Process Automation From Fieldbus to toreal Time Ethernet Safety, reliability IEC61158-2 as the physical layer too slow for Ethernet/IP frames Unsafe cables towards wireless solutions Factory automation Performance, cost Ethernet is the most effective solution with costless integration with the high-level TCP/IP-based systems Real-time Ethernet The Industrial Electronics Handbook, Second Edition, Industrial Communication Systems CRC press, February 28, 2011 ISBN-10: 1439802815 ISBN-13: 978-1439802816 1
Ethernet: why now? 2
Ethernet: evolution 3
ETHERNET ADVANTAGES Simple interfacing with higher levels (supporting of TCP/IP traffic) Standard technology that is widespread, updated and supported by PC 10Base5 10BaseT 100BaseT GigaEthernet Hardware costs are decreasing (some processors support Ethernet interface, as it occurs with CAN) Availability of ITC instruments (Information Communication Technology) PC-based analyzers (Es. Wireshark -Ethereal-), simulators Network analyzers with high performance Emerging nodes and controllers use web and java technologies Soft-PLC, web-sensor,... Support of related technologies (optical fiber, wireless 802.11 WiFi) couplers, bridges vs. subnetworks 4
ETHERNET MEANS INTERNET AND WEB TECHNOLOGIES Ethernet IEEE802.3, internet (IP), web technologies Network with a lot of nodes (7 10 13 global addresses) Protocols over Ethernet Internet Protocol (IP) Address Resolution Protocol (ARP) Internet Control Message Protocol (ICMP) Transfer Control Protocol (TCP) User Datagram Protocol (UDP) Hyper Text Transfer Protocol (HTTP)... ETHERNET 100BaseT Ethernet frame: 5.76 122μs 8 octets 14 octets 46 1500 octets 4 octets PREAMB. HEADER IP TCP UDP 20 60 H DATA 20 60 8 H H DATA DATA DATA CHK 96 bit Inter-frame 5
ETHERNET MEANS INTERNET AND WEB TECHNOLOGIES 6
ETHERNET IS IS JUST A PHYSICAL LAYER 7
TCP/IP OVER FIELDBUS 8
FIELDBUS OVER TCP/IP Modbus and Modbus over TCP Same application layer over different physical layers One of the first use of Ethernet at the field level (non time critical applications) Modbus (Modicon Schneider Electric 1980) Physical layer: serial link (e.g. RS232, RS485) Performance: baud rate between 1200 and 19200 Configuration: 8bit, no parity, 1 stop Characteristics: one-master, 247 slaves, minimum overhead (4 byte) Modbus over TCP (1997) Physical layer: Ethernet Performence: 10/100 Mbit/s Configuration: TCP link over port 502 (reserved) Characteristics: one-master, 247 slaves, min. overhead (66 byte) RTPS middleware for improved real-time performance 9
ETHERNET IS IS AN OLD TECHNOLOGY Why Ethernet has never been considered at the field level? Ethernet has always been used at higher levels Recent cost reduction and performance increase From Hub to Switch (collision avoiding) The Hub forwards every frame (including fragments) to all (fixed delay) The Switch stores and forwards frames only to the destination (variable delay) More topologies allowed Node Hub Switch 10
STORE AND FORWARD SWITCHES All the frame is stored and is verified as free from errors Maximum reliability The switch introduces a delay that depends on switch electronic circuits (T sid ) and on the length of the frame (T d ) Source Receive Check Transmit t Switch Dest. T d T sid t 11
CUT THROUGH SWITCHES Only the destination address is stored The frame is not checked Low reliability, high speed The switch introduces a fixed delay (like a Hub) Source Cut-Through (6 byte) Dest. Addr. Receive and Relay Src Addr. 52 bytes Switch t Dest. T d T sid t Fragment-free versions 12
SWITCHES AND PRIORITY Switch with queue without priority Frames are forwarded according to their arrival time If the destination port is occupied, a queue results 1 T p 1 2 D 2 t n Queue D t Without priority t T sid The time delay T p also depends on other frames A big jitter could result (up to 123μs in the shown case) 13
SWITCHES AND PRIORITY Switch with queue and priority (IEEE802.1p) Frames are forwarded according to the priority level Each priority class (0-7) has its own queue T p 1 2 3 hi low Queue with priority 1 2 3 t t Switch with priority (IEEE802.1p) D t t T sid Time delay T p still depends on other frames, as transmission is not interrupted by a frame with higher priority! 14
SWITCHES AND VIRTUAL LAN Some Switches support Virtual LAN (IEEE802.1q) The port of the switch can be grouped to form different networks Every frame transport information concerning its own VLAN Switch VLAN 2 Switch VLAN 1 Virtual circuits Separate domains allow efficient multicast transmission A VLAN is not affected by traffic of other VLAN sharing the same switch 15
HUB AND SWITCHES: CONCLUSIONS The Hub forwards with a fixed delay if the network topology is known, then the transmission delay is fixed and known but the Hub suffers from collisions and it is obsolete in ICT The Switch stores, queues and forwards with a variable delay no collisions but it suffers from variable delays If infrastructure is shared with TCP/IP traffic the delay due to the switches can not be neglected (>100 µs) Broadcast messages, normally used in fieldbuses to synchronize the nodes, are suitable in fieldbuses where transmission delays are only due to propagation time of electrical signals (~20 cm/ns) come to the nodes in different time instants in switched ethernet networks are forwarded to all the ethernet nodes and therefore they must avoided other synchronizing methods must be designed for switched ethernet 16
REAL TIME ETHERNET (RTE) RTE = use of Ethernet at field level for real-time applications Characteristics: Determinism Synchronization of communication, I/O and applications (e.g. IEEE1588) Simple protocol stack (TCP/IP is too complicated for a sensor) Compatibility with TCP/IP traffic (same infrastructure) - a part of the bandwidth is reserved for TCP/IP - router or gateway (proxy, firewall) for TCP/IP traffic Difficult coexistence among different RTEs Approaches: Full-software Hardware/software 17
RTE: SOFTWARE-BASED SOLUTIONS TDMA The medium is mainly devoted to this RTE TCP/IP traffic (small bandwidth) is managed by suitable router/gateway Limits and problems: other nodes must be managed through router/gateway Compensation of delays due to cables and switches Bandwidth loss Synchro Node 1 Node 2... Node n TCP/IP Synchro Node ordering and position into slots can vary 18
RTE: HARDWARE-BASED SOLUTIONS Enhanced Hardware with respect to standard Ethernet IEEE802.3 TDMA managed by synchronized devices Data can be modified on-the fly Special switches forward frame with special rules Optimized performance TCP/IP compatible, but incompatible with other RTEs Proprietary hardware (ASIC) RTE Switch RTE port TCP/IP port 19
IEC 61158 and IEC 61784 IEC 61158 Industrial communication networks - Fieldbus specifications IEC 61784 Digital data communications for measurement and control 20
IEC 61158 and IEC 61784 21
IEC 61158 and IEC 61784 22
IEC 61158 and IEC 61784 23
IEC 61158 and IEC 61784 24
DIFFERENT STANDARD AND DIFFERENT APPROACHES 25
DIFFERENT STANDARD AND DIFFERENT APPROACHES Full Ethernet (Modbus/TCP, Ethernet/IP, Profinet IO RT, FFHSE) Standard Ethernet IEEE802.3, switched and full-duplex, with priority and VLAN + coexistence, COTS network devices (cost, technology) - No guarantee of deterministic services Ethernet compatible, but using specific devices (Profinet IO IRT) Special switches with time slicing (mandatory, no other switches allowed) + coexistence, deterministic guaranties, priorization of flow - Uses specific network devices Ethernet as the physical (Ethernet Powerlink) Exclusive use of Standard Ethernet IEEE802.3, gateway for coexistence, HUB + deterministic (TDMA), COTS network devices (cost, technology) - No coexistence, HUB or specific NIC New fieldbus and Ethernet Links (EtherCAT, Sercos III) Different MAC layer to provide real-time, use of specific devices, gateway + deterministic, short cycle time, QoS guaranteed (all data in a single frame) - No coexistence, specific network devices 26
DIFFERENT STANDARD AND DIFFERENT APPROACHES 27