Leave a Comment / Automation IntegratorsMACHINE VISION INTEGRATORS / By Mao Ye, P.E.

When you are setting up your PLC with Ethernet/IP devices, have you ever wondered why you must configure certain Ethernet/IP devices on the PLC, but not the other. This lies in the fundamental differences between Connected and Unconnected configuration for Ethernet/IP.

In this article, we will discuss on the differences between Connected and Unconnected Ethernet/IP devices, Explicit vs Implicit messaging, Scanner vs Adapter, and when one should be using versus another.

To start, CIP stands for Common Industrial Protocol, is a standard for organizing and sharing data for industrial devices. Many popular communication protocol in the industrial automation sector utilize CIP, which encompasses protocols such as CompoNet, EtherNet/IP, DeviceNet and ControlNet.

What makes CIP unique are (something you can honestly not care about)

  • Producer-Consumer Architecture
  • Managed jointly by ODVA and ControlNet International
  • Object-Oriented Approach

This brings us to EtherNet/IP, the most widely used protocol in today’s automation. EtherNet/IP became the most popular protocols for many reasons. For typical reason, it combines the most popular Ethernet with CIP, and can coexist with existing network. This allows EtherNet/IP to take advantage of the existing Ethernet infrastructures and gain huge popularity recently over other CIP based protocol. The second main reason (skip to next paragraph) is the push by AB to make it the most domain protocol. AB is the most domain player in the industrial automation market, and when AB PLC only speaks EtherNet/IP, you must adapt EtherNet/IP onto your hardware/software or be left out.

Anyway, in a EtherNet/IP network, each device has to have a unique IP address. Additionally, in a EtherNet/IP network, there has to be a master, and the rest are called slaves. A master can have as many slaves as possible, but a slave can only have one master. Some people refer to Master as Scanner, and Slave as Adapter. In short, when you heard Master in the domain of EtherNet/IP, think of Scanner that is constantly looking for slaves, or adapter.

Once the slaves and master are determined, we will then discuss ways of communicating between slaves and master.

For any communication protocol, there is a balance between speed and resource allocation. You want certain data to be communicated as fast as possible, those includes I/Os that are directly related to the physical world, such as time-critical sensors that directly affect the performance of the machine. On the other hand, if all traffics are communicated as fast as possible, there won’t be enough computation resource left to scan the PLC ladder logic.

Explicit messaging is used to communicate noncritical information, while Implicit messaging is used to communicate critical information. This goes back to the main reason why when you setup your devices on AB PLCs, you add modules like PowerFlex, but not HMI modules. Information on HMI modules are not time critical, while information on PowerFlex are.

There are exceptions, but most of the time, Explicit messaging are ‘unconnected’, while implicit messaging is ‘connected’. This means a connection is established between the master and the slave through a periodic scan, and any interruption to this connection will result in error. On the other hand, ‘unconnected’ devices only speak when needed. This is the reason why when you disconnect a HMI to the PLC, there is no error on the PLC. However, if you were to disconnect a ‘connected’ device to the PLC, the PLC will indicate an error in the modules section.

The main take away are the following:

  • There can be only one master, and as many slaves as you wish (depending on the available IP Address)
    • A master is usually the decision maker, which is mainly the PLC.
  • Not all devices can be a master.
    • Just because you have a bunch of EtherNet/IP devices, do not mean they can talk to each other. Most of the time you are born into either a master and a slave, and very few devices can do both.
  • Use Explicit messaging for noncritical data and Implicit messaging for critical data.
    • Too many implicit data will slow down everything.
  • IP in EtherNet/IP dose not stand for Internet Protocol, but Industrial Protocol.
    • But honestly who cares…

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