SAE J1939 PDU Format field

[adityag8]

Hi ,

I have a very basic question. In the PGN of J1939 , there is a field called PDU Format which is of 8 bit length which is basically used to know the value in PDU specific field . If the value of PDU Format is less than 240 then PDU specific have destination address and the particular message is destination specific but if its value is between 240 to 255 then the message can only be broadcast and the PDU Specific field contains a Group Extension. But my question is, if PDU Format is only used for specify whether it is a destination specific message or Group extension message then its length must be 1 bit only. As with the help of 1 bit we can easily specify the behavior of message i.e.destination specific or group extension. So why its length is of 8 bit.

Is it only due to increase the range of PGN or what? Please help me to clarify on this.

Thanks,

Aditya Garg

[JefeL]

It is to increase the number of possible PGNs in addition to allowing multiple different PGNs to have different priorities on the CAN bus. The lower the PGN, the higher the priority so when two frames are transmitted simultaneously, the one with the lower ID will win arbitration.