Understanding CAN Transceivers

A CAN transceiver is a small IC (integrated circuit) that bridges the gap between your microcontroller’s CAN controller and the physical CAN bus. It’s essential—you can’t connect an ESP32 directly to a CAN bus; you need a transceiver in between.

What a CAN Transceiver Does

Your ESP32 has a built-in CAN controller (called TWAI on ESP32—that’s Espressif’s acronym for “Two-Wire Automotive Interface,” which is their legal-safe name for CAN). This controller handles the CAN protocol logic. But the controller communicates with logic-level signals (0V/3.3V). The CAN bus itself uses differential signaling on two wires—CANH (CAN high) and CANL (CAN low)—with voltage levels established by the bus biasing and termination network.

The transceiver’s job: convert between logic levels and bus levels.

graph TD
    A["ESP32 TWAI Controller<br/>(Logic Level)"] -->|3.3V Signals| B["CAN Transceiver"]
    B -->|Differential Signals| C["CAN Bus<br/>(CANH, CANL)<br/>(set by bus biasing)"]

Dominant and Recessive: How CAN Works

Theory background: For a thorough explanation of CAN’s arbitration scheme and why dominant/recessive bits work, see Chapter 1’s Introduction to CAN. Here we’ll focus on what you need to understand to build your ESP32 CAN node.

How CAN Encodes Bits

CAN doesn’t use “high = 1, low = 0” on a single wire like normal logic. Instead, it uses two wires (CANH and CANL) and encodes bits as a difference between them:

• A dominant bit represents a logic 0
• A recessive bit represents a logic 1

Key concept: The CAN bus’s idle (recessive) voltage is set by external infrastructure—in LCC, by a device like the RR-CirKits LCC Power-Point or SPROG POWER-LCC—not by your transceiver. Your transceiver only nudges the bus around that idle level.

Recessive (Idle) State

In the recessive (idle) state, no node is actively driving the bus. Both CANH and CANL sit at nearly the same voltage, defined by the bias network in your LCC infrastructure. The exact idle voltage depends on what device provides the bias.

Dominant (Transmission) State

When any node wants to send a dominant 0 bit, its CAN transceiver briefly creates a small voltage difference between the two wires:

• CANH is nudged up above the idle level
• CANL is nudged down below the idle level

Example: From my test setup (with a transceiver connected to LCC infrastructure), measurements showed:

The exact voltages you observe will depend on your specific infrastructure and transceiver. The key principle remains: the idle voltage is set by the bias network, and the transceiver creates the differential around that point.

Visualizing the Differential Signal

Here’s what this looks like on an oscilloscope during actual CAN bus communication:

In this capture:

• Yellow trace (CANL): Shows the low line varying as nodes transmit
• Cyan trace (CANH): Shows the high line varying in the opposite direction
• Purple trace (CANH-CANL): The differential signal - notice how clean and digital it is!

Key observation: While the individual CANH and CANL voltages aren’t constant (they shift due to bus activity and bias variations), the differential signal (CANH-CANL) is very clean. This is the magic of differential signaling - noise and voltage shifts that affect both wires equally cancel out when you subtract them, leaving a robust digital signal.

The CAN receiver in your transceiver looks only at this differential voltage, not the absolute voltage on either wire. This is why CAN is so reliable in electrically noisy environments like model railroads and industrial settings.

The Transceiver’s Role

Your transceiver does one essential job: convert the ESP32’s logic signals (0V/3.3V) into small differential voltages on the CAN bus. It pushes CANH and CANL apart to send a dominant bit, then releases them to send a recessive bit. Everything else—the bias voltage, termination, ground reference—comes from your LCC infrastructure (such as the RR-CirKits LCC Power-Point or SPROG POWER-LCC).

The Wired-AND Behavior

This dominant/recessive behavior is the foundation of CAN’s arbitration—if two nodes transmit simultaneously, the one sending 0 (dominant) wins without any other node knowing a collision happened. This is covered in detail in Chapter 1.

Why You Need the LCC Power-Point (or Similar Device)

Your transceiver alone can’t run the bus. You need external infrastructure to provide:

• Bias network: Sets the idle (recessive) voltage that CANH and CANL rest at when no one is driving
• Ground reference: A stable common point between all nodes
• Termination resistors (120Ω): At each physical end of the bus to prevent signal reflections

The first two are provided by devices like the RR-CirKits LCC Power-Point or SPROG POWER-LCC. Without them, the bus has no defined idle state, reflections corrupt signals, and nodes disagree on what voltages mean.

In this section, we focus on connecting the three signal wires—CANH, CANL, and GND—from your ESP32 to the LCC infrastructure. The infrastructure handles the rest.

Ground Connection: The Third Signal Wire

Your transceiver’s ground connection is just as critical as CANH and CANL. While a learning bench might work with USB providing a common ground, a proper LCC installation requires an explicit ground wire connecting your ESP32 to the LCC bus infrastructure.

This ground wire:

• Provides a stable common reference for all transceivers on the bus
• Ensures consistent behavior regardless of USB or power supply topology
• Maintains proper signal margins as specified by the CAN standard

The next section shows exactly where to connect this wire on your breadboard and LCC interface.

Next: We’ll wire this up on a breadboard and connect to your LCC bus.