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CAN bus always needs a resistor to shunt it (termination)?

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exmadscientist:
Always put a (preferably split) terminator at every transceiver. Every single one. No exceptions.

You can always just not populate it. It's a lot easier to skip loading a R-C-R network if you don't need it than to have to bodge in a terminator because Something Changed and you need one now in a place you didn't need it before....

T3sl4co1l:
In the design, yeah, doesn't hurt.  In a product, leaving a jumper to disable the terminator for mid-bus devices is a good idea. :)

Tim

Doctorandus_P:
As written before, signalling on CAN bus has a "dominant" and a "recessive" state.
In the "dominant" state CANH is driven to a higher voltage, while CANL is driven to a low voltage (usually GND).

In the "Recessive" state None of the CAN wires are driven in any way, and a resistor is required to pull the CAN bus wires together.

So the termination resistors on a CAN bus have 2 functions.

Using a capacitor in series with a resistor at the end of the cable is a trick used with RS485. It makes it easier to have a reasonably high DC bias voltage on the bus (and thus noise resistance) when the bus is not driven by any of the nodes, while still having a low AC resistance close to the intrinsic cable impedance, to dampen reflections.

When an RS485 bus is Idle, and the termination resistors pull the bus wires together, there is an "undefined" state on the RS485 bus. RS485 receivers react to relatively small voltage levels (50mV to 200mV or so) and easily pick up noise without an extra biasing network on the bus. The simplest bias network are a few resistors of a few hundred Ohm to a few kOhm to pull the wires apart, Lower resistors form a higher load on the bus, and may cause distortions, while with higher value resistors the created offset voltage decreases. With a capacitor in series with the termination resistor, you have it both ways.

But that is for RS485. For CAN the capacitor has no advantages.

SparkyFX:

--- Quote from: exmadscientist on August 20, 2020, 05:26:01 pm ---Always put a (preferably split) terminator at every transceiver. Every single one. No exceptions.

You can always just not populate it. It's a lot easier to skip loading a R-C-R network if you don't need it than to have to bodge in a terminator because Something Changed and you need one now in a place you didn't need it before....

--- End quote ---
Should the design goal be to put components of the exact same design on a CAN network, you need a method to tell them what their message IDs will be anyway, otherwise there will be arbitration problems. So practically you need to know at the design stage which component will be at the ends or in the middle and determine which ones get a termination resistor and which ones don't. Often this is just defined by function, you could also include it in the wiring, like an endcap or in the connector.

Theoretically you could form a ring and gain redundancy against discontinuity, in which case you would need to know where to place the termination resistors or define where the discontinuity will be.

rvalente:
Check this article

https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/07/14/the-importance-of-termination-networks-in-can-transceivers

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