9.3.1.1 Differential Signal

CAN is a differential bus where complementary signals are sent over two wires, and the voltage difference between the two wires defines the logical state of the bus. The differential CAN receiver monitors this voltage difference and outputs the bus state with a single-ended output signal.

Figure 26. Typical SN65HVD23x Differential Output Voltage Waveform

The CAN driver creates the difference voltage between CANH and CANL in the dominant state. The dominant differential output of the SN65HVD23x is greater than 1.5 V and less than 3 V across a 60-Ω load. The minimum required by ISO 11898 is 1.5 V and the maximum is 3 V. These are the same limiting values for 5-V supplied CAN transceivers. The bus termination resistors drive the recessive bus state and not the CAN driver.

A CAN receiver is required to output a recessive state with less than 500 mV and a dominant state with more than 900-mV difference voltage on its bus inputs. The CAN receiver must do this with common-mode input voltages from –2 V to 7 V. The SN65HVD23x family receivers meet these same input specifications as 5-V supplied receivers.

9.3.1.2 Interoperability Of 3.3-V CAN in 5-V CAN Systems

The 3.3-V–supplied SN65HVD23x family of CAN transceivers are electrically interchangeable with 5-V CAN transceivers. The differential output is the same. The recessive common-mode output is the same. The dominant common-mode output voltage is a couple hundred millivolts lower than 5-V–supplied drivers, while the receivers exhibit identical specifications as 5-V devices.

Electrical interoperability does not assure interchangeability however. Most implementers of CAN buses recognize that ISO 11898 does not sufficiently specify the electrical layer and that strict standard compliance alone does not ensure interchangeability. This comes only with thorough equipment testing.

Table 3. Function Table (Driver)⁽¹⁾

Table 4. Function Table (Receiver)