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CAN Bus Communications Decoded: Adding Termination Resistors

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Adding Termination Resistors


00:00 - At either end of our CAN bus wiring we need to include termination resistors to give the correct bus voltage levels and limit signal reflections.
00:08 We can do this either by having a device that includes termination resistors at either end of the main bus trunk or adding our own resistors in manually.
00:16 When you're planning out your CAN network you need to read the documentation of each device on the network thoroughly and determine if the device includes CAN resistors or whether they can be switched on and off.
00:28 I'm always very hesitant to use a device in a setup if it has hard wired CAN termination resistors that can't be disabled because this means the device must be at one end of the main bus trunk which can make harness planning quite difficult.
00:44 Devices with switchable CAN transistors are great because they can either be connected to a node or to the end of the main bus trunk, meaning we don't have to manually include a termination resistor in the harness build.
00:57 If the planning doesn't work out like this however and there's not a device at either end of the trunk that includes a termination resistor, adding one in is a fairly simple task.
01:07 We do this by adding an extra 4 pin female DTM connector at the end of the bus trunk, which is populated with a switched 12 volt source and a power ground, CAN high and CAN low.
01:20 We can then build a male DTM 4 pin connector with an integrated 120 ohm resistor between the CAN high and the CAN low pins which when inserted in the harness provides the correct termination resistance.
01:33 I like to also include the 12 volt and ground in this connector because it gives us a lot of flexibility in the future.
01:39 Let's have a look at how to create the male side of that termination connector.
01:44 I've got our DTM 4 pin here and the idea is that we're going to take our 120 ohm quarter watt resistor and we're going to crimp on the female DTM pins which match the male DTM connector onto the ends of that and we're going to insert those into the connector.
02:03 We're then going to put some sealing plugs in place and we're going to fill up the back cavity here with some potting compound.
02:12 Now what that potting compound is going to do once our resistor and pins have been inserted there is it's going to keep everything really rigid and make sure that the solid core wiring on the end of our resistor here isn't going to crack or fracture from vibration.
02:25 So to do this, first part of the plan is going to be to crimp our DTM terminals onto the end of our resistor legs here.
02:34 Now our resistor leg wiring is actually just under the size where it's going to crimp nicely into our DTM pins here so what I'm going to do to get around that is I'm simply going to bend over a small section of that leg wiring.
02:50 Now when I'm doing this, I'm just bending over a small section because I want to keep the length here long enough that when we insert it into our connector, it's not hard up against the end of the plastic body there.
03:03 So just using my pliers to get me a good bend point, I'm just going to start that.
03:07 And then bend the rest of it over.
03:13 And just tighten that up a little bit just so that we can get it into our connector there.
03:19 Now I can pop our pin in place like so.
03:26 Into our crimping tool.
03:32 And get that crimped down.
03:36 Just a wee tug test there, that's really nice and snug, it's absolutely not going to go anywhere.
03:41 So we'll do exactly the same on the other side.
04:02 So now we've got our DTM female pins there crimped on, I'm going to bend those legs at 90° right by the resistor body.
04:12 And that's actually going to give us a good spacing to insert that into the back of our DTM 4 pin connector.
04:18 Now my standard pin out for connectors like this is that pin 1 is high, pin 2 is low, pin 3 is plus 12 volts and pin 4 is ground.
04:30 You might have your own standard for this, as long as it's documented, anything you'd like to use is fine.
04:36 That's just the standard we use here at HPA because it's a common connector that we put on all the electronic devices that come in, it lets us plug them into a standard bus connection box and get everything talking for demonstrations nice and easily.
04:50 So I'll get these pins inserted in the back in spots 1 and 2 here and I'm just going to get them started and then I'm going to do the rest of the insertion just with my pick tool because if I try and push on the back of the resistor here, what I'm actually going to do is just bend the legs.
05:07 So just piece by piece I'm going to get these down into place.
05:16 And then looking at the front of the connector, yep they're coming through in the right spot so I'm good to push those all the way through now.
05:30 So I've got that resistor inserted in there now, our pins are all the way to the front of the connector so they're fully inserted and our resistor is just really nicely sitting in the back of the connector there.
05:41 So when we fill this up with potting compound, it's going to keep everything nice and rigid and make sure those legs don't ever fracture.
05:47 Now before we go ahead and mix up our potting compound and get it in the back here, we're going to have to put some sealing plugs in place.
05:54 We've got 2 unused positions in this connector and if we pop our potting compound into the back here now, it's actually going to leak forward and cause a bit of a mess.
06:03 So DTM connectors have specific sealing pins.
06:07 So I can just use a pair of pliers and get those into place.
06:21 Just like that, so that's going to be fully sealed now, so we're ready to mix up our potting compound and get it into the back of the connector.
06:29 To do that, I just use the back of a scrap plastic bag because it's going to be thrown out anyway.
06:36 Potting compounds I like to use are the ResinTech RT125, Hellerman make one called the V9500 or Raychem make one called S2500 as well.
06:46 Any of those potting compounds is going to be fine.
06:49 All it's going to do is keep that resistor nice and rigid and stop anything vibrating and cracking over time.
06:56 So I'll get some of this squeezed out of our tube here, get it mixed up and into the back of that connector.
07:16 Now I've just mixed this up manually with my pick tool here.
07:19 If you've got any of the spare mixing tubes, if you've got a stock of those then that will actually make the procedure a little bit easier because you can just put the mixing tube on there and squirt directly into the back of our connector body here.
07:31 But because this compound actually self levels really nicely, we're going to call that fully mixed up and I'm just using my pick tool, going to scoop some and get it into the back of our connector here.
07:51 As I'm doing this, I'm just working it around the resistor to make sure it gets underneath it and all the way fully around it and we're not going to have any air pockets trapped in there.
08:01 Now the tidier you can be, the better when you're doing this but we are going to use a rag just to come back and clean everything up afterwards so don't stress too much about it if you're getting some potting compound on that outside flange of the connector there.
08:27 So I've got enough potting compound in there now, I'm just going to clean off the end of my pick because nothing's worse than when the potting compound hardens onto the pick.
08:36 And then we're just going to come in and clean off the flange of this connector as well.
08:46 Excellent, and that potting compound will self level really nicely so we either want to leave this connector in this position for 24 hours to really harden up nicely, I actually like to grab a little portable bench vice, get that set up reasonably vertically and just gently tighten that connector into place and then just pop this somewhere where it's not going to be touched for the next 24 hours.
09:13 Now I've used a 4 pin DTM connector there because the idea is that there'll be 12 volt and ground in the matching connector on the harness as well.
09:21 But you can omit the 12 volt and ground and just use a 2 pin DTM connector, however the added flexibility you get with popping 12 volt and ground into it can be a real timesaver down the line.
09:33 That said, if the harness planning doesn't allow for it, then it's not a complete deal breaker.
09:38 In this course section, we've had some good practical discussion of the theories and techniques you'll need to know to work with a CAN system in your project.
09:46 In the next sections of the course, we'll lay out the specific processes we use when working with an unknown OEM CAN bus as well as how we approach building your own CAN network setup from scratch.

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