resistor

[can can canopen]

Hi All,

The question is regarding the CAN BUS. Why 120Ohm resistor is used for the termaination,what happens if there is no resistor or if we place a high value resistor.

Thanks in advance.

[Tom Haggerty]

The 120 ohm resistor is specified by the ISO standard (11898) for CAN networks.  The hardware for CAN has been designed to use this value.  It should be noted that not all nodes on the CAN bus need the resistor only nodes designated as the "ends" need this resistor.  There should be two nodes that have the termination resistor.  All other nodes should be between the end nodes and not have the resistor.  I hope this helps!

Tom 

[Tom Haggerty]

If there are no resistors used, there is nothing to develop the signal and CAN communication is impossible.  If the resistors are too large, then noise (electrical interference) can affect the signals and you may see more frequent error frames.  A smaler resistor is also not good.

Tom

[pBerg]

Hi can can canopen,

Tom is correct. The High-Speed CAN protocol involves communication that goes both ways through the same line. You need termination resistors across the lines to prevent signals sent through the lines from reflecting at their termination points. The ISO 11898 CAN standard requires that a cable have an impedance ("resistance") of 120ohm, and so by using 120ohm termination resistors at the ends of the cable, this cable impedence is matched, thus preventing signal reflection in the lines.

For Low-Speed CAN, pages 4-10 through 4-12 of the NI-CAN Hardware and Software Manual have excellent information on termination and formulas for determining the necessary termination resistance for CAN tranceivers.

For a little more information on impedance matching for CAN, you may reference the <link no longer exists>.