Ethernet/IP 學習筆記六

EtherNet/IP defines two primary types of communications: explicit and implicit (Table 1)

Explicit Messaging in general has a request/reply (or client/server) nature. This type of communication is used for non-real-time data, normally for information. Explicit messages include a description of their meaning (expressed explicitly), so the transmission is less efficient, but very flexible. It may be used by an HMI to collect data, or by a device programming tool. In CIP terms, with Explicit Messaging you request a service of a particular object, e.g., a read or a write service. For EtherNet/IP, Explicit Messaging uses TCP. Explicit Messaging can be done with or without prior establishment of a CIP connection.

Implicit Messaging is also often referred to as “I/O” and is time-critical in nature. Typically this type of communication is used for real-time data exchange, where speed and low latency are important. Implicit messages include very little information about their meaning, so the transmission is more efficient, but less flexible than explicit. The interpretation of the transmitted data is fast. With Implicit Messaging you establish an association (a “CIP connection”) between two devices and produce the Implicit Messages according to a predetermined trigger mechanism, typically at a specified packet rate. The devices both know and agree on the data formats they will use (i.e., the format is “implied”). For EtherNet/IP, Implicit Messaging uses UDP and can be multicast or unicast.

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