Modbus is a physical hardware technology and communication between devices. If two devices can communicate physically, including serial port and TCP, they can communicate with each other.
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The ultimate goal of developing OPC is to establish a set of data transmission specifications in the field of industrial control. The existing OPC specifications cover the following five areas.
① Online data monitoring. OPC realizes efficient and flexible data reading and writing between application programs and industrial control equipment.
② Alarm and event handling. OPC provides a mechanism to send notifications to OPC customers when the OPC server is abnormal and when the OPC server sets events.
③ Historical data access. OPC realizes the reading, operation and editing of historical database.
④ Remote data access. With the help of Microsoft's DCOM (Distributed Component Object Model) technology, OPC has achieved high-performance remote data access capability.
⑤OPC also includes security, batch processing and historical alarm event data access.
Modbus is developed for PLC communication at the end of 1970. These limited data types can be understood by PLC at that time, but they do not support large binary object data.
For nodes, there is no standard method to find the description information of data objects. For example, it is determined whether the register data represents a temperature between 30- 175 degrees.
Because Modbus is a master/slave protocol, there is no way to require devices to "report anomalies" (except TCP/IP protocol based on Ethernet, which is called open-mbus)-the master node must query each node device circularly to find out the data changes.
In applications where bandwidth may be important, this method will consume bandwidth and network time in applications, such as low-speed wireless links.
Modbus can only handle 247 addresses on a data link, which limits the number of devices that can be connected to the main control site (excluding Ethernet TCP/IP again).
Modbus transmission is carried out by buffering data between remote communication devices, which limits the continuity of communication and avoids the problem of buffering loopholes in transmission.
Modbus protocol itself does not provide security against unauthorized commands or intercepted data.
Baidu encyclopedia -OPC
Baidu Encyclopedia -Modbus Communication Protocol