CAN-Breakout-Box Termination

[fabian.gutjahr@fmc-ag.com]

Hello,

With the CAN breakout box from NI there are two adjustable resistors (120 and 60 ohm), if you now measure between CAN_H and CAN_L you have the set resistors. For termination you have to set a 120 ohm resistor between high and low at the "start" and "end" of the bus, so you would have to measure 60 ohm (two parallel 120 ohm resistors). So you have to set 60 Ohm for the normal CAN bus of the breakout box, do I see that right? For what is then the 120Ohm resistor meant that are then 240Ohm resistors at each end?

Greetings

Fabian

[Minions]

The CAN termination is expected to be 120-ohms at the driver and 120-ohms at the receiver with a 120-ohm matched impedance cable between for proper operation.  This would equate to a resistance reading of 60-ohms from either end. 

When used in a multi-port application, it gets a little more complicated.  The termination should only be used at the end points of the communication path; otherwise, the impedance becomes too much for the drivers and the signals will not be read properly. 

Some pieces of equipment are already pre-loaded with the 120-ohm termination resistor for their expected usage.

[fabian.gutjahr@fmc-ag.com]

Ok, I didn't quite understand that yet, what are the "endpoints of the communication path" , for example if I have two CAN sensors and one NI-CAN interface (without the onboard, software-selectable termination) connected to the breakout box, are additional termination resistors needed? Is there a circuit diagram for the breakout box, perhaps it is then easier to understand.

[Minions]

If you look up CAN bus in Wikipedia, there is a good diagram for High-Speed CAN Network ISO 11898-2 that depicts how the termination is to be used with one set of 120-ohm resistors at both ends of the bus.

[fabian.gutjahr@fmc-ag.com]

Yes, as described above in the question, it is clear to me how this happens in principle with the termination. But it is unclear to me how the resistors in the breakout box are connected. The red circled is therefore the breakout box, right? (Of course still with several open CAN connections) Then you measure between High and Low 60 Ohm.

Therefore the switch is set to 60 Ohm.

Termination Switch Breakout Box