One of our customers wanted to add a message which included a 12 bit SPN.
This did not seem possible using any combination of bytes and bits.
(The bit length field is limited to 7)

mwalker,

SPNs only really make since when discussing or setting up DTCs which are contained in DM1 and DM2 messsages. The SPN can be 12 bits (maximum length is 19 bits). The DTCs along with the DM1 and DM2 messages are outlined in the J1939-73 spec. You can add DTCs in the database tab of the tool.

PGNs, which are part of the header in J1939 parameter messages, are limited to 16 bits and are all predefined. If you need a special message to send parametes, there are some PGNs set asside for proprietary messages.

In a message, each data byte is 8 bits but you can use two bytes to represent a parameter if its bigger than 8 bits.

When you say message, what do you mean? Is this going to be an alarm or event type message or something like a parameter that is sent all the time?

Thanks,

Brian McRae

I use the term SPN to refer to the parameter within the data bytes of a particular PGN, although, as you say, SPNs only seem to be used in the context of DTCs.

There seem to be very few SPNs that would give this problem. The only example I came across was spn2660 - Joystick 1 X-Axis Position where the data length is 10 bits.

It is easy enough to get around the problem but maybe it should be possible to specify a data bit length greater than 7.

I see it now. You are having difficulty in the Database tab adding a J1939 Parameter Group which contains a parameter that spans a byte boundary.

I just tried to enter the PGN 64982 which contains the SPN 2660 and it would not let me set the bit length to 10. The CANCapture database shows byte 1, bit position 7, length 10 bits which is correct. I can't set the same in the powervision tool.

The standard has a fairly detail explination about how this bit length parameter over a byte boundary can be handled in J1939-71.

I will enter the bug in our issue tracking system.

This has been an issue with me for PowerVision 2.3 as well. There are so parameters that cannot be built into the J1939 database if they are greater than 8 bits in length, but not ending on a full length of a byte. It would be nice to see an update for this for 2.3 for use with the PV750.