Modbus Communication

Description

Modbus is a client/server protocol for communication between devices connected in a network. Modbus devices communicate using a client/server technique in which only one device (client) can send request messages. The other devices in the network (server) respond, returning the data requested by the client or executing the action contained in the message sent. A server is a device connected to a network that processes information and sends the results to the client using the Modbus protocol.

The client device can send messages to individual servers, or to all the servers connected to the network (broadcast), whilst server devices can only respond to messages individually and to the client device. The Modbus standard employs the RTU code for data transmission.

Data Format (RTU)

The coding type used defines the structure of messages transmitted on the network and the way in which this information is deciphered. The coding type is usually chosen according to specific parameters (baudrate, parity, stop), plus certain devices only support specific coding types. Use the same coding type for all devices connected to a Modbus network.

The protocol uses the RTU binary method with the serial frame configured as follows:

• 8 bits for data

• Parity bits: none (configurable)

• 2 stop bits

Parameters can be changed by:

• Thermostat user interface

• Sending data via Modbus protocol directly to an individual device or in a broadcast using the address 0 (broadcast)

Modbus Commands Available and Data Areas

The following commands are implemented:

Configuration

The address of a device within a Modbus message is set via parameter Adr.

The address 0 is used to broadcast messages that all servers recognize. The server only responds to the individual requests.

The device configuration parameters are as follows:

The changes made to the parameters are taken into account after powering Off and On the cooling units.

Setting the Modbus Parameters from the User Interface

1. Press and hold SET for at least 5 seconds.

2. Scroll to PA1. Press SET to enter PA1 password using the keys and . Press SET to confirm.

3. Scroll to Adr.using the keys and and press SET. The default value is 1. Change the address value using the keys and . Press SET to confirm or press to cancel.

4. Scroll to PtY and press SET. The default value is E. Change the value using the keys and . Press SET to confirm or press to cancel.

5. Scroll to bAU. The default value is 96 (9600 bauds). Change the value using the keys and . Press SET to confirm or press to cancel the List of Modbus Resources.

List of Modbus Resources