CAN Bus Communications Decoded: Step Three: Get the Data Displayed
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Step Three: Get the Data Displayed
03.20
| 00:00 | Step 3 of the process is going to be getting the CAN data displayed on our laptop here. |
| 00:05 | This is typically the trickiest of any CAN reverse engineering process as there can be a lot of things that go wrong in this step but we'll work through it in a reasonably methodical way and make sure we can get that CAN data up on the laptop. |
| 00:19 | So I'm going to open up our CAN analyser software here and I'm going to set our speed to 500 Kb per second. |
| 00:28 | I've chosen that speed because I've had a bit of a browse of the documentation for this device and it looks like by default they're set to 500 Kb. |
| 00:37 | Now this isn't a guarantee that that is going to be what this device is set to because it has actually been used in a vehicle before so it might have been changed. |
| 00:45 | It is possible to change it on this device. |
| 00:47 | But it's a good starting point and if we're not seeing any response from the bus, we can start looking at different bus speeds. |
| 00:54 | So we'll accept that there. |
| 00:56 | So I'm not going to put in any filtering either. |
| 00:59 | This section here allows me to add some filters on particular CAN data frame PIDs so it'll only show CAN data frames that are within that range. |
| 01:10 | But currently I just want all the traffic displayed, so we're going to leave that as the entire range of 11 bit PIDs which is 0 to 7FF or 2047 in decimal. |
| 01:23 | So we'll accept that and open up our software here. |
| 01:28 | And I can see we might have a little bit of an issue that we need to investigate straight away. |
| 01:34 | The traffic light on our CAN analyser here is actually blinking rapidly. |
| 01:40 | Now that LED in this device is hooked directly to the electrical signals that are on the bus so that means there must be some sort of electrical signal on the bus that is actually flicking back and forth between different levels so that's not typical behaviour. |
| 01:56 | I've got another key showing me here on the laptop is that our analyser software here is telling me we've got an error on the bus and it's saying bus heavy. |
| 02:04 | Now bus heavy in this instance means that the CAN bus is basically loaded up with traffic or it's completely jammed and nothing's going to get through. |
| 02:13 | In this situation I could go down a couple of routes. |
| 02:19 | I suspect what the problem is going to be is that our CAN bus keypad here has a termination resistor built into it but our analyser actually doesn't. |
| 02:28 | So we've got a little bit of an interesting bus setup here but it is still a trunk, it needs to have that termination resistor at either end. |
| 02:37 | So I've only got one at the moment so what I'm going to do is take our little CAN bus termination resistor and I'm going to plug that into the last port on our bridge here and by the looks of it I think that has actually done the trick because our traffic LED is now no longer flashing. |
| 02:57 | The software is now telling us, we've got bus light which means the traffic on the bus is very light and we've actually seen a received message come through here as well. |
| 03:08 | That's great because to receive a valid message like that it must mean that we've got our CAN bus speed set correctly as well. |
