Profinet how to achieve real-time sex

At ordinary times we have heard of literary works to "originate from life and higher than life." Profinet is based on industrial Ethernet, with the words "from Ethernet and above Ethernet". So, how does profinet do "above Ethernet"?

To be more sharp than ordinary Ethernet, it is important to achieve real-time profinet, and to achieve real-time, first of all to solve the problem of certainty. As mentioned earlier, there is a problem of media access violation in normal Ethernet, which does not meet the "certainty" standard. So what are the general approaches to conflict resolution?

1. Since the Profibus bus and profinet are closely related, first look at how the bus is the solution. Profibus communication mode is master-slave, using "token" control, like intersection traffic lights function;
2. Can find ways to improve the common Ethernet CSMA/CD, or wireless local area network (WLAN) used by the CSMA/CA;
3. The use of channel multiplexing in the WAN is divided into frequency division multiplexing (FDM) and Time Division Multiplexing (TDM). Frequency division multiplexing divides a high-bandwidth channel into multiple channels with a smaller bandwidth, as if multiple lanes are divided on a single road, and time-division multiplexing transmits the channel multiplexing by allocating the communication packets to different timeslot.

Profinet Communication mode is the producer/consumer, each communication node is equal, can send and receive data at any time, so the bus of the kind of "token" control can not be applied, only in the latter two scenarios. This also makes up the aforementioned profinet different from the Profibus.

1 , Exchange technology

The common Ethernet device may have the data collision, namely the so-called CSMA/CD caused the data transmission delay, thus cannot guarantee the communication certainty, also cannot discuss the communication real-time nature. And the scope of the conflict domain mentioned earlier is size. Ordinary Ethernet uses a traditional switch or hub to form a local area network, and using a hub connection makes the entire LAN a conflict domain for data. Is it possible to improve on ordinary Ethernet from the point of view of conflict, in order to highlight the "advanced" of profinet? The answer is yes.

The profinet uses a switch that supports the IEEE 802.Q standard, or the PROFINET device itself integrates a dual network port with switching capability, allowing multiple devices and one device to use the same bandwidth (100M) so that the area of the data conflict shrinks to the area of the device itself. Because a port may have more than one user communicating with that port, a media access violation is possible, a switch that supports the IEEE 802.Q standard also takes precedence to schedule the send order, narrowing the conflict domain to a specific port, and drastically narrowing the data conflict area from two aspects. Ensure the certainty of communication.

For example, in the intersection of the two roads intersection, the four directions of the car can not be passed at the same time, through traffic lights to guide, the switch equivalent to the construction of an overpass, or even a multi-storey overpass, so that multiple directions of the vehicle can simultaneously pass. And in the local area network using the IEEE 802.Q standard-enabled switch, the communication network is built like the Shanghai-Nanjing Expressway, in one direction to divide a number of lanes (bidirectional 8 lanes), and stipulate that the cart in general must not occupy the inner lane, not only greatly improve the efficiency of the traffic, but also greatly improved the safety of the traffic.

2 , circular communication

Figure

, for the controller, equipment composed of profinet IO system, the way to work is the input, processing, output, the process of circulation, cycle. Processing is circular, T1 is the cycle of the PLC program scanning time, and the communication is also cyclic, T2 is the bus refresh time, it is said that each cycle (CYCLE1...N), the controller and the device will communicate. From this point of view, Profinet uses the concept of time-slot in the division multiplexing, which will greatly improve the communication efficiency of the whole network. Time slots This topic will be described in detail in a later chapter.

3 , thin protocol stacks

The previous mentioned Profinet IO system is circular communication, in a cycle (bus refresh time) do nothing more than send and receive, including the following process:

1. Create a variable in the producer's application;
2. The variable is sent to the communication partner (consumer) in the form of a profinet communication message;
3. Get the variable again in the consumer's application.

The time spent in this process is T1 to T5, which is explained in detail below.

Ordinary Ethernet communication uses the TCP/IP protocol, and the device processes the message at the MS level. For Profinet Real-time communication, the profinet uses a 100Mbps (Fast Ethernet) transfer rate to transmit a maximum of profinet packets of 1522 bytes, the transmission time T3 about 125μs, compared to the processing time using TCP/IP protocol messages , the transmission time T3 is very short. Therefore, to shorten the refresh time, the most effective way to streamline the communication protocol stack, that is, shorten T2 and T4. This simplification of information to accept the difficulty of the practice like I skipped the NBA Finals live broadcast, live screen and host of the explanation so that people can quickly, very direct, very "real-time" feel the scene of the atmosphere, to get immersive effect, let me after watching the game after the event, Triumphantly to the honest class of the fans classmates show off. If only text live, need to spend some time to imagine the game picture through the text comprehension, do you think it is necessary to skip class to be a hardcore fan?

In addition, profinet streamlines the structure of the communication stack, uses special messages and protocols for time-critical data, and does not use TCP/IP protocol stack processing, which greatly shortens communication time and ensures real-time communication. This is also a real-time message type, which will be described in detail later in this section.

All in all, the profinet uses a streamlined communication stack structure, 100Mbps Fast Ethernet, a switch or swap-enabled device, a packet with a priority identity VLAN tag, and a protocol mechanism for its cyclic communication, which is why profinet is real-time.

Profinet how to achieve real-time sex