The your work of little resistance is used — CAN terminal resistor
[introduction] resistor of terminal of CAN bus line, just as its name implies is added in resistor of bus line endmost namely. Although this resistor is small, but there is very main effect however in communication of CAN bus line.
Resistor of terminal of CAN bus line, just as its name implies is added in resistor of bus line endmost namely. Although this resistor is small, but there is very main effect however in communication of CAN bus line.
The action of terminal resistor
The action of resistor of terminal of CAN bus line has two:
1, increase interference rejection capacity, ensure bus line enters recessive state quickly;
2, improve signal quality.
Increase interference rejection capacity
CAN bus line has recessive ” of “ dominance ” and “ two kinds of condition, “ dominance ” represents “0” , “ recessive ” represents “1” , send and receive by CAN implement decision. Graph 1 it is a CAN send and receive implement composition of a picture of typical and in-house knot, bus line of link of CANH, CANL.
Graph 1
When bus line dominance, send and receive implement in-house Q1, Q2 guides, generation presses difference between CANH, CANL; Recessive when, q1, Q2 ends, CANH, CANL is in passive condition, press difference for 0.
If bus line does not have load, value of block of resistor of cent of recessive time difference is very great, external the energy that disturbs a need minimum can make bus line enters dominance (send and receive commonly implement dominance door is restricted the least tension only 500mV) . To promote the interference rejection when bus line is recessive capability, can increase a difference to divide laden resistor, and block is worth as far as possible small, in order to put an end to the influence of major noise energy. However, enter dominance to avoid to had needed gift of great voltaic bus line, block value also cannot pass small.
Ensure enter recessive state quickly
During dominance condition, the parasitism capacitance of bus line can be charged, and when regaining recessive position, these capacitance need to discharge. If any block sex load were not placed between CANH, CANL, capacitance can be passed only send and receive implement discharge of in-house the resistor that need branch. We are in send and receive implement the electric capacity that a 220PF adds between CANH, CANL has imitating test, rate is 500kbit/s, if weaveform pursues 2, graph 3.
Graph 2
Graph 3
From the graph 3 see, dominance restores to be as long as to recessive time 1.44μS, in sampling dot the reluctance below higher case can correspond, if communication rate is higher, or parasitism electric capacity is larger, make sure communication is normal very hard.
Discharge to make capacitance of bus line parasitism fast, ensure bus line enters recessive state quickly, need places a laden resistor between CANH, CANL. After the resistance that increases a 60Ω , if weaveform pursues 4, graph 5. See from inside the graph, dominance restores recessive time cut to 128nS, comparative with dominance set-up time.
Graph 4
Graph 5
Improve signal quality
Signal falls in higher changeover rate case, when signal edge energy encounters impedance not to match, can produce signal reflex; The geometrical structure of cross section of transmission line cable produces change, the diagnostic impedance of wire cable can change subsequently, also can cause reflection.
In the extreme of cable of bus line wire, sharp change brings about impedance reflex of signal edge energy, the meeting on bus line signal produces ringing, if ringing range is too big, can affect communication quality. Online cable extreme increases a terminal resistance that agrees with impedance of wire cable feature, can draw this part energy, avoid the generation of ringing.
We had an imitating test, rate is 1Mbit/s, send and receive implement the double skeining thread that CANH, CANL receives a 10m to control, send and receive implement termination 120Ω resistor assures recessive changeover time, extreme does not increase load. If extreme signal weaveform pursues 6, signal ascendant edge appeared ringing.
Graph 6
If extreme of double skeining thread increases the resistance of a 120Ω , extreme signal weaveform is improved apparently, ringing disappears, if pursue 7.
Graph 7
Be in commonly linear model develop attacks in, wire cable two end are to send end namely, also be sink, two end need reason wire cable to add resistance of a terminal each.
Why to choose 120Ω
The diagnostic impedance of cable of any a wire can pass experimental means to reach. The one aspect of the matter of wire cable receives square wave generator, other one aspect of the matter receives adjustable resistor, observe the weaveform on resistor through oscillograph. Adjust the volume that resistor block is worth, the signal that goes up till resistor is a good square wave that does not have ringing, right now resistance can think the diagnostic impedance with wire cable is consistent.
Cable of major car line is single-track. If you use the typical wire cable that two cars use, twist them make geminate skeining thread, can get according to afore-mentioned methods diagnostic impedance is 120Ω about, this also is the value of terminal resistor block that CAN standard recommends.