CAN Bus Monitor

[Paul_Knight_Lockheed_Mart]

I have two USB-8473 connected together, "CAN_H" to "CAN_H", "CAN_L" to "CAN_L" both are connected to my UUT Can Bus.  Communicating with CAN0 to the UUT works.  Communicationg with CAN1 to the UUT works.  I get multiples update message from the UUT.

However if I am using CAN0 to communicate to the UUT and CAN1 as a "listen only" monitor using MAX's CAN Bus Monitor, I see no data in MAX's CAN Bus Monitor.

Is there any arbitration that needs to be setup?

[Hooovahh]

You didn't specify how your wiring is setup. I suspect what is happening is some kind of odd collision happening.  

Does your wiring resemble this image.

Notice how there is nodes on the bus.  You don't have three CAN-H wires coming together in one place, you only tap into the bus and I think according to the CAN spec you are supposted to be no more than 15cm away from the bus.  Also notice how on each end of the bus you have a 120 ohm resistor.

CAN is generally more robust than these strict guidelines, but if communication is funky, this is the first thing I question.

[Paul_Knight_Lockheed_Mart]

I have attached a rough drawing on my setup.

[Hooovahh]

Well that doesn't look bad.  I'd see no reason to suspect an issue due to wiring.  Are you sure you are using the NI monitor tool right?  I don't have any CAN hardware to test with but there is a Start button, dumb question I know but did you press that after making the change to be listen only?  Do you see data in this monitor tool when you aren't listen only?  I've used the NI CAN monitor tool with listen only successfully in the past so that feature worked at one point.

[Paul_Knight_Lockheed_Mart]

What I am trying to do is debug a LabWindows program.  When I execute it, I see data in BusMonitor but the UUT does not respond.  So I have a LabVIEW program that when I execute it the UUT responds, but I don't see anything in the BusMonitor.

From what I can see both programs use the NI-CAN Library the same wait.  Both use ncWrite and ncRead.  The data looks the same, arbitration IDs look the same.

One program works, one doesn't

[Hooovahh]

Very strange, again I don't have the hardware but was there any place that an error message could be seen?  You aren't accidentally trying to use the same hardware in two places are you?  Beyond that I am out of ideas, sorry.

[Paul_Knight_Lockheed_Mart]

No I only using the hardware in one at a time.

Thanks for your help.

[Paul_Knight_Lockheed_Mart · Accepted solution]

There was a baud rate difference between my LabWindows program and the LabVIEW program.  One was set to 100000 and one was 1000000.  The correct value was 1000000.  Everything is working now.