Terminal resistance is an important configuration of CAN bus communication, and it is an important component affecting CAN bus communication. The CAN bus terminal resistance is generally 120Ω. In fact, when designing, two 60-ohm resistors are connected in series, and there are generally two 120Ω nodes on the bus. Why does the CAN bus need to add a terminal resistor?
The role of terminal resistance:
(1) Improve anti-interference ability and quickly pass high-frequency and low-energy signals.
(2) Ensure that the bus enters the recessive state quickly and quickly passes through the energy of the parasitic capacitance.
(3) Improve the signal quality and place it at both ends of the bus to reduce the reflected energy.
Improve anti-interference ability
CAN bus has "dominant" and "recessive" two states, "dominant" represents "0", "recessive" represents "1", which is determined by the CAN transceiver. The figure below is a typical internal structure diagram of a CAN transceiver, with CANH and CANL connected to the bus.
picture
When the bus is "dominant", Q1 and Q2 inside the transceiver are turned on, and a voltage difference is generated between CANH and CANL; when it is "recessive", Q1 and Q2 are cut off, CANH and CANL are in a passive state, and the voltage difference is 0 .
When the bus is loaded, the resistance value of the differential resistance is very large when it is "recessive", and the external interference only needs a very small amount of energy to make the bus enter the "dominant" (the minimum voltage of the dominant threshold of a general transceiver is only 500mV, and the voltage difference is 500mV, the bus is judged as "dominant"). When there is differential mode interference on the bus, there will be obvious fluctuations on the bus, and these fluctuations have no place to absorb them, and a dominant bit will be created on the bus.
Therefore, in order to improve the anti-interference ability when the bus is recessive, a differential load resistor can be added, and the resistance value should be as small as possible to eliminate the influence of most noise energy. However, in order to avoid requiring too much current bus to enter "dominant", the resistance value should not be too small.
Ensures that the bus enters the recessive state as soon as possible
During the dominant state, the parasitic capacitance of the bus is charged, and when returning to the recessive state, these capacitances need to be discharged. If there is no resistive load placed between CANH and CANL, the capacitance can only be discharged through the differential resistance inside the transceiver. This impedance is relatively large. According to the characteristics of the RC filter circuit, the discharge time will be significantly prolonged. In order to quickly discharge the bus parasitic capacitance and ensure that the bus enters the recessive state quickly, a load resistor needs to be placed between CANH and CANL.
Absorbs reflected signals to improve signal quality
signal reflection
In the circuit, signal reflection refers to the phenomenon that the signal encounters an impedance mismatch in the transmission line or circuit, causing part of the signal to be reflected back. Such reflections can cause distortion and interference of the signal, negatively affecting the performance and reliability of the circuit. Reflection will occur where the impedance changes suddenly, which means that we can take the following measures to avoid this situation:
(1) Use controllable impedance interconnection;
(2) There is at least one termination matching at both ends of the transmission line;
(3) Select the wiring topology to minimize the impact of branches;
(4) Minimize any abrupt changes in geometry.
impedance matching
Therefore, we generally add termination matching resistors on the signal line to avoid signal reflection and enhance anti-interference ability, and the CAN bus is no exception. On the CAN bus, we often terminate with 120 ohm resistors for impedance matching.