Functions
uint16_t	Crc16 (const uint8_t *data, std::size_t length) noexcept
	Compute the Modbus-RTU CRC-16 over a byte buffer.

std::size_t	AppendCrc (uint8_t *frame, std::size_t length) noexcept
	Append the CRC-16 to `frame[length..length+1]` and return new length.

bool	CheckCrc (const uint8_t *frame, std::size_t length) noexcept
	Verify the CRC at `frame[length-2..length-1]` matches the body.

std::size_t	BuildReadHolding (uint8_t unit_addr, uint16_t reg_addr, uint16_t reg_count, uint8_t *out, std::size_t out_capacity) noexcept
	Build a Function-03 (Read Holding Registers) request.

std::size_t	BuildWriteSingle (uint8_t unit_addr, uint16_t reg_addr, uint16_t value, uint8_t *out, std::size_t out_capacity) noexcept
	Build a Function-06 (Write Single Register) request.

std::size_t	BuildWriteMultiple (uint8_t unit_addr, uint16_t reg_addr, const uint16_t regs, uint8_t count, uint8_t out, std::size_t out_capacity) noexcept
	Build a Function-16 (Write Multiple Registers) request.

constexpr uint32_t	InterFrameGapUs (uint32_t bps) noexcept
	Minimum idle time t3.5 between two Modbus-RTU frames, in microseconds.

constexpr uint32_t	InterFrameGapMs (uint32_t bps) noexcept
	`InterFrameGapUs(bps)` rounded up to whole milliseconds (for `UartInterface::delay_ms()`).

bool	IsExceptionReply (const uint8_t *frame, std::size_t length) noexcept

DriverError	ParseException (const uint8_t frame, std::size_t length, uint8_t expected_addr, uint8_t expected_fn, uint8_t code) noexcept
	Parse a 5-byte exception reply (Modbus Application Protocol V1.1b3, section 7).

DriverError	ParseReadHolding (const uint8_t frame, std::size_t length, uint8_t expected_addr, uint8_t expected_count, uint16_t regs) noexcept
	Parse a Function-03 response into `regs[]`.

DriverError	ParseWriteSingleEcho (const uint8_t frame, std::size_t length, const uint8_t request, std::size_t req_len) noexcept
	Validate the echo of a Function-06 (Write Single) request against the just-sent request.

DriverError	ParseWriteMultipleEcho (const uint8_t *frame, std::size_t length, uint8_t expected_addr, uint16_t expected_reg_addr, uint16_t expected_count) noexcept
	Validate the response of a Function-16 (Write Multiple) request.

Variables
constexpr uint32_t	kRtuBitsPerChar = 11
	Bits per RTU character used for the timing: start + 8 data + parity/stop + stop (section 2.5.1).

constexpr uint32_t	kFixedGapAboveBps = 19200
	Above this baud rate the spec fixes t3.5 at 1750 us instead of 3.5 character times.

constexpr uint32_t	kFixedGapUs = 1750
	The fixed t3.5 used above 19200 baud, in microseconds.

constexpr std::size_t	kExceptionFrameLength = 5
	Length of a Modbus-RTU exception reply: address, function \| 0x80, exception code, CRC-16.

constexpr std::size_t	kReplyHeaderLength = 3
	Number of reply bytes a receiver reads before it can tell an exception reply (bit 7 of the function code set) from a normal one: address, function code, and one more byte.

Function Documentation

◆ AppendCrc()

std::size_t alicat_basis2::modbus::AppendCrc	(	uint8_t *	frame,
		std::size_t	length
	)

inlinenoexcept

Append the CRC-16 to frame[length..length+1] and return new length.

◆ BuildReadHolding()

std::size_t alicat_basis2::modbus::BuildReadHolding	(	uint8_t	unit_addr,
		uint16_t	reg_addr,
		uint16_t	reg_count,
		uint8_t *	out,
		std::size_t	out_capacity
	)

inlinenoexcept

Build a Function-03 (Read Holding Registers) request.

Returns the total frame length (header + payload + CRC).

◆ BuildWriteMultiple()

std::size_t alicat_basis2::modbus::BuildWriteMultiple	(	uint8_t	unit_addr,
		uint16_t	reg_addr,
		const uint16_t *	regs,
		uint8_t	count,
		uint8_t *	out,
		std::size_t	out_capacity
	)

inlinenoexcept

Build a Function-16 (Write Multiple Registers) request.

regs carries count 16-bit values in big-endian-on-the-wire order.

◆ BuildWriteSingle()

std::size_t alicat_basis2::modbus::BuildWriteSingle	(	uint8_t	unit_addr,
		uint16_t	reg_addr,
		uint16_t	value,
		uint8_t *	out,
		std::size_t	out_capacity
	)

inlinenoexcept

Build a Function-06 (Write Single Register) request.

◆ CheckCrc()

bool alicat_basis2::modbus::CheckCrc	(	const uint8_t *	frame,
		std::size_t	length
	)

inlinenoexcept

Verify the CRC at frame[length-2..length-1] matches the body.

◆ Crc16()

uint16_t alicat_basis2::modbus::Crc16	(	const uint8_t *	data,
		std::size_t	length
	)

inlinenoexcept

Compute the Modbus-RTU CRC-16 over a byte buffer.

Parameters

data	Pointer to the bytes to checksum.
length	Number of bytes.

Returns: Little-endian CRC-16 (low byte first when transmitted).

◆ InterFrameGapMs()

constexpr uint32_t alicat_basis2::modbus::InterFrameGapMs ( uint32_t bps )

constexprnoexcept

InterFrameGapUs(bps) rounded up to whole milliseconds (for UartInterface::delay_ms()).

◆ InterFrameGapUs()

constexpr uint32_t alicat_basis2::modbus::InterFrameGapUs ( uint32_t bps )

constexprnoexcept

Minimum idle time t3.5 between two Modbus-RTU frames, in microseconds.

3.5 character times of 11 bits at bps, rounded up; a fixed 1750 us above 19200 baud (Modbus over Serial Line V1.02, section 2.5.1.1). bps == 0 is treated as "unknown" and gets the 4800-baud (longest) gap.

◆ IsExceptionReply()

bool alicat_basis2::modbus::IsExceptionReply	(	const uint8_t *	frame,
		std::size_t	length
	)

inlinenoexcept

Returns: true when frame (at least 2 bytes) is an exception reply (function code bit 7 set).

◆ ParseException()

DriverError alicat_basis2::modbus::ParseException	(	const uint8_t *	frame,
		std::size_t	length,
		uint8_t	expected_addr,
		uint8_t	expected_fn,
		uint8_t *	code
	)

inlinenoexcept

Parse a 5-byte exception reply (Modbus Application Protocol V1.1b3, section 7).

Parameters

	expected_fn	Function code of the request (the reply carries `expected_fn \| 0x80`).
[out]	code	Exception code, written only when the frame is a valid exception reply.

Returns: ModbusException for a valid exception reply; InvalidFrame for a wrong length, BadCrc for a CRC mismatch, ResponseMismatch for another address or function.

◆ ParseReadHolding()

DriverError alicat_basis2::modbus::ParseReadHolding	(	const uint8_t *	frame,
		std::size_t	length,
		uint8_t	expected_addr,
		uint8_t	expected_count,
		uint16_t *	regs
	)

inlinenoexcept

Parse a Function-03 response into regs[].

expected_count is the number of 16-bit registers the master asked for.

◆ ParseWriteMultipleEcho()

DriverError alicat_basis2::modbus::ParseWriteMultipleEcho	(	const uint8_t *	frame,
		std::size_t	length,
		uint8_t	expected_addr,
		uint16_t	expected_reg_addr,
		uint16_t	expected_count
	)

inlinenoexcept

Validate the response of a Function-16 (Write Multiple) request.

◆ ParseWriteSingleEcho()

DriverError alicat_basis2::modbus::ParseWriteSingleEcho	(	const uint8_t *	frame,
		std::size_t	length,
		const uint8_t *	request,
		std::size_t	req_len
	)

inlinenoexcept

Validate the echo of a Function-06 (Write Single) request against the just-sent request.

The slave must echo the full request.

Variable Documentation

◆ kExceptionFrameLength

constexpr std::size_t alicat_basis2::modbus::kExceptionFrameLength = 5

inlineconstexpr

Length of a Modbus-RTU exception reply: address, function | 0x80, exception code, CRC-16.

◆ kFixedGapAboveBps

constexpr uint32_t alicat_basis2::modbus::kFixedGapAboveBps = 19200

inlineconstexpr

Above this baud rate the spec fixes t3.5 at 1750 us instead of 3.5 character times.

◆ kFixedGapUs

constexpr uint32_t alicat_basis2::modbus::kFixedGapUs = 1750

inlineconstexpr

The fixed t3.5 used above 19200 baud, in microseconds.

◆ kReplyHeaderLength

constexpr std::size_t alicat_basis2::modbus::kReplyHeaderLength = 3

inlineconstexpr

Number of reply bytes a receiver reads before it can tell an exception reply (bit 7 of the function code set) from a normal one: address, function code, and one more byte.

◆ kRtuBitsPerChar

constexpr uint32_t alicat_basis2::modbus::kRtuBitsPerChar = 11

inlineconstexpr

Bits per RTU character used for the timing: start + 8 data + parity/stop + stop (section 2.5.1).

Functions

Variables

Function Documentation

◆ AppendCrc()

◆ BuildReadHolding()

◆ BuildWriteMultiple()

◆ BuildWriteSingle()

◆ CheckCrc()

◆ Crc16()

◆ InterFrameGapMs()

◆ InterFrameGapUs()

◆ IsExceptionReply()

◆ ParseException()

◆ ParseReadHolding()

◆ ParseWriteMultipleEcho()

◆ ParseWriteSingleEcho()

Variable Documentation

◆ kExceptionFrameLength

◆ kFixedGapAboveBps

◆ kFixedGapUs

◆ kReplyHeaderLength

◆ kRtuBitsPerChar