Reference — Data Types

Modbus carries only bits (coils, discrete inputs) and 16-bit registers (holding, input). This adapter synthesizes richer types from those primitives. Every signal declares its type (and, for multi-register types, byte/word order); the adapter assembles registers into the value on read and splits the value back into registers on write. All conversion is in modbus_adapter/codec.py.

Tables

Table	FC (read)	Access	Element
coil	1	read/write	1 bit → bool
discrete	2	read-only	1 bit → bool
holding	3	read/write	16-bit register
input	4	read-only	16-bit register

Types

type	Registers	On-wire JSON (read)	Write input	Notes
bool	1 bit, or 1 register + bit	boolean	boolean	coil/discrete; or a single bit of a holding/input register via bit
int16	1	number	number (int)	signed
uint16	1	number	number (int)
int32	2	number	number (int)
uint32	2	number	number (int)
int64	4	number	number (int)
uint64	4	number	number (int)
float32	2	number	number	IEEE-754 single
float64	4	number	number	IEEE-754 double
string	count	string	string	UTF-8, null-padded; count registers (2 chars each)

Byte & word order (multi-register types)

A register is 16 bits (2 bytes). A 32/64-bit value spans 2–4 registers; devices differ in how the bytes are laid out. Two independent knobs cover the four real-world layouts:

• wordOrder — big (default): most-significant register first; little: registers reversed (word swap).
• byteOrder — big (default): high byte first within each register; little: bytes swapped.

wordOrder / byteOrder	Layout (32-bit ABCD)
big / big	ABCD
big / little	BADC
little / big	CDAB
little / little	DCBA

Scaling & bit extraction

• scale / offset (numeric types): read value = raw·scale + offset; write inverts it (raw = round((value − offset) / scale) for integer types). With no scale/offset an integer type stays an integer on the wire; with a scale it becomes a float.
• bit (0–15, holding/input + type: bool): publishes a single bit of a register as a boolean. Bit writes (read-modify-write) are not supported and are skipped with a warning.

Published identity

Each SouthboundSignalUpdate / read result carries:

• signal.name — the configured signal name (also the sanitized data-class channel token).
• signal.id — a stable canonical id, u<unitId>/<table>/<address>/<type> (e.g. u1/holding/24/float32).
• signal.address — the protocol-native handle: { unitId, table, address, type, wordOrder?, byteOrder?, bit?, count? }.

Value typing notes

• Integers use the full 64-bit range; consumers whose JSON parser uses IEEE-754 doubles (e.g. JavaScript) may lose precision above 2^53.
• bool is a JSON boolean; string a JSON string; everything else a JSON number.
• Modbus has no device-side timestamp, so sourceTs is null and serverTs is the adapter’s read time.