum (xor sum of type, length and value)
+     │    │  │  └────── Value
+     │    │  └───────── Length (of data)
+     │    └──────────── Type
+     └───────────────── Preamble (constant)
+
+## Line encoding packets
+Niimbot uses three types of packet to print image.
+
+All three packet type has repeat count. So, repetition of same data can be reduced to one packet.
+
+Currently niimprint's encoder only uses line packet without optimization.
+
+_(Common packet fields are omitted for clarity.)_
+
+### Blank packet
+Blank packet (packet type 0x84) encodes blank line.
+
+    |5555|84|03|0000|0a|8d|aaaa|
+                │    └ Repeat count
+                └───── Line count
+
+This packet will print 10 blank lines.
+
+### Line packet
+line packet (packet type 0x85) encodes line as bitmap.
+
+    |5555|85|12|000a|00|01|0f|02|0000000000000001fffe0000|91|aaaa
+                │    │  │  │  │  └ Bitmap data
+                │    │  │  │  └─── Repeat count
+                │    │  │  └────── Pixel count (right)  (bit count of 0x00000000)
+                │    │  └───────── Pixel count (middle) (bit count of 0x00000001)
+                │    └──────────── Pixel count (left)   (bit count of 0xfffe0000)
+                └───────────────── Line count
+
+This packet will print 2 bitmap lines.
+
+### Points packet
+points packet (packet type 0x83) encodes line as array of 1D points.
+
+    5555|83|0e|000c|00|01|03|07|003f0040004d004e|f8|aaaa|
+                │    │  │  │  │  └ Point data (0x003f, 0x0040, 0x004d, 0x004e)
+                │    │  │  │  └─── Repeat count
+                │    │  │  └────── Pixel count (right)
+                │    │  └───────── Pixel count (middle)
+                │    └──────────── Pixel count (left)
+                └───────────────── Line count
+
+This packet will print 7 lines with points.
+
+### Todo
+
+!!! question
+
+    How does B21 encodes more then 96 pixels? 

mkdocs.yml

@@ -117,3 +117,4 @@ nav:
   - Home: index.md
   - FAQ: faq.md
   - Printers & Tapes: hardware.md
+  - Protocol: protocol.md