DeviceNet Networking

CanOpen is an open low-cost communications link based on the reliable CAN technology to interconnect industrial devices (such as limit switches, photoelectric sensors, valve manifolds, motor starter, process sensors, panel displays, operator interfaces, etc.) via a single network. This eliminates expensive wiring and failure due to the increase of number of connections. It also reduces the cost and time to wire and install industrial automation devices while providing reliable interchangeability of components from multiple vendors. The direct connectivity provides improved communication between devices as well as important device-level diagnostics not easily accessible or available through hardwired I/O interfaces. This technology is used extensively in the U.S., Australia, New Zealand, Japan, China, and it is rapidly gaining popularity in the European region.

CanOpen is based on the ISO standard for automotive networking known as Controller Area Networking (CAN), which is used in virtually all industries, e.g. automotive, manufacturing, agricultural, medical, building controls, marine, aerospace, etc.

CanOpen defines the maximum cable lengths (trunk and drop cables) to ensure the propagation of the transmitted message falls within the acceptable region. The upper boundaries of the trunk cable and drop cable length are listed as below:

Trunk cable length specification

Drop cable length specification

What are the considerations of CanOpen cable installation?

To install the trunk and drop cable, several considerations have to be taken into account.

• If the distance from a trunk line tap to the farthest device connected to it is greater than the distance from the tap to the nearest terminating resistor, then the drop line length must be included as part of the trunk cable length, as well as the drop length calculation.

• The cumulative drop line length refers to the sum of all drop lines, thick or thin cable, in the cable system. This sum cannot exceed the maximum cumulative length allowed for the given communication rate used.

Power Supply and the CanOpen Bus

The CanOpen network allows distribution of power supplies on the network cable system. Follow these general rules to achieve a safe and reliable operation:

• Use power supplies rated at 24VMinimize installation problems by using one power supply with sufficient current to operate all the attached nodes. This must comply with the national and international safety standards.

• Make sure that each power supply incorporates current limit protection.

• Make sure each power supply is temperature compensated.

• Provide over current protection for each segment of your CanOpen cable installation.
  

Termination of the CanOpen Network

The CanOpen network uses the Controller Area network (CAN) bus as the physical layer. This requires that the trunk line of your CanOpen network be terminated with a resistor at each end. This terminates the signal lines.

The terminating resistor:

• Prevents reflection of communication signals on the network.

• Connects the two signal conductors.

• Must be sealed if the end node uses a sealed tee.

• Must be open if the end node uses an open-style tap.

When using the open-style terminating resistor, connect a 121ohms, 1/4W resistor to CAN_H and CAN_L (between blue and white data-pair wires).

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