What is Modbus? Simply put, it is a method used to transmit serial information between electronic devices. Modbus is an open protocol that is free to use for commercial applications. It was developed by Modicon to control their programmable logic controllers (PLCs). The protocol is based on master-slave architecture.
In Modbus communication, the master broadcasts requests to the slave devices which return a response. Slave devices each have a distinct identifying address. The master can send messages and requests to individual slaves or to all of them simultaneously. When a response is required, the message is usually sent to a specific slave device. The slave address is the first byte in a request so the slaves know whether to listen or ignore the message.
Many industries use the Modbus protocol to establish serial communication between devices. It supports the TCP/IP and RS232/RS433/RS485 protocols. Also supported but less often seen is Modbus over UDP.
The serial implementations of Modbus are the ones most used in industry. There are two different variations of serial Modbus:
Modbus ASCII - uses ASCII characters for more human-readable communication. Checksums are derived using a longitudinal redundancy check (LRC). Messages begin with a colon (“:") and end with a newline character (CR/LF).
Modbus RTU is the variation most often employed. It presents data in a compact, binary format. A message is comprised of consecutive bytes with no spaces between them. Error checking is performed by appending a cyclic redundancy check (CRC) checksum to commands. Modbus RTU software identifies the end of a message by a 3.5 character space that acts as a delimiter.
If you are a software developer working with either Modbus ASCII or RTU, you could greatly benefit by using professional-grade Modbus software that can help you monitor and analyze data sent with the protocol.
Modbus Protocol Analyzer is a Modbus utility that can be instrumental when troubleshooting issues with driver or application development. It can also be used for testing and optimization efforts in a Modbus network.
This professional Modbus monitoring software enables you to:
A typical Modbus implementation is made up of a single controller or master and multiple sensors or slaves. They communicate with each other through a serial interface which is controlled by specialized serial software. Sometimes, several applications may need access to the port simultaneously. In order to accomplish this, you need to use Modbus splitter software.
Virtual COM Port Driver Pro by Eltima is an excellent solution for splitting Modbus data.
QModMaster is a Qt-based app that simulates a master Modbus device. It’s a free tool that provides the user with a bus monitor with which to analyze traffic and a graphical user interface for ease of use.
SimplyModbusMaster - is an application that enables you to simulate a Modbus master using either the ASCII or RTU format. All information is displayed using a single window.
Simply Modbus Slave - Modbus slave software monitors master requests and can also be used in the RTU or ASCII Modbus formats. It uses one window for configuring and displaying settings. Another window is used for the data.
Modbus Poll is another application that is used to simulate a Modbus master device. It can be very helpful for developers working with Modbus slave devices. With this program, you can monitor multiple Modbus slaves or data areas at the same time. Testing is facilitated by creating and sending strings of test data in the app’s test center and reviewing the slave response in hex format.