Beckhoff EtherCAT EL6752 Documentation page 95

Error handling and diagnostics
Fig. 77: 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 drop line lengths and the total bus length (a rough estimate is not sufficient) and compare the
values with the topology rules (depending on the baud rate).
Screening and earthing
The power supply and the screen should be carefully earthed at the power supply unit, once only and with
low resistance. At all connecting points, branches and so forth the screen of the CAN cable (and possibly the
CAN GND) must also be connected, as well as the signal leads. In the Beckhoff IP20 Bus Couplers, the
screen is grounded for high frequencies via an R/C element.
EL6752 Version: 2.1 95