The CAN bus terminal resistor, as the name suggests, is the resistor added to the end of the bus. Although this resistance is small, it plays a very important role in CAN bus communication.
The role of terminating resistors
There are two functions of the CAN bus terminating resistor:
1. Improve the anti-interference ability to ensure that the bus quickly enters the recessive state;
2. Improve signal quality.
Improve anti-interference ability
The CAN bus has two states of "dominant" and "recessive", "dominant" represents "0", "recessive" represents "1", which is determined by the CAN transceiver.
If there is no load on the bus, the differential resistance value is very large when it is recessive, and the external interference only needs a very small amount of energy to make the bus become dominant (the minimum voltage of the common transceiver dominant threshold is only 500mV). In order to improve the anti-interference ability when the bus is recessive, a differential load resistor can be added, and the resistance value is as small as possible to avoid the influence of most of the noise energy. However, to avoid requiring too much current from the bus to go dominant, the resistance should not be too small.
Ensuring fast entry into recessive state
During the dominant state, the parasitic capacitances of the bus are charged, and when returning to the recessive state, these capacitances need to be discharged. If no resistive load is placed between CANH and CANL, the capacitance can only be discharged through the differential resistance inside the transceiver.
Improve signal quality
When the signal is at a high slew rate, when the signal edge energy encounters an impedance mismatch, signal reflection will occur; if the geometry of the cross-section of the transmission cable changes, the characteristic impedance of the cable will also change, which will also cause reflections .
At the end of the bus cable, the rapid change of impedance causes the energy reflection of the signal edge, and ringing occurs on the bus signal. If the ringing amplitude is too large, the communication quality will be affected. Adding a terminal resistance consistent with the characteristic impedance of the cable at the end of the cable can absorb this part of the energy and avoid ringing.
During the dominant state, the parasitic capacitances of the bus are charged, and when returning to the recessive state, these capacitances need to be discharged. If no resistive load is placed between CANH and CANL, the capacitance can only be discharged through the differential resistance inside the transceiver. We add a 220PF capacitor between CANH and CANL of the transceiver for simulation experiments, and the bit rate is 500kbit/s
Why choose 120Ω
The characteristic impedance of any cable can be obtained experimentally. One end of the cable is connected to a square wave generator, the other end is connected to an adjustable resistor, and the waveform on the resistor is observed through an oscilloscope. Adjust the resistance value of the resistor until the signal on the resistor is a good square wave without ringing, and the resistance value at this time can be considered to be consistent with the characteristic impedance of the cable.
Most automotive cables are single wire. If you take two typical cables used in automobiles and twist them into twisted pairs, you can get a characteristic impedance of about 120Ω according to the above method, which is also the recommended terminating resistance value of the CAN standard.
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