RTU framing¶

Modbus RTU is the binary, serial-line variant of Modbus. Each transaction is a single ADU (Application Data Unit) on the wire:

| slave_address (1 byte) | PDU (1-253 bytes) | CRC-16 LE (2 bytes) |

The PDU is function_code (1 byte) | body (variable). Frame boundaries are signaled by idle silence on the bus — at least 3.5 character-times of no activity between frames, and no more than 1.5 character-times of silence within a frame. At baud rates above 19200 the spec clamps both gaps to fixed values (1.75 ms and 0.75 ms respectively).

Why this matters¶

Modern operating systems cannot reliably deliver microsecond-accurate idle-gap detection on commodity USB-serial hardware. A naïve framer that watches for "no bytes for N microseconds" will drop frames or fuse them together depending on scheduling jitter.

anymodbus solves this two ways:

1. Tx-side, enforced. Before sending a request, we sleep until at least 3.5 character-times have passed since the last bus activity. This is honest and deterministic.
2. Rx-side, length-aware. For every standard function code, the response length is computable after at most 3 bytes (function code byte tells us if it's an exception, byte count tells us the body length). We read by length, not by timing. The timing-gap fallback fires only for unknown function codes.

See Length-aware framer for the state machine.

Wire defaults¶

• Spec-conformant: 8 data bits, even parity, 1 stop bit (8E1).
• Common in real devices: 8N1 (no parity, two stop bits to keep the bit count steady) or 8O1.

There is no portable default. The open_modbus_rtu(...) convenience opener takes baudrate= and parity= as required keyword arguments — you must pick. Mismatched parity silently drops every frame, so making this an explicit choice up front is worth the small ergonomic cost.

Inter-frame timing reference¶

(Character time = 11 bits / baud, accounting for start, 8 data, parity, stop.)