Example: MODBUS Communication Between Siemens S7-1200 and V20 Inverter

For MODBUS communication between S7-1200 and V20, the S7-1200 PLC needs to add a CM1241 RS485 communication module.

The V20 inverter is connected to the PLC via RS485 cable, using the standard MODBUS communication protocol. Through MODBUS communication, the PLC sends commands to the V20 inverter to start, stop, and adjust the frequency.

The specific wiring is shown in the figure below:

▲ Figure 1-1: Wiring of S7-1200 CB1241 and V20 Communication

Figure 1-2: Wiring of S7-1200 CM1241 and V20 Communication

The network topology diagram for communication between V20 and PLC, the matching terminal network termination resistors can be purchased from Siemens distributors. The order number for the termination resistor is: 6SL3255-0VC00-0HA0, the specific wiring is shown in the figure below:

▲ Figure 1-3: PLC and V20 Communication Wiring Diagram

Common MODBUS registers for V20, as shown in Table 2-1:

▲ Table 2-1: Common MODBUS Registers for V20

V20 Parameter Settings

Communication related parameter settings for the V20 inverter, as shown in Table 2-2:

▲ Table 2-2: Related Parameters for V20 Inverter

Note: You can also directly choose to connect macro CN011, after which the default settings are as shown in Table 2-3:

▲ Table 2-3: Parameter Settings for Connecting Macro CN011

The PLC programming is as follows:

Initialize the Modbus communication interface, ensuring that the baud rate and parity on the PLC side match the settings of the V20.

Note:

PORT: Communication port ID.

BAUD: Baud rate.

PARITY: Parity setting.

MB_DB: Background DB block 2 for MB_MASTER, write the required control word through register 40100.

Note:

1. The 10th bit of register 40100 must be set to 1 to allow PLC control.

2. Upon first power-up, write the hexadecimal 047E, and then write 047F to start the drive.

MB_ADDR: Modbus RTU slave address.

MODE: Mode selection, 0 for read, 1 for write.

DATA_ADDR: Register address.

DATA_LEN: Data length, depending on the accessed parameter (in words).

DATA_PTR: Pointer to the CPU’s storage address, from which data is read or written. 3. Write the speed set value into register 40101, where the hexadecimal 0-4000H corresponds to 0-100% of the P2000 base frequency.

4. Read two words at once through address 40110, which directly reads the status word (40110) and actual frequency (40111).

5. Read ramp-up time through address 40322.

6. Write ramp-up time through address 40322.