Low-Speed Can - National Instruments NI-CAN User Manual

Chapter 1 Introduction

Low-Speed CAN

NI-CAN User Manual
Bus Off State
If the transmit error counter increments above 255, the CAN device
transitions into the bus off state. A device in the bus off state does not
transmit or receive any frames, and thus cannot have any influence on the
bus. The bus off state is used to disable a malfunctioning CAN device
which frequently transmits invalid frames, so that the device does not
adversely impact other devices on the network. When a CAN device has
transitioned to bus off, it can only be placed back into error active state
(with both counters reset to zero) by manual intervention. For
sensor/actuator types of devices, this often involves powering the device
off then on. For NI-CAN network interfaces, communication can be started
again using a function such as
Low-speed CAN is commonly used to control "comfort" devices in an
automobile, such as seat adjustment, mirror adjustment, and door locking.
It differs from "high-speed" CAN in that the maximum baud rate is 125K
and it utilizes CAN transceivers that offer fault-tolerant capability. This
enables the CAN bus to keep operating even if one of the wires is cut or
short-circuited because it operates on relative changes in voltage, and thus
provides a much higher level of safety. The fault tolerance feature means
that communications capability is maintained even if any of the ISO11519
wiring failures occur. The transceiver solves many common and frequent
wiring problems such as poor connectors, and also overcomes short circuits
of either transmission wire to ground or battery voltage, or the other
transmission wire. The transceiver resolves the fault situation without
involvement of external hardware or software. On the detection of a fault,
the transceiver switches to a one wire transmission mode and automatically
switches back to differential mode if the fault is removed.
Special resistors are added to the circuitry for the proper operation of the
fault-tolerant transceiver. The values of the resistors depend on the number
of nodes and the resistance values per node. For guidelines on selecting the
resistor, refer to the cabling appendix of your NI-CAN getting started
manual.
Because the low-speed transceiver switches to a fault tolerant mode on fault
detection and continues to maintain communications, NI-CAN provides a
special attribute,
NC_ATTR_LOG_COMM_ERRS
enables the reporting of such errors in the Read queue of the Network
interface rather than in the Status returned from a function call. The default
value of this attribute is
.
ncAction
(for high-speed interface). Refer to the
NC_FALSE
1-8
, which when set to
NC_TRUE
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