Error handling and diagnostics
FC5101 and FC5102
73
Version: 2.0
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 shield
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
line.
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 load 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 (e.g. 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.
A free channel on a Beckhoff FC5102 CANopen PCI card is appropriate for such a trace - Beckhoff make the
necessary trace software available on the internet. Alternatively, it is of course possible to use a normal
commercial CAN analysis tool.
Protocol problems can be avoided if devices that have not been conformance tested are not used. The
official CANopen Conformance Test (and the appropriate certificate) can be obtained from the CAN in
Automation Association (
).
