Example 4: READ_VAR Function for Reading Bits

General

Programming exchanges with Modbus slave devices is done with the help of the READ_VAR and WRITE_VAR communication functions only (the SEND_REQ function is not supported on the TER port).

Example with READ_VAR

Description of the objects used in the example:

Object		Description
%MW0.0		Transmission demand of the request
%M20		Request in progress
%MW100:10		Reception buffer
%MW200:203		Report zone:
	%MW200	Session and activity bit number (X0)
	%MW201	Error code
	%MW202	Time-out in units of 100 ms
%M30		Bit set to 1 after a successful exchange
%MW204		Counter of requests sent
%MW205		Counter of good requests
%MW206		Counter of bad requests
%MW207		Error code of the last bad request

Presentation of the program:

!(*Read of the bits %M0 to %M8 from the Nano at address 37 *)

IF %MW0.0 AND NOT %M20 THEN

%MW200:4:=0;%MW202:=50;SET %M20;

READ_VAR(ADDR('0.0.0.37'),'%M',0,8,%MW200:4,%MW100:10);

(*8 bits %M0..%M7 are read in the slave 37 and placed in the %MW100 word of the master*)

END_IF;

!(*Analysis of the results*)

IF %M20 AND NOT %MW200.0 THEN

INC %MW204;RESET %M20;RESET %MW0.0;

IF %MW201=0 THEN INC %MW205;SET %M30;

ELSE INC %MW206;%MW207:=%MW201;RESET %M30;

END_IF;