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Overview
This tutorial provides basic information about the Modbus Communication Protocol as well as a process for Modbus configuration in the Lookout SCADA software package. The detailed steps show how to set up the Modbus driver, test communication between the driver and a device, configure Lookout objects and expressions to reference Modbus registers on the device, and display data on the Lookout panel. This tutorial also includes basic troubleshooting steps to help you monitor and optimize Modbus communications.
General Modbus Information
The Modbus communications protocol is a widely-used serial communications standard for many Programmable Logic Controllers (PLCs) and other industrial control devices. Modbus can function in full-duplex RS232 mode or half-duplex RS485 mode, as well as newer implementation variants such as TCP/IP. The communication is master-slave / server-client driven.
When using serial, you can set up controllers to communicate on standard Modbus networks using either of two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode, etc) during configuration of each controller. The mode and serial parameters must be the same for all devices on a Modbus network. The selection of ASCII or RTU mode pertains only to standard Modbus networks. It defines the bit contents of message fields transmitted serially on those networks. It determines how information is packed into the message fields and decoded.
When controllers are set up to communicate on a Modbus network using ASCII (American Standard Code for Information Interchange) mode, each eight-bit byte in a message is sent as two ASCII characters. The main advantage of this mode is that it allows time intervals of up to one second to occur between characters without causing an error.
When controllers are set up to communicate on a Modbus network using RTU (Remote Terminal Unit) mode, each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII for the same baud rate. Each message must be transmitted in a continuous stream.
Modbus TCP/IP allows for devices to be configured and accessed on an Ethernet network while still using the characteristics of the Modbus protocol. So you can implement a server-client architecture and have multiple clients access the same devices simultaneously, versus the peer-to-peer or master-slave connection of RS232 or RS485.
Regardless of the type of protocol or mode you use, Lookout provides several Modbus drivers to implement the communications easily and quickly create an application. For more detailed information about the Modbus protocol, refer to the Modbus Organization web site.
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