CAN bus impedance measurement?

CAN bus impedance measurement?

CAN Bus Termination. There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.

CAN cable impedance?

The wires are a twisted pair with a 120 Ω (nominal) characteristic impedance. This bus uses differential wired-AND signals.

CAN bus voltages?

Measured on a machine that is running, it will usually range between 2.7 and 3.3 Volts. Value should normally be in between 1.5 and 2.5 Volts. Measured on a machine that is running, it will usually range between 1.7 and 2.3 Volts.

CAN bus shorted to ground?

Shorts and opens: The CAN controllers will tolerate a short circuit of one of the two lines to ground because of the characteristics of the differential bus. It cannot tol- erate both CAN bus wires shorted to ground or to each other. It will tolerate one of the CAN lines being open or disconnected.

Which coding technique is used in CAN protocol?

The CAN Bus protocol can be summarized in the following manner: The physical layer uses differential transmission on a twisted pair wire. A non-destructive bit-wise arbitration is used to control access to the bus. The messages are small (at most eight data bytes) and are protected by a checksum.

CAN communication 120 Ohm?

In a low speed CAN each device should have a 120 Ohm resistor. In a high speed CAN-Bus (>100Kbit, used in automotive) only each end of the main loop should have a 120 Ohm resistor. You should measure 60 Ohms over these 2 wires, because there are two 120 Ohms resistors in parallel (parallel resistance calculator).

Why is there a 120 ohm bus impedance?

Why it’s 120 is simply a function of the design limited by physical size. It isn’t specifically important which value they picked within a broad range (for example, they could have gone with 300 Ohms). However, all devices in the network have to conform to the bus impedance, so once the CAN standard was published there can be no more debate.

How big of a wire do I need for a CAN bus?

It’s designed to be 120-Ohm at the nominal baud rate, but adherence to that impedance specification pretty much goes out the window at the connection points since 120-Ohm connections aren’t ever used by equipment that employs CAN-Bus. Consequently, pretty much any shielded, twisted pair wire 20AWG or larger will do.

How does balanced line operation work on a CAN bus?

On CAN bus systems, balanced line operation, where current in one signal line is exactly balanced by current in the opposite direction in the other signal provides an independent, stable 0 V reference for the receivers.

What is the total voltage of a CAN bus?

You can think of CAN as a open collector bus implemented as a differential pair. The total of 60 Ω is the passive pull-together of the CAN bus. When nothing is driving the bus, the two lines are at the same voltage due to the 60 Ω between them.