When Ethernet is compared with other fieldbuses that are recognized by the industry as deterministic, there is a fierce debate about Ethernet and determinism. Some people think that the industrial Ethernet system is less deterministic than other specialized industrial field bus systems. Here we will analyze to confirm that Industrial Ethernet is actually deterministic under some common conditions.
First of all, we must study the definition of certainty. Deterministic systems are considered predictable. For example, a wastewater treatment system can be a stable and predictable deterministic system with an application response time of 500 ms, whereas a multi-axis CNC motion system may require an application response time of 1 ms. The above two examples illustrate that determinism is a factor that varies with customer and process requirements in each specific application. The essence of certainty is that each operation that meets the current application requirements is predictable and consistent.
Many systems claim to be deterministic, but if they are carefully studied, the determinism of Ethernet can outperform the best system. Take a distributed input/output system (DIO) as an example. The slave I/O controller must be controlled by a master PLC. Loss of communication in the DIO system will cause the process to lose control. Any delay caused by changes to the network expected to change DIO ART (application response time) performance can cause problems, just because adding or removing a device, or extending the network with additional cables may require unpredictable logic and time changes .
Although each fieldbus has its packet payload, Ethernet can still find the best balance between expanding process and shop floor networks and reducing network load and performance loss. Because of the limitation of the number of nodes and devices, other field buses must pay for the expansion in a very strict way.
This article examines network transport components and compares the functionality of Ethernet with other fieldbus systems that are currently considered to be deterministic systems.
Fieldbus Comparison <br> Most of the distributed input/output (DIO) fieldbus systems that are considered deterministic are logical ring/physical bus token-passing networks such as Profibus, Modbus Plus, and others. Taking Modbus Plus DIO as an example, assuming that the PLC request or I/O device response transmission time is fixed, the time required to transmit a message request or response can be calculated. In other deterministic networks such as Profibus, as the network length exceeds a specific network length boundary, the transmission time will be prolonged due to a decrease in the transmission speed. In these examples, the fieldbus itself is still considered to be deterministic, because the network transmission message delivery time between any two given nodes can be calculated and stabilized.
Ethernet was originally a bus-connected network that was abandoned due to randomness and uncertainty and was considered unsuitable for many industrial applications. Because CSMA/CD Ethernet may cause the message transmission time to change due to the MAC layer conflict backoff algorithm retransmission timer, and there is too much conflict that causes the message to be abandoned at the MAC layer, Ethernet relies on higher layer protocols. (such as TCP) or application to resend the message. This was once a major disadvantage of Ethernet's competition with existing deterministic fieldbuses as benchmarks.
While Ethernet has made great progress, obstacles such as network access and bus contention have been eliminated. Due to the Ethernet switching technology introduced by Kalpana in 1995, and the IEEE 802.3x full-duplex standard, conflicts and bus contention have been resolved. Any device operating on a full-duplex Ethernet network can send and receive simultaneously at any time without risk of collisions. In full-duplex operation, Ethernet CSMA/CD conflicts are not needed and are therefore disabled.
Comparing Ethernet and fieldbus in terms of data volume, device count, and network distance, Ethernet has several advantages. In terms of the number of devices, Ethernet adopts the IP subnet mask and there is no practical limit on the number of terminal devices. For example, a Class A network with 24 master bits and an 8-bit subnet mask can provide more than 16.7 million node addresses, which we can think of as a physically unreachable subnet size. In point-to-point message distribution, each node can communicate directly with any other node. Thus, the transmission of query or response messages is not significantly affected by the number of devices on the network because there is no sequential message distribution employed in the token delivery topology in switched Ethernet. The token-passed, logical-loop fieldbus must send the token sequence to each device, so the response time increases as the number of devices increases.
In the above simple comparison, Ethernet has a great advantage over the existing fieldbus, but there are still factors that may affect the certainty of Ethernet transmission. Below we will analyze these effects and examine methods that can be used to counteract these effects in an appropriately-combined industrial Ethernet network.
In a dedicated fieldbus/token pass system, network traffic is often limited to a specific message type and the sequential flow between network devices. In an Ethernet system, some messages that can flexibly implement freeform peer-to-peer communication may need to be broadcast to determine the location of the required resources that make up a complete message request. Address Resolution Protocol (ARP) is a secondary protocol used to bind Ethernet hardware MAC addresses to logical software stack IP addresses. The ARP request is designed to be broadcast to all devices on the IP subnet or VLAN broadcast domain. However, ARP requests and other protocol broadcast messages may be destructive when excessive. Processing broadcast requests is a basic function of IP Ethernet, even if the ARP request is for another terminal device. Many other common protocols (such as NetBIOS or IPX) also provide broadcast services. They sometimes also generate broadcasts in reverse to all other NetBIOS hosts on the subnet. This is the case when the Microsoft Windows NetBIOS domain or workgroup host browser election is initiated. Happening. There is also a situation where a host configured in another network tries to locate its basic resources. The misconfigured host PC may suddenly issue 10 or more broadcasts per second, trying to use the unusable network domain controller. , Server shares, and other resources are registered or registered, which may be destructive for some devices.
If ARP or other broadcasts are excessive, these broadcasts can be destructive. They can cause buffers for all terminal devices on the subnet to be congested. Delays can even block important unicast or multicast automation application messages and legitimate UDP such as BootP or DHCP. Normal processing of broadcast requests.
Ethernet switches have evolved to use broadcast rate limiting to control excessive broadcast traffic. This feature will throttle excessive broadcast traffic above the configured level. The use of managed industrial Ethernet switches that support broadcast rate limits allows the switch to protect terminal equipment from excessive broadcasts and to ensure that any broadcast storm disruption is minimized, making it impossible to influence industrial applications. There is a configuration rule that allows two general broadcasts per second on the switch port of each device on the subnet, plus each target device broadcasts two times per second as a conservative figure. The principle of obtaining these two figures is: for each of two application services (such as ARP and DHCP services), one broadcast is allowed each time, and the IP standard broadcast interval is once per second.
For example, if a device is communicating with five other devices on a subnet of 60 hosts, the broadcast rate limit will be 130 broadcasts per second.
(Number of Subnet Devices × 2) + (Number of Target Devices × 2) = Rate Limit Considering the case of power restoration after a power failure, all devices may start at almost the same time. They not only perform duplicate address checks but also attempt to acquire addresses and Configure the information and try to find its designated counterpart. Client broadcasts are typically limited to 1 s intervals, and some devices may exchange frame types between IEEE 802.3 and Ethernet II within this 1 s boundary. All clients will also use ARP to find the MAC address of their peer to collect the MAC address information needed to initiate the TCP connection. The value of the broadcast rate limit configuration must be fair enough so that all devices can receive broadcasts from all connected devices on the subnet.
Ethernet can achieve deterministic performance by operating in full-duplex mode and alleviating the damage caused by excessive broadcast or multicast. With modern terminal equipment, Ethernet can usually transmit at 100 Mbs. The packet size in the automation application protocol is usually within 500 bytes, and the transmission time for sending such a 500-byte packet is 0.00004 s or 40 ms. There are also other factors, such as normal propagation speed (NVP), which are necessary to calculate the propagation time of a bit along a given length of media. NVP is measured as a percentage of light speed. The NVP for most Category 5e cables is between 0.65-0.70, ie they will transmit bits at speeds up to 70% of the speed of light. For all practical applications, NVP is 477 ns on a 100 m cable segment. Such a small time value can be ignored.
As mentioned earlier, the traditional fieldbus that can achieve deterministic performance also has some variables, but it does not affect the total transmission time. For example, on Modbus Plus, the number of devices affects the token rotation time. On Profibus, the total length of the network will affect the transmission speed. However, in either network, once the network is established and stable, the transmission time will be stable.
Although there are many ways to control broadcast, multicast, and congestion, most industrial Ethernet networks actually have virtually no possibility of these problems because data packets in automation applications are small. Smaller data packets require shorter transmission times and easier insertion of frames. In our tests, we sent 78 samples of Modbus TCP/IP request packets, including Ethernet MAC overhead (inter-packet time slots, headers, FCS), sent through a series of Ethernet networks running in full-duplex mode. Switch. The results are shown in FIG. Figure 2 shows the result of the same test repeated with a 275-byte Modbus TCP response. As can be seen from Figure 1 and Figure 2, as the number of switches on the path increases, the transmission time will increase accordingly. However, please note that the actual total transmission time is still a small value even after several switches have passed.
This shows that, similar to the dedicated deterministic fieldbus, once the switch path is determined, the transmission time will reach a consistent steady state, and the amount of change may be only a few microseconds. Adding a switch on the path between two devices is a roughly uniform increase in total transfer time.
Test setup
The test was conducted using multiple managed and unmanaged Industrial Ethernet switches. The data packet generator used is Spirent Smartbits 200, as shown in Figure 3.
The Modbus request and Modbus response data packets are transmitted through the increasing number of switches after being sent out from the output interface of the SmartBits 200, and then received by the input interface of the SmartBits 200 to measure the round-trip time. Each packet has a nominal 96-bit inter-packet time slot (IPG) between frames to simulate the actual data flow. The elapsed time is measured on a single system clock reference on the SmartBits 200. Since SmartBits, which is used exclusively for testing, uses specialized ASICs to generate traffic, its data flow is kept constant and there is no effect of operating system fluctuations in the software-based packet generator.
Priority queuing effect
Even with broadcast rate limiting, multicast filtering, and port priority configured, it is possible that a single largest low priority Ethernet frame happens to begin buffering at the switch input just before a high priority automated application message. As shown in Figure 4. The largest Ethernet frame will continue to be cached, and automated application message packets will be forced to be queued. This situation may be rare, but it may happen. In this case, the maximum queuing delay experienced by automating the application of data packets on the 100 Mbs link will be 121 μs, which is not enough to damage the automation application and is completely within the deterministic reasonable tolerance.
For deterministic design
Network design also plays an important role in the maintenance of deterministic Ethernet. As mentioned earlier, the only real threat to certainty when operating in full-duplex mode is damage caused by unnecessary protocols or excessive broadcast or multicast. If you are developing a distributed input/output (DIO) network over Ethernet and need truly deterministic performance, consider separating the DIO device on the dedicated PLC communication adapter from the dedicated switch (Figure 5).
Please note that the fiber optic interface on modern Industrial Ethernet switches has no practical limit on distance, unlike other deterministic fieldbuses. When using multimode fiber media, there can be a distance of 2 km between adjacent two switches of a DIO network. When using single-mode fiber media, the allowable distance of more than 20 km between two adjacent switches is practically sufficient for any industrial application. Compared to Profibus, it does not need to reduce the speed to achieve.
In summary, the above shows two ways to achieve Ethernet determinism: The first is to use end-to-end full-duplex operation and use the configuration options of the IE switches to mitigate excessive broadcasts and unnecessary multicast. Prioritize automated application traffic. This allows determinism within 1 ms of almost any automation application; the second option is to use a dedicated DIO LAN for deterministic I/O control over Ethernet. This approach allows Ethernet to operate in a closed, controlled environment with minimal additional cost for determinism.
Due to these design and configuration options, Ethernet can achieve greater flexibility than traditional deterministic fieldbus without restrictions and performance loss. With the reduction of cost and the increase of product selection range, Ethernet is becoming the choice of fieldbus. With very little planning and using several configurable features of Industrial Ethernet switches, Ethernet can easily enter the field currently occupied by dedicated fieldbuses.
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