Capacitance of CAN bus line crosses 3 kinds of old solutions
[introduction] when engineers are looking for bus line of CAN bus line to transmit unusual reason through weaveform, via because drop,often can be being encountered the edge crosses delay to cause the condition of a sampling mistake, and drop because bus line capacitance is brought about greatly too,the edge crosses delay is commonly. The article will take you to know the problem that capacitance creates greatly too and solution.
Bus line capacitance crosses the phenomenon of big question
If the graph is shown 1 times, the capacitance of CAN node can affect the capacitance of whole network, capacitance jumps over big edge to jump over delay, cause a sampling error.
Graph 1
If the graph is shown 2 times, the fall below the circumstance that the CAN signal that is 1M baud rate is 120pf in capacitance along, decay time is 95.7ns. Because capacitance value is little, pursue so the weaveform of 2 and graph it is better that 1 photograph is compared, the transmission of message is stabler also.
Graph 2
Bus line capacitance lives the matter of big question
Graph 3 it is CAN is sent and receive implement knot composition of a picture, CAN is sent and receive implement the arbitration to realize CAN and wrong processing, can use the structure of one-way drive only, the ascendant follow that causes CAN weaveform has drive, and drop the edge is to pass whole bus line and terminal resistor discharge to arise, so the first action of the terminal resistor of bus line is “ discharge ” .
Graph 3
Solution one: Sacrificial amplitude law
Reduce terminal resistance, bus line accelerates discharge rate, with the method of ” of “ sacrifice amplitude improvement drops along. Attention, if the graph is shown 4 times, the terminal resistor of total paralell connection must not be less than 24 Europe, online cable is fine (lead attenuation is bigger) when, total shunt resistance should rise appropriately, the lowermost limit that ability assures to the amplitude that need branch is not less than 0.9V.
Graph 4
Solution 2: Change small electric capacity, the lead of resistor of small direct current
The 90% capacitance of CAN pass big question, lead is caused, the electric capacity that controls lead so is the most important, make sure lead electric capacity is inside the limits of 40-70PF/m, the price that it is a gender compares more expensive program. If the graph is shown 5 times, it is the parameter graph of a piece of lead, in select material when the parameter picture that had better want lead, the degree of stand or fall of such wire with respect to be clear at a glance.
Graph 5
Besides, if the graph is shown 6 times, the cross section of lead accumulates the block with dc block to be worth size to concern, so transmission distance jumps over long lead cross section to accumulate should bigger.
Graph 6
Solution 3: Have undee plastic through repeater
Through the graph 7 can see, weaveform is before course repeater, because bus line electric capacity is too large, drop the edge becomes special delay, formed hook state, cause a sampling error easily so. And after passing repeater, message weaveform is given out afresh after course plastics, can see undee whole case is good, what can assure message is normal send and receive.
Graph 7
Summary
Face bus line capacitance to pass big case, general the problem that these 3 kinds of methods can solve the spot to appear, and in repeater this one, the repeater of bridge of CAN segregation net that ZLG sends far electron to produce collect line implement series, have 2/5/8 the interface of CAN bus line of electric segregation, can realize the relay function of CAN-bus network. Use this series product to be able to be achieved add laden node and the effect that lengthen communication distance, realize the function that the network relays enlarge is allowed.
