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CAN Termination Meter

Run the CAN Termination Meter MicroSim Fullscreen

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Description

Switch 120 ohm terminators on and off on a CAN bus and watch a resistance meter between CAN_H and CAN_L respond. Then diagnose six meter readings as healthy, one terminator missing, none or a broken wire, an extra terminator, or a short circuit. Used in Chapter 4.

The schematic shows the two bus wires, CAN_H and CAN_L, with a resistance meter at one end, two motors, and three places where a 120 Ω terminator can sit. Each terminator joins the same two wires, so the terminators are in parallel and the meter reads 120 Ω divided by the number that are switched on. The symbols are drawn by circuit-lib.js, the same library as the Chapter 3 circuit MicroSims. No current is animated, because the power is off for this measurement.

In this model the broken wire is next to the meter, so it cuts the meter off from every terminator and the reading is open. The 0.3 Ω reading for a short and the 62 Ω reading are illustrative values.

Lesson Plan

Grade level: Middle school and up (age 12+). Time: 10 to 15 minutes.

  1. Predict first. The bus starts with a terminator at each end. Before you look at the meter, work out two 120 Ω resistors in parallel.
  2. Explore. Switch off End B, then switch on all three. Notice 120 Ω with one, 60 Ω with two and 40 Ω with three. Then move a terminator from an end to the middle. Does the meter notice?
  3. Six readings. The terminators are hidden. Choose a diagnosis for the reading and press Check. You get one try for each reading, and 5 of 6 is mastery.
  4. Think about it. Why is "more terminators" not safer?

Always measure termination with the power off.

Specification

The learning objective, evidence of mastery and content for this MicroSim are specified in Chapter 4: Serial and CAN Communication.