RS485 & Modbus
What Is RS485 And Modbus?
Many of our products can be equipped with RS485 terminals either as standard or an optional extra. RS485 is a physical standard for serial communication. In it's simplest form it comprises 2 wires which allow the exchange of data between two or more devices. If a device is RS485 enabled it will be equipped with internal electronics for sending and receiving data signals.
Figure 1: Sample Wiring On The Physical Layer (FieldLogger Master)
Modbus is a widely used serial communication protocol for the exchange of data between devices. It provides a standard method of message framing. The Modbus RTU format follows the commands/data with a CRC error check to ensure the reliability of data. Modbus RTU is the most widely used protocol and can be found in Variable Speed Drives, Instruments, Sensors, Programmable Logic Controllers, Human Machine Interfaces and more.
In summary RS485 is the physical connection between devices. Modbus is the common language used to exchange data.
Difference Between RS485 Master And Slave
You may notice some of our products such as Data Loggers are capable of operating as a Master. The Master device initiates data requests from Slaves, who then respond to the request by sending their data back. Our RS485 equipped temperature controllers function as Slave only, meaning they can only respond to requests.
RS485 Masters can process data from slaves either by recording it i.e. a Data Logger or using it to apply some logic i.e. a PLC (programmable logic controller).
RS485 Slaves are used for a variety of reasons:
- Simplicity: Minimise wiring on longer cable runs (2 wires only can send and receive a variety of data points and commands)
- Integration: Centralising process control (one PLC can control many serial devices)
- Visualisation: Centralising data from remote points (displaying process information on HMI, SCADA, DCS)
In some instances the terms Client (Master) and Server (Slave) can be found in technical literature. A basic overview can be seen below:
Master or Client - There can only be one e.g. one main Data Logger, initiates data requests
Slave or Server - There can be multiples e.g. several RS485 Instruments, respond to data requests
Figure 2: FieldLogger (Master), Mixed IO Modules (Slaves)
Find Out More About RS485
You can read more about RS485 and serial networks by downloading the below document from Novus Automation. Some key takeaways:
"Unterminated networks are low power, low cost, and simple to build. The disadvantage, of course, is that data rates must be quite slow or cable length must be short for the network to operate reliably. A network up to 100 m long and communicating at 19,200 bps is expected to be reliable even when no termination resistors are installed."