Architecture

The driver provides four Modbus modules that support five Modbus network types. Three networks serve as clients, where the host (controller and station) act as a Modbus master device. In the other two networks, the controller and station serve as servers (slaves), where the station exposes Modbus data and responds to Modbus queries. All Modbus networks use the standard Framework network architecture. For more information, refer to the Niagara Drivers Guide.

The Modbus protocol defines the message structure and format used in communication transactions. Modbus devices communicate using a master-slave relationship in which only the master device can initiate a communication transaction. A Modbus network supports only one master device. For most integrations (modbusAsync, modbusTcp), the remote host serves as the master device. Other devices function as Modbus slaves.

Modbus provides two slave components: modbusSlave or modbusTcpSlave. Either slave station can act as a server. Usage of these components is expected to be infrequent. When used, basic Modbus principles remain the same.

Similar to other Framework integrations, the Modbus driver uses proxy points to provide monitoring and control. To help clarify station configuration, this document describes items specific to Framework components.

Async networks

Figure 1. ModbusAsyncNetwork

This network requires a serial port (typically, RS-485) on the host platform, which connects to a Modbus RTU (Remote Terminal Unit) or ASCII network and functions as the Modbus Master. Lighting Control, Meter, VSD Control and I/O Device function as slaves on the same network.

Communications rates are typically at 9600 baud, and the network transmission mode (protocol) may be either Modbus RTU or Modbus ASCII (either one is supported). If Modbus RTU over RS-485, up to 31 slave devices may be attached—or more, if repeaters are used. The address range for Modbus devices on a serial network is from 1 to 247, however (as noted), networks are typically smaller. Depending on the number of available COM ports, a host may support multiple ModbusAsyncNetworks.

The station acts as Modbus master to all other Modbus devices on the attached COM port. A ModbusAsyncDevice represents each child device, and has a unique Modbus address (1 to 247), as well as other Modbus config data and starting addresses for Modbus data items (coils, inputs, input registers, holding registers). A Modbus network usually has many child ModbusAsyncDevices.

TCP/IP networks

Figure 2. ModbusTcpNetwork

This Modbus network automatically binds to the TCP/IP setup of the host platform’s Ethernet LAN adapter. The host appears as the Modbus Master on a network of Modbus TCP slave devices. The Network connectivity protocol is Ethernet/IP.

In addition to specifying the TCP software port used (typically 502), various global properties on the network’s property sheet are specific to Modbus TCP. For example, you can configure the default order for float and long numeric data (overrideable within each child device).

A ModbusTcpDevice represents each child device, and has a unique IP address, as well as other Modbus config data and starting addresses for Modbus data items (coils, inputs, input registers, holding registers). There are typically many child ModbusTcpDevices.

A ModbusTCPGateway is a network-level object that also represents a particular device: a Modbus TCP-to-Serial gateway, where this device has an IP address reachable by the station. On the gateway’s far side are serially-connected Modbus devices (typically Modbus RTU via RS-485). Child device objects under the gateway (network) represent each serial Modbus device (RTU or ASCII).

TCP/IP gateway networks

Figure 3. ModbusTcpGateway

This type of network includes a Modbus TCP/Serial gateway between the Modbus Master and the serial Modbus devices.

In addition to the IP address and TCP port used by the gateway, global properties on the network’s property sheet specific to Modbus configure TCP gateway devices. For example, you can configure the default order for float and long numeric data (overrideable within each child device).

The station acts as a Modbus master to the serially-connected Modbus devices on the gateway’s far side. A ModbusTcpGatewayDevice represents each child device, and has a unique Modbus address (1 to 247), as well as other Modbus config data and starting addresses for Modbus data items (coils, inputs, input registers, holding registers). There are typically many child ModbusTcpGatewayDevices.

Slave networks

Figure 4. ModbusSlaveNetwork

This type of network requires a serial port on the host platform, which connects to a Modbus RTU or ASCII network where the station functions as a Modbus server (slave) to queries received from a serially-connected Modbus master device. In each uniquely-addressed ModbusSlaveDevice you specify the ranges for available Modbus data items (coils, inputs, input registers, holding registers). In some cases, only a single child ModbusSlaveDevice represents the station.

TCP/IP slave networks

Figure 5. ModbusTcpSlaveNetwork

This type of network automatically binds to the TCP/IP setup of the host platform’s Ethernet IP LAN adapter, assumes the IP address of the station, and functions as a server. The Host (Modbus Slave) can appear as multiple devices.

The station acts as a Modbus server (slave) to queries received from a Modbus TCP master device. A ModbusTcpSlaveDevice represents each uniquely-addressed child device. You specify ranges for available Modbus data items (coils, inputs, input registers, holding registers). In some cases, only a single child ModbusTcpSlaveDevice represents the station.