Low-Speed CAN

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 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 Low-Speed/Fault-Tolerant CAN.

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, Log Comm Warnings, which when set to true enables the reporting of such warnings 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 false, which enables the reporting of low-speed transceiver warnings in the status returned from a function call.