modbus.parse_tcp

modbus.parse_tcp

Function

Parse Modbus TCP response frames, verify MBAP length and extract common response fields.

Syntax

Parameters

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

Return value

Type	Description
Object	Returns the TCP 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 `tcp`.
transaction_id	Int	Yes	MBAP transaction number.
protocol_id	Int	Yes	MBAP protocol number, standard Modbus TCP is usually `0`.
length	Int	Yes	MBAP length field, representing the number of bytes of `unit_id + pdu`.
valid_length	Bool	Yes	Whether the input byte length is exactly the same as MBAP length. When the input is longer, the current frame is still parsed according to MBAP length.
unit_id	Int	Yes	MBAP unit ID.
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

valid_length indicates whether the input length is exactly consistent with MBAP length; when the input is longer, the current frame will still be parsed according to MBAP length.