Higher Level Protocols - CAN System Engineering - page 174

Table 4.3 CANopen pinning for the nine-pin DIN-Sub-D connector
Pin Signal Description

1 - Reserved

2 CAN_L CAN_L bus line (dominant low)

3 CAN_GND CAN ground

4 - Reserved

5 (CAN_SHLD) Optional CAN shield

6 (GND) Optional ground

7 CAN_H CAN_H bus line (dominant high)

8 - Reserved

9 (CAN_V+) Optional CAN external positive supply (dedicated for supply of

transceiver and optocouplers, if galvanic isolation of the bus node
applies)

achievable bus length depends also on the internal delay times of the CAN control-
ler and the CAN transceiver. For the exact design of the physical transmission, the
formulae for line theory or appropriate empirical formulae should be used.
Basically, a linear bus topology with a 120-Ω termination resistor is required.
Longer bus lengths may require the usage of a higher termination resistor. Not ter-
minated bus lines are tolerable only to a certain degree, due to the reflections that
could lead to a falsification of the bit value.
There are no detailed regulations on which bus cables or connectors should be
used. However, there are recommendations for the pin assignments for a variety of
connectors available. One of the widely used connectors is the nine-pin DIN-Sub-D
connector (DIN41652). Table 4.3 shows the pin assignment given in the recom-
mendation CiA 303-1.
CANopen uses the CAN protocol as described in ISO 11898-1. Although usage
of remote frames is not recommended, they are basically allowed in some CANo-
pen protocols. The various implementations of remote frames in CAN controllers
could lead to problems when using CANopen protocols.
Some CANopen protocols use the base frame format (11-bit identifier) exclu-
sively for data and remote frames, whereas other CANopen protocols allow the us-
age of the extended frame format (29-bit identifier). Error and overload frames are
transparent for CANopen protocols. They are automatically processed by the CAN
controller and lead, in case of an error frame, to an automatic retransmission of the
interrupted frame or to a shutdown of the device (bus-off). Under rare conditions,
it is to be kept in mind that a frame can be sent twice. Because of this situational
condition, relative data or toggle commands shall not be transmitted.

4.1.2 Device Model

A CANopen-compatible field device features, from the hardware point of view,
at least one CAN driver module and a CAN controller which implements the
CAN protocol. Every CANopen device has to have its own object dictionary.