(Controller Area Network)

Transcription

1 CAN (Controller Area Network) CAN is open technology supporting multiple applications Chips available today from Intel, Motorola, Philips/Signetics, NEC, Hitachi, Siemens Volumes from multiple industry usage insures downward price pressure -over 5 million chips in 1995 Network has flexibility for now and future Master/slave, multiple master, and peer-to-peer Currently in use in automobiles An excellent proxy for industrial applications Temperature extremes, shock/vibration, high noise environment 1

2 General Features drop lines zero drops Trunk line, drop line configuration Node removal without breaking trunk line Up to 64 addressable nodes Signal and 24Vdc Power in same cable Selectable Data Rates (125k, 250k, 500k) Both Sealed and Open-Style connections zero node separation 121 ohm terminator at each trunk line end 2

3 Speeds, Distances, and Drops Data Rate 125K Trunk Distance (thick cable) 500m (1640 ft) Drop Length Max drop Cumulative 6m (20 ft) 156m (512 ft) 250K 250m (820 ft) 6m (20 ft) 78m (256 ft) 500K 100m (328 ft) 6m (20 ft) 39m (128 ft) NOTE: Thin cable may be used as trunk. Maximum distance is 100 meters, regardless of data rate. 3

4 Power and Signal 24vdc PS Two twisted pair Signal pair: low loss, high velocity with foil shield Power pair: up to 8A capacity with foil shield NOTE: Class II NEC Code limits current to 4A on any segment Overall braid with drain wire Sensors can be powered direct from bus Opto-isolation for self powered devices e.g. drive, PLC, weigh scale, etc. Multiple power supplies can be used used for additional power or as back-up 4

5 Typical Sealed-Style Taps Tee Tap Field Installable trunk line connectors -screw type -crimp type Drop lines - molded connectors - 0 to 20 ft. - mini or micro at sensor Trunk line segments -molded mini-connectors Multiport Tap 5

6 CAN in the Stack ISO Layer 7 -Application { Application Layer } DeviceNet Application Layer Specification ISO Layer 2 -Data Link Data Link Layer CAN Protocol Specification ISO Layer 1 -Physical ISO Layer 0 -Media { Physical Signalling Transceiver Transmission Media DeviceNet Physical Layer Specification 6

7 Data Link Layer Characteristics CSMA/NBA - Carrier Sense Multiple Access with Nondestructive Bitwise Arbitration Any node can access bus when quiet Data portion of packet can be 0 to 8 bytes long Non-destructive bit-wise arbitration allows 100% utilization and message priority based on 11-bit packet identifier CAN provides automatic error detection, signaling, and retries CAN Data Frame Overview S O F 11 bit IDENTIFIER Control Field Length 0 to 8 bytes Data CRC A C K E O F Arbitration Field Data Field SOF - Start of Frame LEN - Data Length Code CRC - Cyclic Redundancy Code (CRC 16) ACK - Acknowledgment 77

8 CAN Arbitration Example Node 1 Transmits: xxxx 11 E O F Node 2 Transmits: Node 2 losing arbitration and stops transmitting! Node 2 still ACKs message. 01 As seen on the wire: xxxx 01 E O F Arbitration Field 8

9 CAN Error Detection Bit Errors Transmitting node checks bit on bus versus what it sent Stuff Error After 5 consecutive bits of same value, transmitter must insert opposite value bit Acknowledgment Error All nodes respond in the ACK slot if they receive the message properly CRC Error 16 bit transmitted value recalculated by receiving node Form Error check for delimiter and other packet formats violations 9

10 DeviceNet Supports Multiple Network Models and Data Movement Master/Slave Peer-to-Peer Multi-master Change of State data Cyclic Data Production Strobed Polled data Producer/Consumer Paradigm allows for multiple model support vs. limited Source/Destination 10

11 Network Models - Master/Slave The simplest and most understood; polling The PLC or scanner is the master and I/O devices are the slaves The slaves speak only when spoken to Only one master per network ( single master ) Deterministic but not repeatable Network examples - Remote I/O, Profibus DP, Interbus-S, Seriplex, LON PLC 1,4,... 3,6,.. I/O 1 2,5,.. I/O 2 I/O 3 11

12 Network Models - Peer-to-Peer Peer-to-peer are generally token pass networks Each device can send messages only when they have the token The token gets passed based on node number (round robin) or possibly via user defined priority list No sense of mastership or priority Not deterministic Network examples - DH+, DH485, LON, Profibus FMS PLC1 PLC4 PLC2 PLC3 12

13 Producer/Consumer Data is identified as to it's content No more Source/Destination requirements No sense of mastership Allows the functionality of the Source/Destination models: Master/Slave and Peer-to-Peer Additional models are allowed because relationships can be built dynamically: Multicast - one to many, many to one Change of state Cyclic, time based Superior performance because bandwidth is not wasted Network examples - DeviceNet, ControlNet, FIP, Fieldbus Foundation 13

14 Network Models - Multi-master & Multicast PLC1 PLC2 MMI #1 I/O 1 #2 DRIVE 1 DRIVE 2 DRIVE 3 Transaction #1 - position reference from I/O Rack #1 is broadcasted to PLC1, PLC2, and the MMI at the same time Transaction #2 - speed command is sent to all three drives at the same time 14

15 Data Movement - Change of State Rather than a master going through a polling list (scanning), devices report data (input or output) on a change-of-state basis as the events happen Change of State is more efficient for discrete applications Network traffic is significantly reduced Performance is greatly improved Background heartbeat for device health Can be used in Master/Slave, Peer-to-Peer, or Multi-master environments PLC #2 #3 I/O 1 #1 I/O 2 I/O 3 15

16 Data Movement - Cyclic Data Production Devices report data on a user-configured time increment basis (input or output) Cyclic Data Production is more efficient for applications with slowly changing I/O (analog) Network traffic is reduced Performance is repeatable Can be used in Master/Slave, Peer-to-Peer, or Multimaster environments PLC every 25 ms every 500 ms I/O 1 I/O 2 I/O 3 every 2000 ms 16

17 Scan Cycle STROBE: This multicast message starts off the scan cycle. Strobable slaves respond based on their latency. POLL: Sent out even as strobe responses are being received, as bandwidth allows. INTER SCAN DELAY: User selectable minimum quiet time to allow other devices access to the network. Scanner will not start another scan cycle (even if packet processing is complete). Strobe/responses Polls/Responses New Cycle S... (up to 63) P1 R1 P2 R2... (up to 63) P R...packet processing time... ISD S 17

18 C.O.S./Cyclic Traffic Devices report only when necessary More efficient since only data changes are transmitted Configurable for Ack or no Ack Used along with Poll & Strobe Optional heartbeat can be utilized to assure node is still active C C C A C A C 18

19 Application Layer ISO Layer 7 -Application { Application Layer } DeviceNet Application Layer Specification ISO Layer 2 -Data Link Data Link Layer CAN Protocol Specification ISO Layer 1 -Physical ISO Layer 0 -Media { Physical Signaling Transceiver Transmission Media DeviceNet Physical Layer Specification 19

20 Functions of Application Layer CAN Identifier Assignment establishes priority in arbitration process used by receiving nodes to screen for their messages Two Messaging Types I/O Messages for Time Critical control data Explicit Messages for typical Client/Server functions Fragmentation supported for data greater than 8 bytes Duplicate Node Address Detection each node must pass before going on line Device Application Data Consistency Identity data: Type, Vendor, Cat. #, Serial #,... Comm Link data: Node Address, Baud Rate,... Device Config data: e.g., drive - Accel/Decel, Freq Control,... 20

21 Device Configuration Support Provide for User Friendly configuration Computer based, notebook, palmtop, etc. Electronic Data Sheet (EDS) standardized ASCII file format provides description of device attributes name, ranges, eng. units, data type, etc. public attributes from device profiles vendor specific attributes device vendor does once all software vendors use as input Parameter Object same content as EDS, but on board the device itself no mismatches between tool and device/version 21

22 Device Interchangeability Standardized Data Sets (profiles) for various device types Allows user to substitute logically equivalent devices among vendors only provides for same device types, data structures and meaning NOT complete functional interchangeability User still has to determine application equivalency accuracy, life, mechanical strength and mounting, environmental ratings, response time, capacity, etc. this is the same as when replacing a device connected to an I/O point substitution with another vendor s product not likely if user has utilized any vendor specific parameters System can key devices to prevent illegal substitutions For example, system can be configured to not accept a reflective photoeye for a diffuse photoeye 22