Comparing CANopen and EtherCAT FieldBus Networks

Comparing CANopen and EtherCAT FieldBus Networks September 23, 2013

Posted by Servo2Go.com in Technical Support Information.
Tags: CANopen, EtherCAT, RS232, RS422, RS485
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Servo2Go offers a variety of network options for connecting servo drives in a multi-axis configurations. Choosing the right network depends on a variety of factors such as required bandwidth, update rate, performance, and cost. The network options supported in AMC Digiflex Performance servo drives are outlined below.

CANopen (CAN)

A robust serial protocol that is low cost and offers enhanced diagnostic and control capabilities with reasonable bandwidth. Digiflex Performace CANopen drives also support RS232 as a secondary communication channel. DriveWare software can run over the RS232 channel during operation to monitor quantities in real time making system design and commissioning fast. Visit http://www.can-cia.org/ for more information. Some of the strengths of CANopen are:

3-wire bus is all that is needed to connect drives together (CAN_H, CAN_L and GND).
Differential transmission for noise immunity.
Up to 1Mbit/sec speeds possible.
Up to 128 nodes per CAN network.
Robust message arbitration with collision detection/prevention built into the physical layer.
Many microcontrollers have built in CAN ports.
CAN Hardware for many different platforms readily available (Desktop, PC/104, etc.)
Many different operating systems supported (Windows, VxWorks, Linnux)
Bi-directional (non-polled) communication possible.
PVT – Position,Velocity Time trajectory interpolated by the drive from points sent by the host. Reduces overhead at the host. Countless trajectories possible.
Coordinated motion capabilities.

How Fast Can Messages Be Sent? The average CAN message is 130 bits, so it takes 130μsec per message PER NODE to physically send out a message. Different CAN message types improve on this time, but update rates close to 1Khz are possible.

RS232 (serial)

An economical single-axis solution that offers reasonable diagnostics better suited toward low-bandwidth applications. All AMC Digiflex Performance servo drives support this communication interface.

Many PCs have serial port hardware. No extra host hardware needed.
Simple 3 wire bus (TX, RX, and GND).
Speeds up to 115.2K baud are possible.

How Fast Is RS232? – RS232 messages can be longer than CAN. A read command to a 16-bit index takes 8 databytes (64 bits / 115.2K = 560μsec). The reply is another 12 bytes (96 bits / 115.2kK = 834μsec). Total message time (both command and reply is 560μsec + 834μsec = 1.3msec. The result is much slower than a 1Mbit/s CAN bus.

RS422/RS485

An economical multi-axis solution that yields reasonable diagnostics and control capabilities at higher bandwidths than RS232.

Can support multiple nodes (up to 32)
Operates at speeds higher than RS232 (115.2K) baud (921.6K) possible with host hardware supporting it.
Full-Duplex (RS485 4-wire only) or Half-Duplex (RS422 2-wire).

How fast can I go with RS485? – Max speed is 921.6K baud. RS485 messages can be longer than CAN. A read command to a 16-bit index takes 8 databytes (64 bits / 921.6K = 70μsec). The reply is another 12 bytes (96 bits / 921.6K = 104μsec). Total message time (both command and reply is 70μsec + 104μsec = 174μsec. This is almost as fast as a 1Mbit/s CAN bus.

EtherCAT™

A high-performance Ethernet based deterministic network protocol developed by Beckhoff. Visit the EtherCAT Technology Group http://www.ethercat.org for more information. Some important features of EtherCAT are:

Transmission rates up to 200Mbit/sec.
Based off standard Ethernet for 100baseT.
Real-time down to the I/O level.
Multiple topologies possible – Line, Star, Tree, Daisy Chain + Drop Lines. Can be used in any combination.
Requires no special Ethernet hardware – Standard Network Interface Cards (NIC) can be used for EtherCAT.
CANopen over EtherCAT (CoE) allows use of CANopen protocol and feature set over EtherCAT.
Off-the-shelf EtherCAT masters available that implement full language features of IEC 61131.
Open protocol – Designers can implement their own EtherCAT master.
Secondary USB port for direct connection to DriveWare. Can monitor quantities using Driveware without interrupting over EtherCAT.

How Fast Is EtherCAT?

EtherCAT is based off 100BaseT physical layer and can send multiple datagrams per EtherCAT packet. Cycle times can reach lower than 1msec.