This is analogous to a wired logical AND connection. The recessive state will only exist on the bus if all transceivers connected to the bus are transmitting a recessive state, because the recessive state is weakly biased, while the dominant state is strongly biased. Additionally, the CANH and CANL signals are commonly referred to as complementary singles since you need to know the voltage potential of both signals to determine the logical state of the bus. Since the difference between the two signals is used to define the state of the bus, this signaling type is known as differential signaling. Lastly, for V diff values between 0.5V and 0.9V, the bus state is undefined. Alternately, V diff values greater than 0.9V indicate that the bus is in a dominant state. When the V diff value on the bus is less than 0.5V, the bus is considered to be in a recessive state. By subtracting the voltage potential of the two bus pins, you can determine the logical state of the bus using Equation 1. During the dominant state, the CANH bus pin is biased to a higher voltage potential (~3.5V) and the CANL bus pin is biased to a lower voltage potential (~1.5V). Figure 2 shows these two states.Īs you can see, in the recessive state both the CANH and CANL bus pins are biased to the same level: ~2.5V. The recessive state corresponds to a logic high level on the transmit input pin of the transceiver. The dominant state occurs when a logic low level is applied to the transmit input pin (usually called TXD) of the transceiver. The CAN bus has two logical states: dominant and recessive. Essentially, the transceiver provides differential drive and differential receive capability to and from the CAN bus.įigure 1 : CAN controller and CAN transceiver It also converts the differential signal on the bus back to a single-ended logic signal (RXD) for input into the CAN controller. During normal operation, the CAN transceiver converts the single-ended logic-level output signal (TXD) from the CAN controller to a differential signal. The two types of signals that are processed by the CAN transceiver are single-ended signals (TXD and RXD) and differential signals (CANH and CANL). A discrete implementation of this is shown in Figure 1. Every CAN application consists of a microcontroller with built-in CAN controller and a transceiver that is tied to the bus. To effectively explain the different types of signals, it’s useful to first understand a typical CAN application. This means that data is sent one bit at a time through two complementary signals on the controller area network high (CANH) and controller area network low (CANL) bus wires. In this post, I’ll focus on the signaling levels found on a CAN bus, so that designers will understand the origin of CAN’s reputation for noise immunity.Īs stated in my first post, CAN is a serial, two-wire, differential bus technology.
#Can bus high and low series
Welcome to the second post in this series on the controller area network (CAN), which is increasingly being used in industrial applications.
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