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Wiring Fundamentals: CAN Communications

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CAN Communications

03.46

00:00 - In the last few years it has become increasingly common for network communications to be included in an EFI system design.
00:07 As it provides the opportunity to get signals from one place in the vehicle to another without needing to run a dedicated wire for each.
00:14 This reduces the cost, weight, and bulk of our wiring harness substantially.
00:18 A great example of this in action is a dash display system.
00:22 Traditionally a dash display would have needed a signal wire going to it for each parameter we wanted to display.
00:28 Engine RPM, coolant temp, oil pressure, et cetera.
00:31 If the dash display can instead be connected to a communications network, where it can read all the desired data, the number of wires we need to connect to it dramatically decreases.
00:41 Another benefit of a communications network setup like this is the decisions we make about which data the dash receives are no longer physically tied to the construction of the wiring harness.
00:51 Once it's connected to a communications network, we can make changes to the data it reads and displays without modifying the harness further.
00:59 The most common network communication protocol you will strike is CAN.
01:02 Which is an acronym for controller area network.
01:05 It's also commonly referred to as CAN bus.
01:08 The details of the CAN protocol and its electrical signalling and data structures are outside the scope of this course.
01:13 But we do need to discuss how to physically wire and make the connections between the devices that will communicate on the bus.
01:19 The wiring requirements to implement a CAN bus are relatively straightforward but some basic rules do need to be followed.
01:26 A CAN bus consists of two wires, CAN high and CAN low.
01:30 Which are twisted together along their length to form what we call an unshielded twisted pair.
01:35 This bus runs through the length of the wiring harness and any device that needs to communicate on the bus taps into it at the closest available point.
01:44 You can think of this bus running through the length of the wiring harness as a trunk, and the points where devices tap into it as branches.
01:51 At either end of the trunk we connect the CAN high and CAN low wires together via a 120 ohm resistor, which we call the termination resistor.
02:00 Often devices will have termination resistors built into them, meaning they need to be directly connected to the bus at either end.
02:08 This is where things start to get a little bit complicated and usually we have to stray away from strict adherence to the CAN bus protocol.
02:15 We'll look at a couple of situations to clarify.
02:17 The most simple implementation of a CAN bus is one where we only have two devices that need to communicate.
02:23 We can run and unshielded twisted pair of wires from one device to the other connecting the CAN high pins together and the CAN low pins together.
02:31 If the devices have integrated termination resistors then the job is done and the devices will communicate.
02:37 If we need to add in termination resistors ourselves, this can be easily achieved by connecting a 120 ohm resistor between the CAN high and CAN low wires when we perform the crimp to attach the connector pins.
02:49 If we then want to add a third device that communicates on the bus, we run an unshielded twisted pair of wires from its CAN high and CAN low pins and splice them into the main trunk of the bus at the nearest convenient point.
03:01 However if this third device has an integrated termination resistor, we're now breaking the rules outlined in the CAN protocol as there is a device connected to the centre of the bus that has a termination resistor.
03:13 Often there's no real solution to this problem but you need to be aware of it so if it does cause you issues, you know the cause.
03:19 Our saving grace is that the length of wires we're running in the wiring harness are quite short when compared to other uses of a CAN bus in the industry.
03:27 The termination resistors have a larger effect as the length of the wires increases.