In this lesson, you will learn how to correctly place and size termination resistors on a CAN bus to eliminate signal reflections and ensure reliable communication.
Why Termination Matters
In our previous lessons, we explored how CAN uses differential signaling (CAN_H and CAN_L) to send bits. Because CAN is a high-speed bus, the electrical signals travel as waves along the wires. When a signal wave reaches the end of a cable, it hits an "impedance mismatch" where the wire ends abruptly. Without protection, this energy reflects backward like a ripple in a pool hitting a wall, creating noise that corrupts the data bits.
To prevent this "echo," we use termination resistors to absorb the wave's energy at the ends of the bus.
Placing the Resistors
The rule for CAN termination is simple but strict: place the resistors at the two physical ends of the bus only.
Even if you have ten nodes on your network, you only ever use two resistors total. Placing them in the middle of a wire or at every single node creates improper loading and ruins the differential voltage levels, leading to the exact errors (like Bit Stuffing or CRC failures) we discussed in earlier lessons.
Calculating the Value
CAN cabling typically has a characteristic impedance of 120 ohms. To match this, we place a 120Ω resistor between CAN_H and CAN_L at both terminal ends of the bus.
Think of your bus setup like this:
Node 1 (120Ω) ---- Bus Line ---- Node 2 ---- Node 3 (120Ω)
If you accidentally place 120Ω resistors at every node in a 5-node system, your total bus resistance would drop significantly, causing the CAN transceivers to draw too much current and fail to generate the required "Dominant" voltage levels. By sticking to exactly two 120Ω resistors, you maintain a 60Ω load across the bus, which is the standard requirement for stable, high-speed differential signaling.
Key Takeaway: To prevent signal reflections, always place exactly one 120Ω resistor at each of the two physical ends of your CAN bus, regardless of the total number of nodes.