Terminating Resistor CAN

[Al Skierkiewicz]

To back up Alan and Oz, the termination of the CAN bus has more to do with transmission line theory than anything else. A typical twisted pair of #22 wire with approx. 1/3 turn per inch has a stable impedance (above 1 kHz) of about 60 ohms. As the bus can have several receivers and transmitters, it is not practical to treat this system with matched sources and loads so the transmission line is terminated at each end with a nominal 120 ohm resistor. The receivers and transmitters then present a high impedance to the line (except when transmitting data). As the bus length is typically much less than 1/10 wavelength of the highest frequency, the two resistors do appear to properly terminate the line in 60 ohms. By terminating the line in it's characteristic impedance, reflections at each end are reduced and the overall signal degradation is minimized. The greatest problems we see in the field with teams using CAN is the improper termination resistors and failure to properly install the resistors. The ability to use the termination on the PDP, while it may not directly fit your design, is perhaps the best method to prevent termination problems from affecting your operation.