Error Confinement - National Instruments NI-CAN User Manual

Chapter 1 Introduction
NI-CAN User Manual
• Form error
• Acknowledgment error
Bit Error
During frame transmissions, a CAN device monitors the bus on a bit-by-bit
basis. If the bit level monitored is different from the transmitted bit, a bit
error is detected. This bit error check applies only to the Data Length Code,
Data Bytes, and Cyclic Redundancy Check fields of the transmitted frame.
Stuff Error
Whenever a transmitting device detects five consecutive bits of equal
value, it automatically inserts a complemented bit into the transmitted bit
stream. This stuff bit is automatically removed by all receiving devices.
The bit stuffing scheme is used to guarantee enough edges in the bit stream
to maintain synchronization within a frame.
A stuff error occurs whenever six consecutive bits of equal value are
detected on the bus.
CRC Error
A CRC error is detected by a receiving device whenever the calculated
CRC differs from the actual CRC in the frame.
Form Error
A form error occurs when a violation of the fundamental CAN frame
encoding is detected. For example, if a CAN device begins transmitting the
Start Of Frame bit for a new frame before the End Of Frame sequence
completes for a previous frame (does not wait for bus idle), a form error is
detected.
Acknowledgment Error
An acknowledgment error is detected by a transmitting device whenever it
does not detect a dominant Acknowledgment Bit (ACK).

Error Confinement

To provide for error confinement, each CAN device must implement a
transmit error counter and a receive error counter. The transmit error
counter is incremented when errors are detected for transmitted frames,
and decremented when a frame is transmitted successfully. The receive
error counter is used for received frames in much the same way. The error
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