How To Read/Write Data from 2 Modbus Slave Devices & Multiple Registers

 To read/write data from two Modbus RTU slave devices and access multiple registers, you need to configure your S7-300 or S7-1200 as the Modbus RTU Master. Below is a step-by-step guide:

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1. Hardware Requirements

For S7-300 (Modbus RTU Master)

• CP341 (with RS485 interface) + Modbus RTU Loadable Driver
  OR
• RS232/USB to RS485 Modbus Converter (like Anybus, Moxa, or Westermo)

For S7-1200 (Modbus RTU Master)

• CM1241 (RS485 Module) (if your PLC doesn't have a built-in RS485 port)

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2. Software Configuration in TIA Portal or STEP 7

Function Blocks for Modbus RTU

• MB_COMM_LOAD → Initializes the serial port.
• MB_MASTER → Sends requests to read/write Modbus registers.

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3. Steps to Read/Write Multiple Registers from Two Slaves

Step 1: Configure the Modbus Communication Port

• For S7-300, configure the CP341/CP340 module.
• For S7-1200, use MB_COMM_LOAD to initialize RS485.

MB_COMM_LOAD Parameters

Parameter	Description
PORT	RS485 (CM1241 or CP341)
BAUD	9600, 19200, etc.
PARITY	Even/Odd/None
STOP_BITS	1 or 2

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Step 2: Read Registers from Modbus Slaves

Use MB_MASTER to read multiple registers from each slave device.

MB_MASTER Parameters

Parameter	Description
REQ	Trigger signal (rising edge)
MODE	1 (Read Holding Registers)
DATA_ADDR	Start register address
DATA_LEN	Number of registers to read
SLAVE	Slave ID (e.g., 1 or 2)
DONE	High when data is received
ERROR	High if communication fails
DATA_PTR	Pointer to store data

Example

1. Slave 1 (ID = 1), Read Registers 40001-40010

   • SLAVE = 1
   • DATA_ADDR = 0
   • DATA_LEN = 10
   • Store data in DB10.DBB0

2. Slave 2 (ID = 2), Read Registers 40011-40020

   • SLAVE = 2
   • DATA_ADDR = 10
   • DATA_LEN = 10
   • Store data in DB10.DBB10

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Step 3: Write Data to Multiple Registers

To write to a Modbus slave, use MB_MASTER with MODE = 2 (Write Multiple Registers).

• Example: Write data to Slave 1 at address 40050-40055
  • SLAVE = 1
  • MODE = 2
  • DATA_ADDR = 50
  • DATA_LEN = 6
  • Source data from DB20.DBB0

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4. Execution Logic

• Call MB_COMM_LOAD once at startup.
• Continuously call MB_MASTER in a cyclic loop (OB1).
• Use a state machine or timer to alternate between Slave 1 and Slave 2.
• Use the DONE bit to trigger the next read/write operation.

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5. Summary

• Use MB_MASTER twice (once for each Slave).
• Configure correct Slave ID and Register Addresses.
• Ensure polling intervals are set properly.
• Implement error handling (timeout, retries).

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Next Steps

• Do you need a sample PLC program for Modbus RTU communication?
• What are the register addresses for your Modbus devices?