9.11 Settings for communications links

The Modbus communication link enables the device’s temperatures, status information, setpoints and malfunctions to be read by a higher-level automation system. Via the Modbus connection, Gebwell’s heat pumps can be controlled by sending a heat request in percentages (%) or a setpoint in Celsius degrees (°C).

The master-slave protocol means that one master device and at least one slave device are connected to the same bus at the same time. The master always begins data transfer. Slave devices do not communicate with each other, nor do they send requests or other messages to the master device unless the master specifically requests this. In a precise transmission, the master first sends a request to one particular slave device and waits for a response. The slave device has a unique address from 1 to 247.

Taurus heat pumps support the following Modbus versions: serial port (RTU) and Ethernet (TCP).

Modbus communication takes place using the RS-485 protocol. The recommended network structure is to connect the devices directly or in short branches to a single trunk cable. “Branch” refers to the distance between a device and the trunk cable. Branches should be as short as possible to avoid signal reflection. According to the Modbus guidelines, branches should never be longer than 20 metres. The network topology cannot be a star or ring. It also cannot be a trunk bus with connected stars or clusters. All such structures must be removed from the network.

Twisted pair cables should be used for Modbus RTU data transfer, and the cables must meet the requirements specified in the EIA-485 standard for double-wire systems.

The cable’s maximum standardised length depends on the data transfer rate and the cable’s properties, such as its characteristic impedance and thickness. The Modbus guidelines specify a maximum length of 1,000 metres if the cable’s cross-sectional area allows for this. At baud rates of 19,200 or more, the recommended characteristic impedance is 100 ohms.

For serial traffic, the required parameters are the baud rate, the parity and the stop bits. The data transfer settings of all devices on the same bus must be the same, and they must be set for each device individually. If the parameters are set incorrectly, the slave device will not be able to respond to requests sent by the master device.

The address specifies the slave device. Every device must have a unique address. The address can be from 1 to 247.

Modbus TCP communication takes place over the TCP/IP network using port 502. As a communications media, a standard Ethernet cable (e.g. CAT5 twin cable) is used. Master and slave devices must be within the same IP space.

See section Modbus registers.

Set the settings in Modbus module so that they match the settings of the higher-level automation system.