Example 3: SEND_REQ Function with Bit Read Request

At a Glance

The example shows the reading of 2 output bits from address 0 by a third-party device whose slave address is 5.

Question

The communication function is as follows:

SEND_REQ(ADDR('0.3.0.5'),16#9F,%MW300:50,%MW450:4,%MW400:50)

Before sending the function it is necessary to initialize the following words:

Words	Value of the Words	Description
%MW300	:= 16#0001	Corresponds to the read function of n output bits (byte 0 = 16#01, byte 1 = 16#00)
%MW301	:= 16#0296	Corresponds to the identification of the Modbus function
%MW302	:= 0	Reserved
%MW303	:= 16#0000	Address of the first bit to read (1)
%MW304	:= 16#0200	Number of bits to read (1)
%MW453	:=10 (bytes)	Length of the data to be transmitted in bytes

Legend:
(1)	The most significant bytes and the least significant bytes must be inverted.

Response

The slave response, contained in the %MW400:50 reception buffer, is of type:

Word	Value	Description
%MW400		:= 0 if action taken := 1 if action not taken
%MW401 if %MW400:= 0	:= 16#0001	Corresponds to the read function of n output bits (byte 0 = 16#01, byte 1 = 16#00)
%MW401 if %MW400:= 1	:= 16#0007	Incorrect request parameters
	:= 16#0004	Incorrect question parameters
	:= 16#0681	Byte 0 =16#80 + function code (16#01) Byte 1 = 16#06 Modbus error code (the slave is busy) 16#01 Modbus error code (the function is unknown) 16#03 Modbus error code (the data is invalid)
%MW402	:= 16#xx01	Contains the Modbus response data: Byte 0 = 16#01: length received in bytes (1 byte) Byte 1 = 16#xx: value of the bits For example, if bit 1 = 1 and bit 2 = 1, then byte 1 = 16#03