Bill, I wanted to post a follow-up to what you and I spoke about in this thread last November. I actually get a single Error Frame when using the USB-8473 @ 500kbps on a Toyota car (which is definitely a 500kBaud bus). The single Error Frame almost always appears just after I turn on the Ignition:
01 00 00 7A
Indeed, if I enable the "Stream to Disk" function in MAX's "Options," I can leave the Ignition switched on, and over the course of several minutes, several text files named Stream 0, Stream 1, Stream 2, and so on will appear (each 1MB in size), and when I search all of those files, I usually can only find a single instance of the Error Frame in the Stream 0 file. (I tested the car with all doors closed, running my cables through an open driver's side window, testing only by turning the Ignition ON or OFF.)
My cable (measured from the tip of the 9-pin D-SUB to the tip of the OBD2 connector I use at the other end is 99cm (just under 1m), so as you said before, I shouldn't have to use a 120-ohm terminating resistor across CAN-H & CAN-L. Even so, I put the 120-ohm resistance (measuring it first to ensure I was right) across the CAN wires (at the end of the cable closest to the OBD2 connector) and then tested again. I still get the same Error Frame when turning on the ignition.
I then hooked up a digital scope, setting SEC/DIV to 2.5us as you directed, with the CH1 probe on CAN-H and the CH2 probe on CAN-L, viewing both CAN-H & CAN-L signals separately on the scope screen. Without the 120-ohm terminating resistor, I could see the bits fine when I played with the Trigger Level adjustment. With the 120-ohm resistor across CAN-H/L, I could still see the bits, although there was much more signal ghosting on my scope screen when I did added it. On the scope, CAN-H exactly mirrored CAN-L, with CAN-H rising as high as 1.8v and CAN-L going as low as 1.8v.
There were occasions with my scope hooked up that I got as many as 16 Error Frames back-to-back, which appeared as follows:
Error Frame 5.7059 01 00 00 7B
Error Frame 5.7059 01 00 00 77
Error Frame 5.7064 01 00 00 7B
Error Frame 5.7064 01 00 00 77
Error Frame 5.7069 01 00 00 7B
Error Frame 5.7069 01 00 00 77
etc.
Not sure what all this means, or if my "single" Error Frame is anything to worry about. But I wanted to report it nonetheless.
Thank you.
Bill_in_Detroit wrote:
James:
In response to what you would look for using a scope:
High Speed CAN is a differential voltage logic system. When the signal is recessive, the signals on CAN_H and CAN_L are roughly equivalent at a value of about 2.5 volts DC relative to ground. When the signal goes dominant, the CAN_H rises to about 3.5 volts DC and the CAN_L drops to about 1.5 volts DC, giving a net differential voltage of about 2.0 volts DC during the dominant bit. A discussion of this and about termination can be found in the Microchip document number AN228 which can be found at this link -> CAN Physical Layer.
What you would look for with a scope is that the voltages can be clearly seen at these levels during message traffic. Most of the problems that I have seen have shown up pretty quickly on a scope. First of all, you should use a scope with sufficient bandwidth so you can actually see the bit levels change. If termination is incorrect, you will not be able to get a lock on a message as the reflected signals will overwrite each other to some degree and the signal will look like noise to a large extent. For high speed CAN at 500 Kbps, if you set your scope in the 2 or 5 us range you should be able to see the bits clearly.
Good luck with your project. There are other tools on the market that may work better for your purpose than trying to write something on your own, but as this is the NI forum I won't go into that here without first clearing it with NI.
