Siemens SIMATIC S7-200 System Manual page 375

Example of Programming the Modbus Master Protocol
The program will turn on outputs Q0.1 and Q0.2 if there is an error returned from the MBUS_MSG instruction.
Using the Modbus Protocol Library
Network 1
// Initialize and monitor the Modbus
// Master by calling MBUS_CTRL on
// every scan.
// The Modbus Master is set for 9600 baud
// and no parity. The slave is allowed 1000
// milliseconds (1 second) to respond.
Network 2
// On the first scan, reset the enable flags
// (M2.0 and M2.1) used for the two
// MBUS_MSG instructions.
Network 3
// When I0.0 changes from OFF to ON, set
// the enable flag for the first MBUS_MSG
// instruction (M2.0).
Network 4
// Call the MBUS_MSG instruction when the
// first enable flag (M2.0) is ON. The First
// parameter must be set for only the first
// scan that the instruction is enabled.
//This instruction writes (RW = 1) 4 holding
// registers to slave 2. The write data is taken
// from VB100 VB107 (4 words) in the CPU
// and written to address 40001 - - 40004 in
// the Modbus slave.
Network 5
//When the first MBUS_MSG instruction is
// complete (Done goes from 0 to 1), clear
// the enable for the first MBUS_MSG and set
// the enable for the second MBUS_MSG
// instruction.
//If Error (MB1) is not zero then set Q0.1 to
// show the error.
Network 6
// Call the second MBUS_MSG instruction
// when the second enable flag (M2.1) is ON.
// The First parameter must be set for only
// the first scan that the instruction is
// enabled.
//This instruction reads (RW = 0) 4 holding
// registers from slave 2. The data is read
// from address 40010 - - 40013 in the Modbus
// slave and copied to VB200 - - VB207
(4 words) in the CPU.
Network 7
//When the second MBUS_MSG instruction
// is complete (Done goes from 0 to 1), clear
// the enable for the second MBUS_MSG
// instruction.
//If Error (MB1) is not zero then set Q0.2 to
// show the error.
Chapter 12
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