Electronics > Projects, Designs, and Technical Stuff
CAN bus always needs a resistor to shunt it (termination)?
zzattack:
What designates an 'end' of a bus within a branching topology? Would you place the terminating resistors at the nodes spanning the longest possible path? Or do you use Ohms law to determine a suitable resistor value to place at every single node?
Rerouter:
For canbus, the ends are usually loosely defined as the ends of the wires that are furthest away from one another, seeing as canbus is preffered as a piggyback bus instead of a trunk and branch topology, its usually clear where the ends are,
Other ways are 1 on the most active node, and some fractional ones at the ends of each branch if its closer to a star topology, trying to keep the total load around 60 ohms, though honestly I have never seen a bus fail in the wild with only 1 terminated 120 ohm node
Then there are intelligent can trancievers that can automatically switch in a termination resistor if it detects it is lacking one, but they tend to cost about 3x as much, from memory an optofet and a resistor was about the same price and a bit harder to kill.
Edit: Remove that capacitor, It will only cause you greif, can is a differential pair, so it doesnt need added capacitance to soak up differential mode noise
SparkyFX:
--- Quote from: zenerbjt on August 20, 2020, 06:29:39 am ---Our bus is about 2 metres. The contractor put a 120R at one end, (shunting the CAN bus) and a 120R_in_series_with_10nF at the other end......i take it this is bad?
--- End quote ---
Longer stretches of CAN should be wired as twisted pair, i reckon the approach to use a capacitor in series was trying to address issues with signaling? Because otherwise he might have tried to add the parasitic capacitance in form of a component, which is very unusual and counteracts the termination itself i guess.
--- Quote from: T3sl4co1l on August 19, 2020, 10:45:33 pm ---Even faster CAN (up to what, 10Mb/s?) isn't an impressive bitrate.
--- End quote ---
I heard 2 MBit is like a practical limit. Line length adds to the problem as well. Modern car is 1 MBit.
Rerouter:
CANFD goes up to 12Mbit, but the length and capacitance requirements get a bit hard to handle, also the controllers to interface with the protocol tend to cost more.
T3sl4co1l:
A DC-block terminator is used when the load is well known (it only works up to a certain line length, proportional to C), the pulse length is much longer than the electrical line length, and supply current should be saved (hence the DC block).
It will improve signal quality for an intermediate length bus, have little effect for short lengths (again, CAN is tolerant of stubs) and be insufficient for long lengths (where either C is too small, or the electrical line length is longer than the bit time).
Tim
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version