Beckhoff CX1500-M510 Manual page 174

Error handling and diagnostics
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 length of drop lines
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.
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.
Test 7
Use a DC ammeter (16 amp max.) to measure the current between the power supply ground and the screen
at the end of the network most remote from the power supply unit. An equalization current should be present.
If there is no current, then either the screen is not connected all the way through, or the power supply unit is
not properly earthed. If the power supply unit is somewhere in the middle of the network, the measurement
should be performed at both ends. When appropriate, this test can also be carried out at the ends of the drop
lines.
Test 8
Interrupt the screen at a number of locations and measure the connection current. If current is flowing, the
screen is earthed at more than one place, creating a ground loop.
Potential differences
The screen must be connected all the way through for this test, and must not be carrying any current - this
has previously been tested.
Test 9
Measure and record the voltage between the screen and the power supply ground at each node. The
maximum potential difference between any two devices should be less than 5 volts.
Detect and localize faults
The "low-tech approach" usually works best: disconnect parts of the network, and observe when the fault
disappears.
However, this does not work well for problems such as excessive potential differences, ground loops, EMC
or signal distortion, since the reduction in the size of the network often solves the problem without the
"missing" piece being the cause. The bus loading also changes as the network is reduced in size, which can
mean that external interference "hits" CAN telegrams less often.
Diagnosis with an oscilloscope is not usually successful: even when they are in good condition, CAN signals
can look really chaotic. It may be possible to trigger on error frames using a storage oscilloscope - this type
of diagnosis, however, is only possible for expert technicians.
Protocol problems
In rare cases, protocol problems (such as faulty or incomplete CANopen implementation, unfavorable timing
at boot up etc.) can be the cause of faults. In this case it is necessary to trace the bus traffic for evaluation by
a CANopen experts - the Beckhoff support team can help here.
174 Version: 1.0 CX1500-M510, CX1500-B510