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| photo by siemens |
PROFINET, the next-generation data communication in the field of automation, offers greater flexibility, efficiency, and speed.
But how exactly does PROFINET work?
What exactly happens between the controller, I/O devices, and switches?
01
Let's start with PROFIBUS.
What? Aren't we supposed to be discussing PROFINET? Why bring up PROFIBUS?
In fact, introducing PROFIBUS here is to compare the differences in their "traffic rules" for communication.
In a control system, there are master stations and slave stations. The purpose of communication is to have the slave stations "deliver goods" to the master station, where the "goods" are data packets. Just like delivering packages by courier, it needs to be timely and accurate. This is where the traffic rules come into play.
For PROFIBUS communication, the connection between the master and slave stations is unidirectional. So, how does PROFIBUS data transmission work? We can imagine it as a highway with a single-direction lane.
As a result, only one vehicle can travel in one direction at any given time. It starts from the factory, reaches the first supplier, returns, then moves to the second supplier, and returns again. This process continues until it has visited all the suppliers.
02
The Traffic Rules in PROFINET
Unlike PROFIBUS, PROFINET boasts bidirectional lanes, with connection topologies including star, ring, and linear. PROFINET is relatively more efficient and flexible, enabling the rapid transmission and reception of large amounts of data.
03
Update Rate
The update rate refers to the time interval between the transmission of messages between the controller and I/O devices. It is akin to a supplier delivering goods to a manufacturing plant at regular intervals.
Here, we introduce the concept of delivery volume and delivery time, i.e., how frequently the deliveries are made. In the context of PROFINET, this delivery frequency is referred to as the update rate. Some may ask, "Is a higher update rate always better?"
The answer is no. Using the supplier-manufacturing plant analogy again, too few deliveries cannot meet the production needs of the plant, while too many deliveries can lead to inventory piling up.
In fact, in configuration software, it is possible to individually set the update rate for different I/O slave stations. This exemplifies the flexibility of PROFINET.
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| photo by siemens |
04
Switches and Jitter
001 Switches
Many people have used switches, but what role do they play in PROFINET, and how do they impact the network?
A switch in the context of PROFINET functions akin to a courier transfer station. The word "switch" in this context simply means forwarding. A switch has multiple Ethernet ports that can send and receive data. When a switch receives data, it examines the "destination address" of the data packet and forwards it out through another port based on this address.
But how does the switch know which port to forward the data from?
As we know, every slave station has a MAC address. Inside the switch, there is a MAC address table that keeps track of which devices are connected to each port. The switch decides from which port to send the data based on this table.
002 Jitter (Latency)
While switches can intelligently transmit data packets, just like traffic lights on the road, packets can encounter obstacles.
After receiving a packet, the switch performs a CRC (Cyclic Redundancy Check) to verify if there are any errors in the received packet. If there are errors, the packet is discarded. This process introduces a delay every time a packet passes through a switch, which is known as jitter.
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| photo by siemens |
05
Depth of the Network
In this context, "depth" refers to the number of hops (or switches) between the I/O devices and the controller. It is analogous to the number of traffic lights one would encounter on the way to a manufacturing plant. When designing a network, it is crucial to plan the depth of the network based on the update rate.
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| photo by siemens |
06
Queues and Packet Loss
When a switch receives data, it needs to forward it packet by packet, prioritizing the first packets that arrive. If the amount of data received is too large, the switch will temporarily store the incoming data and forward it packet by packet. The storage capacity of the switch for this purpose is referred to as the queue.
However, the storage capacity of this queue is limited. When the switch's load becomes too high, the queue can fill up, and subsequently arriving packets will be discarded. This phenomenon is known as packet loss.
last：
As we all know, the maximum length of a PROFINET cable cannot exceed 100 meters. Do you know the specific reasons behind this?
The limitation of 100 meters for PROFINET cables stems from several key factors that affect signal integrity and data transmission reliability:
Signal Attenuation: As the signal travels through the cable, it experiences attenuation, which is the gradual weakening of the signal due to resistance in the wire. The longer the cable, the greater the attenuation, leading to reduced data transmission rates and potential data loss.
Clock Jitter: Clock jitter refers to the lack of perfect synchronization between the sending and receiving ends of the signal. Over long distances, this jitter becomes more pronounced, potentially causing data transmission errors and packet loss.
Crosstalk: Crosstalk, or crosstalk interference, occurs when signals in the cable interfere with each other. This can happen due to electromagnetic fields generated by the currents flowing through the wires, causing distortion and degradation of the signal.
Transmission Delay: The time required for the signal to travel through the cable increases with distance. In Ethernet networks, especially Fast Ethernet (such as 100Base-TX), there are specific rules governing the maximum allowable delay to ensure timely conflict detection and resolution, maintaining network stability and throughput.
Ethernet Standards and Rules: The 5-4-3-2-1 rule, for example, limits the number of network segments and repeaters in a ring topology to ensure reliable data transmission. This rule, along with the requirement for the round-trip delay to be within a certain threshold, contributes to the 100-meter limit.
Voltage Tolerance and Signal Integrity: The signal in the cable is represented by binary values (0s and 1s), with specific voltage ranges assigned to these values. As the signal travels, it can experience voltage drops due to attenuation. If the voltage drops below a certain threshold, the receiving end may interpret the signal as noise, leading to packet loss.
The 100-meter limit for PROFINET cables is a result of the combined effects of signal attenuation, clock jitter, crosstalk, transmission delay, and adherence to Ethernet standards and rules. This limitation ensures stable and reliable data transmission over the network.