CAN Physical Layer Analysis
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Signal and Level Analysis with CANwatch reduces installation costs and lost production time
When installing CAN networks it is important to have an immediate report whether the network is functional and reliable. Even simple errors such as short-circuits, missing or multiple bus terminations can lead to costly delays. For this purpose OCTUM electronic and EMS Thomas Wünsche have developed the physical layer analyser CANwatch.
Errors in CAN networks
In the installation of CAN networks a number of errors can occur with respect to the physical layer. Wiring errors, missing or duplicated terminations, slow slopes or insufficient dominant or recessive levels are just a few of them. These errors can lead to completely unfunctional networks or – even worse – to networks which turn out to be unreliable. Widely used CAN protocol chips have only a digital view of the network and do not offer possibilities to give an indication regarding the quality of the analog signal. For these reasons specially trained personnel with measurement equipment including a digital scope was required in the past to insure the analog quality of a CAN network. Without that additional quality tests it could happen that exchange of a component or expansion of the network by a new component could lead to the breakdown of a complete control system. We ourselves – as third party specialist – have more than once been called to troubleshoot CAN networks. To help in these situations the physical layer analyser CANwatch has been developed. It is available in a hand-held housing for use in the installation phase as well as in a rail-mount housing for permanent installation. An immediate report is given on the type of error, should a fault occur in the network. This can considerably reduce the length of time lost through a production system breakdown.
CANwatch performs detection of errors and critical signal characteristics on CAN networks with a physical layer according to ISO 11898. The bus is tested with regard to levels and signal shapes. After being connected to a CAN network, CANwatch detects the current bit rate and starts the analysis. All further dynamic characteristics are evaluated with the bit rate as base. Overshoots, undershoots and slow slopes (specially on the falling edge) can be caused by invalid termination resistors. These deviations from an ideal signal regularly can not be avoided, but are only acceptable in the leading part of a bit time. The deviations must have dropped to an acceptable amount before the sample point to avoid disturbances in the communication. Recessive and dominant levels of the signal are influenced by the transceiver characteristics, termination resistors and line resistance. Low dominant levels can result in a message being not recognized by some of the bus nodes. Besides the problems ‘designed in’ by things like cable selection and network structure, careless mistakes during installation such as incorrect bus terminations or short-circuits in the signal lines are common sources of time consuming trouble-shooting. The error sources mentioned above as well as the error frames which appear as a result are indicated by CANwatch. Since the analysis performed by CANwatch is exclusively focused at the physical layer, it is independant of the various higher layer protocols. CANwatch is also uninfluenced by bus traffic as it performs the signal quality assessment without a CAN protocol chip.
In a network CANwatch functions purely as a monitor and does not burden the bus any more than a normal node. The analysis is performed based on the analog signals generated by bus devices. The differential level is sampled with a resolution of 8 bits and up to 16MHz sample frequency. The sampled signal is analysed by a digital signal processor. A message processing rate of up to 5000 messages a second is achieved by the fast DSP used in the device. The power supply for CANwatch can be provided by the bus or by a battery pack. In stationary applications CANwatch is typically used in a rail-mountable housing and supplied from power supply wires on the bus. For mobile applications hand-held operation in a housing with battery supply is possible. With respect to battery powered applications a low power design has been chosen for CANwatch.
CANwatch can profitably be used for three application fields: Make a network operational, verify that a network is likely to be reliable and provide troubleshooting information if a network fails in the field. The first application – making a network operational – is typically covered by a hand-held device to find out wiring errors. The installer plugs the analyser to the network at different locations during the installation of the devices. This task is finished as soon as all the nodes are able to communicate. The second task – verifying the network quality – is an important new option offered by CANwatch. In the past an installation was assumed to be functional as soon as the devices could exchange messages. There regularly was no indication on how far the system was away from failures due to unsufficient signal quality. CANwatch warns about signal shapes likely to cause problems and thus can be used as an early warning system indicating problems before a failure occurs. The third application is online monitoring of a CAN network and providing troubleshooting application in case of failures. For this task CANwatch is typically used in a rail-mountable housing and permanently installed in the system. Error indications given by CANwatch can aid in reducing down times of a system and service cost. All in all this makes CANwatch a useful helper for installation and trouble-shooting of CAN networks. It is a valuable tool which is likely to pay back within a short time whether in mobile or stationary use.
By Heiko Nagel, Oliver Wagener, OCTUM electronic and Dr. Thomas Wünsche, EMS Thomas Wünsche