2 CAN and RS-485 Physical Layers

Although the SMAART wire bus interface is proven to be reliable, both the CAN and RS-485 physical layers offer a significant improvement in noisy environments due to differential signaling. The difference between the CAN and RS-485 transceiver is the way they drive the bus.

The CAN transceiver uses an open collector output and does not require any driver enable signal. Multiple transceivers transmitting at the same time ruin data integrity but there is not any excessive current flowing through the bus. The CAN bus defines a recessive and dominant state on the bus. These states represent logic values zero or one. Figure 2-1 shows the fundamentals. See the blog article ^[1] for further details.

The RS-485 transceiver uses a push-pull configuration and requires a driver enable (DE) and receive enable (~RE) signal for direction control. The bus driver either actively drives the bus or enters the idle mode. In the idle mode the driver represents high impedance and external or internal fail-safe biasing ensures a default logic value of the bus. Any active driver can override the bus in the idle mode. In the active mode the push-pull driver controls nodes A and B. The differential voltage between the pins is positive or negative. The polarity of the differential voltage represents logic values on the bus. Higher voltage on the node A than on B represents
logic 1. Lower voltage on the node A than B represents logic 0.

Multiple RS-485 transceivers transmitting at the same time cause bus contention. One transceiver may force the signal line high whereas the second forces it low. Modern transceivers use protections but this unwanted event causes excessive power dissipation and ruins data integrity. Figure 2-2 shows fundamentals of the RS-485 bus. See the blog^[2] for additional information on RS-485 bus.