Protocol

The NXT firmware speaks a simple request/reply protocol, documented by LEGO in the MINDSTORMS NXT Bluetooth Developer Kit. nxt-rs implements it in nxt_rs::protocol, without any I/O.

Telegrams

request:  [type] [opcode] [arguments…]
reply:    [0x02] [opcode] [status] [data…]

• All multi-byte values are little-endian.
• A telegram is at most 64 bytes.
• Status 0x00 means success. Other values are listed in Status codes.
• File and module names are 20-byte, NUL-padded ASCII fields (15.3 characters plus the terminator).

Framing

use nxt_rs::transport::frame;

let (buf, len) = frame(&[0x00, 0x0B]).unwrap();
assert_eq!(&buf[..len], &[0x02, 0x00, 0x00, 0x0B]);

Example: PlayTone

80 03 E0 01 F4 01
│  │  └─┬─┘ └─┬─┘
│  │    │     └─ duration 500 ms (0x01F4)
│  │    └─────── frequency 480 Hz (0x01E0)
│  └──────────── opcode 0x03 PlayTone
└─────────────── direct command, no reply

Example: GetOutputState reply

02 06 00 | 01 | 9C | 05 | 01 | 00 | 20 | 68 01 00 00 | 10 00 00 00 | F0 FF FF FF | 00 01 00 00
reply      port B    mode     ratio      tacho limit   tacho count   block tacho   rotation
   opcode     power -100  regulation  run state 0x20   360           16            -16           256
      status ok

Commands

The opcode tables are in Direct commands and System commands. Each command’s struct in nxt_rs::protocol::direct and nxt_rs::protocol::system documents its fields.

Output fields

Timing

• USB: a few milliseconds per round trip.
• Bluetooth: about 60 ms per round trip. The Bluetooth module needs time to switch between sending and receiving. Commands sent without a reply avoid half of that.