modbus.parse_rtu

modbus.parse_rtu

Function

Parse Modbus RTU response frames, check CRC and extract common response fields.

Syntax

Parameters

Parameters	Type	Required	Default value	Description
data	Bytes/Array/String	Yes	None	RTU response frame.

Return value

Type	Description
Object	Returns the RTU response parsing object. See "Return Object Fields" below for fields.

Return object fields

Field	Type	Must exist	Description
protocol	String	Yes	Protocol type, fixed to `rtu`.
unit_id	Int	Yes	RTU slave address.
crc	Int	Yes	The CRC16 value carried at the end of the response frame, parsed with the low byte first.
crc_expected	Int	Yes	The CRC16 value recalculated based on the response payload.
valid_crc	Bool	Yes	Whether `crc` is equal to `crc_expected`. Parsing of the PDU continues even if the CRC does not match.
function_code	Int	Yes	Response function code. The exception response will have the highest bit, for example `0x83`.
exception	Bool	Yes	Whether it is a Modbus exception response.
exception_code	Int/Empty	No	The exception code of the exception response; only written when `exception` is `true`, and `empty` when the exception code is missing in the response.
data	Bytes	Yes	The original data in the PDU after removing the function code. The exception response contains exception code bytes.
byte_count	Int	No	Number of data bytes in the read coil, read discrete input, read holding register, read input register response. Other responses do not write this field.
coils	Array	No	Read coils or read a parsed Boolean array of discrete input responses. Other responses do not write this field.
registers	Array	No	Reading a holding register or reading an input register responds to a parsed 16-bit unsigned register integer array. Other responses do not write this field.
address	Int	No	Write single coil, write single register, write multiple coils, write multiple registers Confirm the starting address in the response. Other responses do not write this field.
value	Int	No	The quantity or value to write in the confirmation response. Function code 5/6 represents the written value, and function code 15/16 represents the written quantity.

Code Example

Notes

The data will not be discarded when the CRC does not match, valid_crc returns false, and the script can decide whether to discard it.
Exception responses will return exception: true and exception_code.