As long as all the devices communicate with Modbus on compatible connections, the Modbus system will work. This is why Modbus can be used over common serial communications methods such as RS-232, RS-485, USB connections, etc. The standard does not define the physical layer the wiring and electrical signals used between the Modbus devices. The physical-layer, the mechanical wiring and electrical specifications for communicating with Modbus, can vary. ' MODBUS is an application-layer messaging protocol… between devices connected on different types of buses or networks.' As an application-layer protocol, the Modbus specification defines how two devices communicate in terms of what an information request looks like, how a meter responds, where data is stored, etc. For example, while a traditional level transmitter may communicate a single process variable to another device on a 4-20 mA output, a Modbus level transmitter might be able to communicate top level, interface level, temperature, and alarm data all on a 3-wire RS-485 network (more on that later). The server executes the required actions and responds to the client.2 The ability to communicate complicated requests and information mean much more can be done with this type of digital communication than with a single analog signal. One device is a client, which initiates a request to another device, the server. Modbus ' provides client/server communication between devices.' This means that Modbus allows two or more devices to communicate data to each other. Modbus is a way for two devices to communicate digitally. To understand what Modbus is, look at each statement in that definition. 'MODBUS is an application-layer messaging protocol… It provides client/server communication between devices connected on different types of buses or networks.' 1 This article will discuss what Modbus communications is, some popular ways to use it, and explain some of the terms often associated with using it.

Precision Digital's PROVU® Series is a good example of this.

One feature common on more advanced digital panel meters is Modbus serial communications, often with RS-485, RS-232, or USB connections available. Digital panel meters are available with a wide range of features and options.

Modbus rtu serial communication protoco