Hi All,
Following is a part of article about Request Specific Address in the forum. viewtopic.php?f=7&t=552&p=1466&hilit=request#p1466
Request Specific Address
Basically, if "Request Specific Address" is checked, the display will request any PGNs marked as "Request" in the J1939/NMEA database that are mapped to the device. For instance, you mapped the Total Fuel Used (SPN 250) parameter to the VD device displayed in your image. If you checked "Request Specific Address", the display would periodically send a request for PGN 65257 from address 0 (the source address of the VD device). This is because PGN 65257 contains the Total Fuel Used parameter and is marked as "Request".

My question is if the PGN requested by Display can be triggered by event ,etc by Button.

Many thanks,
Tim

Hi Tim,

I have more questions than answers. What does VD device mean? Are you trying to transmit the Request message (0xEA00) triggered by an event? Or, do you want to transmit a different PGN from the PowerVision display triggered by an event?

The default (Request Specific Address not checked) is that a PowerVision display, will send the Request message (59904/xEA00) to the global address, FF, (ie., 0xEAFF). For your example to request the Total Fuel used, PGN 65257, assuming the PowerVision display source address = 242, the Request message PDU Format would be xEA, the PDU Specific=xFF, and the Source Address=xF2. The data would be Byte 1 = E5, Byte 2 = FE, Byte 3 = 00.

When the Request Specific Address is checked, the PowerVision display will send an additional Request message with the specific destination address of the specific SourceAddress of the device. Assuming the SourceAddress=2, the additional message PDU Format=xEA, the PDU Specific=x02, and the Source Address=xF2 and the data would be the same as the previous message.

Sorry, I find this easier to understand using hex so you can see the byte for the PDU Specific used as the destination address.

You could confirm this by looking at the traffic on the CAN, with the Request Specific Address checked, you should see two requests for each of the PGNs you have indicated in the Library as Request and mapped to a device. One request message will have the global destination and one request message will have the specific destination with the same PGN for data.

All of the above may not have anything to do with what you are attempting to do, but it is good to understand what the tool and the PowerVision display are doing.

Is there any way to set the pacing on these global and destination specific requests? It seems like they're automatically sent about every 10 ms or so. For some reason, we are told that this is too fast of a request for Fiat engine hours. I know a FFCAN message can be created with adjustable timing loop, but I wanted to check to see if there was another way. Thanks.

Requests for a specific PGN are sent 1 second apart. If there are more PGNs being requested then the time between requests is the second divided by the number of requests. If the request goes to the global address and also to the specific destination address then there are twice as many messages. You would have to have 100 requests for them to be 10 ms apart.
What display are you using and what PowerVision version? I checked this to make sure on a PV780 with PV 2.8.10546. If this is not what you are seeing can you send a CAN capture of the packets?

Sorry, I'm using a PV480 display with 2.8.10546 software. I also meant to type 100 ms between each request.

I looked at my capture of data and see that the cycle of all requests are about 100 ms apart. There are a total of 8 requests and then a repeat. There are 4 PGNs and it does a request for address specific and then a global (FF) request. So, what you wrote makes sense with the number of requests I have in my capture. In actuality, there are 8 total requests per 1 second.

I think the issue with Fiat was something else and they just told us that they were coming in too fast. I think we figured it out though. Their SA of the ECU is 0, the TSC1 message is only used if it comes from address 33, and the hours only respond if the request comes from address 39.

Thanks for the clarification on how the request process works in the background.