“As we have seen, while CAN termination is a fairly simple subject, it can cause communication problems if not handled properly. In the next part of this series, I’ll discuss how split termination can help improve electromagnetic emissions and show CAN bus signal and conducted emission diagrams, with and without split termination.
In this article, I will build a typical CAN driver topology and explain why termination is so important for proper communication with CAN.
The International Organization for Standardization (ISO) 11898 CAN standard stipulates that the physical line of the CAN network is a single twisted pair cable with a characteristic impedance of 120Ω. In addition, the standard specifies that both ends of the bus must be terminated with a resistor equal to the characteristic impedance of the cable.
As I showed in my last article, a typical CAN driver has an “open drain” output structure, which means that the dominant edge is actively driven and the recessive edge is not. Therefore, it is very important to terminate the bus properly as it ensures that the recessive edge decays properly and is used in time for the next bit’s sampling point.
Termination can take many different forms, but Figure 1 shows the two most common bus termination techniques: standard termination and split termination.
Figure 1: Common CAN Termination Techniques
The standard termination consists of a single-resistor termination between CANH and CANL, as shown on the left side of Figure 1. This technique requires placing a single resistor between CANH and CANL that matches the differential mode impedance of the cable (usually 120Ω) on each terminal bus line.
The split termination technique shown on the right side of Figure 1 uses two resistors equal to half the cable’s characteristic impedance (typically 60Ω each) and a capacitor (typically between 1-100nF) placed between the common-mode point and ground between) ).
Although the split termination technique uses more components, it offers the added benefit of creating a low-pass filter for common-mode noise on the network, thus helping to improve electromagnetic emissions. The resistor and capacitor (RC) create an RC low-pass filter whose corner frequency is given by Equation 1:
One thing to keep in mind with split terminations is that it’s important to use well-matched resistors. Any change in resistance will convert the common-mode noise present on the network to differential noise, affecting the noise immunity of the receiver.
Typical concerns I hear about this termination technique include: “Will this filter my CAN bus signal?” and “Do I need to put the corner frequency above the data rate?” Simple answers to both questions negative. Since the capacitor does not place a direct current (DC) load on the differential bus signal – it only filters the alternating current (AC) and common mode signals – and the differential signal determines the bus state, you do not need to change the corner frequency of the filter Set above the data rate.
A sometimes overlooked issue that leads to improper bus termination is when one or more nodes with integrated termination are removed from the network. This results in the CAN bus being half or possibly unterminated. Figures 2, 3 and 4 show CAN bus signals with three different termination situations:
• Figure 2 is an example CAN transceiver properly terminated using standard terminations on both ends.
• Figure 3 shows the same CAN transceiver with only one of the two standard terminals populated.
• Figure 4 shows the same CAN transceiver lacking the two standard terminations.
Figure 2: CAN bus signal with two standard terminations
Figure 3: CAN bus signal with only one standard termination (and one missing)
Figure 4: CAN bus signals without termination (both missing)
As we can see when comparing Figure 3 and Figure 2, when we lose one of the two terminations, the recessive edge takes twice as long to decay (120ns vs 251ns). This delay will increase with larger and more capacitively loaded networks. For the scenario shown in Figure 4, the bus does not decay back to the recessive state even after 18.0µs! For the case where the RC delay is too slow, the sampling point of the next bit will return to a differential voltage below 500mV on the bus appears before, thus causing a bit error.
This RC delay will vary from network to network and depends on the differential loads that the transceivers place in parallel on the network, as well as any capacitance due to routing, protection, and filtering components. Therefore, it is important to place terminations external to all network nodes or on nodes that will never be offloaded, as this will avoid major signal integrity issues that can arise from improper bus termination.
The last thing we need to consider when choosing termination resistors is how to size them. Depending on the possible faults in the system, the resistors need to be rated to handle possible fault currents. Usually the worst failure is the power line shorted to CANH, CANL draws high current when driving the dominant signal. For a 12V supply and 120Ω impedance, the resistor may have up to 100mA of current flowing through it. Therefore, it is important to use resistors rated high enough to handle possible bus failure conditions.
As we have seen, while CAN termination is a fairly simple subject, it can cause communication problems if not handled properly. In the next part of this series, I’ll discuss how split termination can help improve electromagnetic emissions and show CAN bus signal and conducted emission diagrams, with and without split termination.