Communication between S7-1200 and G120 inverters based on USS communication protocol
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**Figure 1**: Control system principle and wiring diagram.
2. **Hardware Requirements**
The S7-1200 PLC has three CPU models: S7-1211C, S7-1212C, and S7-1214C. All of them can communicate with the G120 inverter via the CM1241 RS485 module using the USS protocol.
**PLC hardware used in this example includes:**
- PM1207 power supply (6EP1 332-1SH71)
- S7-1214C (6ES7 214-1BE30-0XB0)
- CM1241 RS485 (6ES7 241-1CH30-0XB0)
- Simulator (6ES7 274-1XH30-0XA0)
**G120 inverter hardware used in this example includes:**
- SINAMICS G120 PM240 (6SL3244-0BA20-1BA0)
- SINAMICS G120 CU240S (6SL3224-0BE13-7UA0)
- SIEMENS Motor (1LA7060-4AB10)
- Operator panel (XAU221-001469)
- USS communication cable (6XV1830-0EH10)
3. **Software Requirements**
The programming software used is Step7 Basic V10.5 (6ES7 822-0AA0-0YA0). We will now explain how to configure the USS communication between the S7-1214C and the G120 drives in Step7 Basic V10.5.
4. **Configuration Steps**
**4.1 PLC Hardware Configuration**
Start by creating a new project in Step7 Basic V10.5 as shown in Figure 2.
**Figure 2**: New S7 1200 project.
In the hardware configuration, add the S7-1214C CPU and the CM1241 RS485 module.
**Figure 3**: S7 1200 Hardware Configuration.
Set the IP address of the Ethernet interface and connect the PG to the PLC.
**Figure 4**: Settings for the S7 1200 IP Address.
**4.2 G120 Parameter Setting**
Below is a table of key parameters set on the G120 inverter:
| Serial No. | Feature | Parameter | Value |
|------------|----------------------------|-----------|-----------|
| 1 | Factory reset | P0010 | 30 |
| 2 | Factory reset | P970 | 1 |
| 3 | Quick start | P0010 | 1 |
| 4 | Motor rated voltage | P0304 | 380V |
| 5 | Motor rated power | P0307 | 5.5KW |
| 6 | Motor rated frequency | P0310 | 50Hz |
| 7 | Motor rated speed | P0311 | 1350r/min |
| 8 | USS command source | P0700 | 5 |
| 9 | Frequency setting source | P01000 | 5 |
| 10 | Min motor frequency | P1080 | 0.0Hz |
| 11 | Max motor frequency | P1081 | 50.0Hz |
| 12 | Start ramp time | P1120 | 10.0S |
| 13 | Delay ramp time | P1121 | 10.0S |
| 14 | End quick start | P3900 | 1 |
| 15 | Expert mode | P0003 | 3 |
| 16 | Reference frequency | P2000 | 50.0Hz |
| 17 | USS baud rate | P2010 | 9 |
| 18 | USS slave address | P2011 | 1 |
| 19 | USS PZD length | P2012 | 2 |
| 20 | USS PKW length | P2013 | 4 |
| 21 | Communication monitoring | P2014 | 0 |
| 22 | Save to E2PROM | P0971 | 1 |
| 23 | Expert mode | P0003 | 3 |
| 24 | Parameter mode | P0010 | 30 |
| 25 | Transfer parameters | P0802 | 1 |
**Table 1**: G120 inverter parameter settings.
*Note: Parameters 17–20 must be consistent between the PLC and the inverter. Otherwise, communication errors may occur.*
5. **USS Communication Principle and Programming Implementation**
**5.1 Basic Principles of S7-1200 and G120 Communication via USS**
The S7-1200 provides a dedicated USS library for communication, including function blocks such as **USS_DRV**, **USS_PORT**, **USS_RPM**, and **USS_WPM**. These are essential for controlling and reading data from the G120 inverter.
- **USS_DRV**: Main block for data exchange with the inverter.
- **USS_PORT**: Interface for the communication module.
- **USS_RPM**: Reads inverter parameters.
- **USS_WPM**: Writes parameters to the inverter.
Each S7-1200 CPU can support up to 3 communication modules, and each CM1241 RS485 supports up to 16 inverters. Thus, up to 48 inverters can be connected.
**5.2 Programming for USS Communication**
**1. Programming the USS_PORT Function Block**
The USS_PORT function block handles communication on the USS network. It is crucial for defining the communication module, baud rate, and data block. For example:
- **PORT**: Specifies the communication module.
- **BAUD**: Matches the inverter's P2010 setting.
- **USS_DB**: Data block for communication.
- **ERROR/STATUS**: Indicates communication status.
The communication interval depends on the baud rate. For example, at 57600 bps, the minimum interval is 36.1ms, and the timeout should be set accordingly.
**2. Programming the USS_DRV Function Block**
This block controls the inverter’s operation and reads its status. Key parameters include:
- **RUN**: Start command.
- **OFF2/OFF3**: Stop commands.
- **SPEED_SP**: Speed setpoint.
- **DRIVE**: Inverter station address.
- **PZD_LEN**: Process word length.
Note: The PKW length must be set to 4 for proper communication.
**3. Programming the USS_RPM Function Block**
Used to read inverter parameters. Ensure that data types match correctly, and avoid initial values of 0 to prevent errors.
**4. Programming the USS_WPM Function Block**
Used to write parameters to the inverter. Again, ensure correct data types and avoid zero initial values.
**5.3 Debugging Tips**
When multiple inverters are connected, ensure that each USS_DRV uses the correct station address. Avoid simultaneous read/write operations. If issues arise, adjust based on site conditions and network setup.