Difference between Fieldbus HSE and TCP/IP over Fast Ethernet


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What is the technical difference between Foundation Fieldbus HSE and TCP/IP over Fast Ethernet? If HSE is some kind of Ethernet, how it can be real-time? Because Ethernet uses CSMA/CD. Anyone knows?
Question 1: "What is the technical difference between Foundation Fieldbus HSE and TCP/IP over Fast Ethernet?"

Foundation Fieldbus HSE uses UDP/IP rather than TCP/IP although non-real-time functions are also supported by TCP/IP

Foundation Fieldbus HSE is an application protocol (layer 7). Contrary to popular belief, Ethernet (layer 1 and 2) and TCP/UDP/IP (layer 3-4) do not provide any interoperability - only coexistence. To make any two devices or software communicate data intelligently you need an application layer protocol to define data types and semantics etc. Application layer interoperability is one of the things that Foundation Fieldbus HSE provides. Rigorous interoperability testing ensure simple integration.

All device information is hierarchically organized in virtual field device "VFD" (control modules/loops), block, parameter, and simple parameter making it easy to locate. Standard data types (mode, diagnostics, process variables, alarms, etc) and standard block types (AI, PID, and DO etc.) with strong typing and standard names further simplifies integration.

Additionally, Foundation Fieldbus HSE includes a true "DCS style" redundancy management protocol. This is real redundancy with redundant devices and redundant Ethernet ports on each device. This is not merely ring-topology which is not real redundancy and moreover is shared media with associated contention. Since each device has redundant Ethernet port, HSE supports redundancy for peer-to-peer communication. That is, HSE is a complete control-network for process automation.

Other useful features provided by the HSE system management is an address annunciation scheme. As soon as an HSE device is connected to a network it annunciates its address to the world. This makes it easy to discover addresses of devices and thus HSE devices are easy to commission

Question 2: "If HSE is some kind of Ethernet, how it can be real-time? Because Ethernet uses CSMA/CD. Anyone knows?"

HSE is unmodified IEEE 802.3 Ethernet and standard IP and UDP. It typically runs on 100 Mbit/s FastEthernet (IEEE 802.3u) media, but it can use faster and slower options as well. Since the introduction of switched Ethernet media and full-duplex the non-deterministic behavior of CSMA/CD (such as on coax and shared UTP) has largely been eliminated. There are thousands of articles on the Internet explaining this. HSE uses a high-level-schedule-driven communication arbitration. Blocks are executed as per a high-level schedule. The results are communicated upon completion. The schedule further reduces the chance of collision. Note that this schedule is not in the data link layer because standard Ethernet data link is used.

Instead this schedule is part of the application process and one purpose of it is to make sure devices do not publish data at the same time.

"Real-time" is relative. It depends on if you are logging movement of glaciers or controlling a missile. For process control real-time is usually 250-1000 ms. With HSE you can expect a cycle time of 100 ms. That is, HSE is well suited for process control, but I would not use if for motion control.

Like your question implied, motion control buses like SERCOS-III and Profinet-IO-IRT, PowerLink, EtherCat, and possibly SynqNet as well use Ethernet wire but the data link layer is modified so it is not Ethernet and thus regular LAN switches cannot be used. Their real-time protocols do not use IP, TCP, or UDP either.