 | Modbus Concepts |
The common language used by many controllers is the Modbus protocol. Modbus is an application layer messaging protocol that provides client/server communication between devices connected on different types of buses or networks. Vision Builder AI supports Modbus serial and Modbus TCP.
The Modbus protocol defines the process a controller uses to request access to another device, how it will respond to requests from other devices, and how errors will be detected and reported. The Modbus protocol establishes a common format for the layout and contents of message fields.
During communications on a Modbus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message.
Controllers communicate using a master-slave technique in which only one device, the master, can initiate transactions, or queries. The other devices, slaves, respond by supplying the requested data to the master, or by taking the action requested in the query. Typical master devices include host processors and programming panels. Typical slaves include programmable controllers.
Controllers can be set up to communicate on standard Modbus serial networks using either of two transmission modes: ASCII or RTU. Vision Builder AI supports both modes. Vision Builder AI also supports the TCP/IP physical layer.
Using the Modbus protocol, a device can read and write data to a set of registers.
Refer to the following table for descriptions of the register sets implemented in the Modbus protocol.
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Note For each of the four primary tables, the protocol allows individual selection of 65536 data items. |
| Tables | Description |
|---|---|
| Discrete Input | Single bit—Read only. This type of data can be provided by an I/O system. |
| Coils | Single bit—Read/Write. This type of data can be alterable by an application program. |
| Input Registers | 16-bit Word—Read only. This type of data can be provided by an I/O system. |
| Holding Registers | 16-bit Word—Read/Write. This type of data can be alterable by an application program. |