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This represents a 2.5V automotive controller area network (CAN) bus. This is an industry standard for ECU communication on high-speed networks. The bus is operating at 500kbps, so this would represent a C-class network. This is a two-wire network that utilizes a twisted-pair of transmission lines. The circuit in blue that pulls high is known as CAN(+) and the circuit in green that pulls low is known as CAN(-).
Every ECU, or node, biases the voltage on the bus wires. When the bus is recessive (no messages being sent) the voltages on each wire are very close to 2.5V. When the bus is dominant (messages being sent), CAN(+) pulls to 3.5V and CAN(-) pulls to 1.5V. The network lines therefore mirror each other. Each deviation from recessive represents a "1" in the binary code. CAN ECUs use hexadecimal coding.
Automotive CAN-C network nodes utilize termination resistors between CAN(+) and CAN(-). Therefore, each node is wired in parallel with all others. The termination resistors are present to mitigate signal reflections at the end of the line. This network's termination resistance is close to 60 ohms. The two ECUs at the top are the dominant nodes that each contain a 120 ohm termination resistor. These two resistors in parallel equal 60 ohms. The four lower ECUs are known as non-dominant nodes and their termination resistors are much higher than 120 ohms and therefore have less of an impact on total circuit resistance.
The oscilloscope pattern represents messages being sent by the upper-left ECU. Only one ECU should be sending a message at any given time. If not, this is known as data corruption. Each ECU in my representation has a double-throw switch to choose between manual logic switching and a logic train. I provided LEDs to represent general network conditions. The blue LED shows that ECU is transmitting information. The green LED will be on solid when connected to the bus and will flash when any ECU is sending messages. I provided switches at the bottom of the schematic to short each bus wire to ground. No communication is possible in this condition, so I provided red LEDs in each ECU to indicate when that occurs.
Each ECU has a switch on its ground connection to remove its power source. Each ECU also has switches to open each of its bus wires to the other nodes. You can manipulate these switches to see the effects on the network. There is an ohmmeter in the middle of the schematic to measure the network's total resistance. Be sure to open each ECU's ground before measuring. Use the ohmmeter to see the effects of opening various network connections. If you measure close to 120 ohms, there is an open on one of the two dominant ECUs.
This is not an exhaustive explanation, nor is this an exact representation of ECU schematics. This is meant to be educational. Please enjoy!!
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