docs: describe generic serial transport framing

docs/generic_serial_transport.md

@@ -0,0 +1,63 @@
+# Generic Serial Transport
+
+The generic serial transport is ZCM’s default framing for byte-stream links (UART, USB serial, etc.). It provides:
+
+- Byte framing and re-synchronization
+- Channel + payload length fields
+- Fletcher-16 checksum for basic corruption detection
+- A simple escaping rule so the frame marker can appear in data
+
+This transport is used by `serial://...` when neither `pkt_size` (packetized mode) nor `raw=true` are specified.
+
+---
+
+## Wire Format
+
+Constants:
+
+- Escape byte: `0xCC` (configurable via `ZCM_GENERIC_SERIAL_ESCAPE_CHAR`)
+- Frame prefix: `0xCC 0x00`
+- Checksum: Fletcher-16, seeded with `0xFFFF`
+
+The unescaped frame layout is:
+
+| Field | Size | Notes |
+|---|---:|---|
+| `0xCC 0x00` | 2 | Frame start |
+| `chan_len` | 1 | Channel name length in bytes |
+| `data_len` | 4 | Big-endian uint32 payload length |
+| `chan` | `chan_len` | Channel bytes (not null-terminated on the wire) |
+| `data` | `data_len` | Message bytes |
+| `checksum` | 2 | Big-endian Fletcher-16 over `chan` then `data` (unescaped bytes) |
+
+---
+
+## Escaping Rules
+
+To ensure the escape byte can appear in the channel name or message payload:
+
+- Any literal `0xCC` byte in `chan` or `data` is transmitted as `0xCC 0xCC`.
+- The receiver interprets:
+  - `0xCC 0x00` as a frame start.
+  - `0xCC 0xCC` as a single literal `0xCC` data byte.
+
+The checksum is computed over the original (unescaped) `chan` and `data` bytes.
+
+---
+
+## Decoder Outline
+
+At a high level, a receiver can:
+
+1. Scan the byte stream for the two-byte prefix `0xCC 0x00`.
+2. Read `chan_len` and `data_len` (big-endian).
+3. Read `chan_len + data_len + 2` bytes, applying the unescaping rule while reading `chan` and `data`.
+4. Compute Fletcher-16 over `chan` and `data` (seed `0xFFFF`) and compare to the transmitted checksum.
+5. If the checksum matches, deliver the reconstructed `(channel, payload)` message.
+
+---
+
+## References
+
+- Implementation: `zcm/transport/generic_serial_transport.c`
+- Related: [Packetized Serial Transport](packetized_serial_transport.md)

docs/transports.md

@@ -66,6 +66,23 @@ be used to *summon* the transport:
 When no url is provided (i.e. `zcm_create(NULL)`), the `ZCM_DEFAULT_URL` environment variable is
 queried for a valid url.
 
+## Serial Transport Notes
+
+The `serial://` transport supports multiple modes:
+
+- Generic serial framing (default): [Generic Serial Transport](generic_serial_transport.md)
+- Packetized mode (set `pkt_size`): [Packetized Serial Transport](packetized_serial_transport.md)
+- Raw mode (set `raw=true`): passes bytes through without ZCM framing
+
+Common `serial://` options:
+
+- `baud=<int>` — baud rate (if omitted, baud setup is skipped)
+- `hw_flow_control=true|false` — enable/disable hardware flow control
+- `pkt_size=<int>` — enable packetized mode; packet payload size in bytes (must be in `[1,253]`)
+- `raw=true|false` — enable raw passthrough (mutually exclusive with `pkt_size`)
+- `raw_channel=<string>` — optional channel name used by the raw wrapper
+- `raw_size=<int>` — raw buffer size (default 1024)
+
 ## Custom Transports
 
 While these built-in transports are enough for many applications, there are many situations