Beckhoff EL6751 Documentation page 174

Error handling and diagnostics
Network terminator and signal leads
The nodes should be switched off or the CAN cable unplugged for this test, because the results of the
measurements can otherwise be distorted by the active CAN transceiver.
Fig. 139: Wiring diagram for test setup
Test 3
Determine the resistance between CAN high and CAN low - at each device, if necessary.
If the measured value is greater than 65 Ohms, it indicates the absence of a terminating resistor or a break
in a signal lead. If the measured value is less than 50 Ohms, look for a short circuit between the CAN lines,
more than the correct number of terminating resistors, or faulty transceivers.
Test 4
Check for a short circuit between the CAN ground and the signal leads, or between the screen and signal
leads.
Test 5
Remove the earth connection from the CAN ground and screen. Check for a short circuit between the CAN
ground and screen.
Topology
The possible cable length in CAN networks depends heavily on the selected baud rate. CAN will tolerate
short drop lines - although this again depends on the baud rate. The maximum permitted drop line length
should not be exceeded. The length of cable that has been installed is often underestimated - estimates can
even be a factor of 10 less than the actual length. The following test is therefore recommended:
Test 6
Measure the lengths of the drop lines and the total bus lengths (do not just make rough estimates!) and
compare them with the topology rules for the relevant baud rate.
174 Version: 3.5 EL6751