canWarnPeriodsTooFast warning

[scottw]

I get the canWarnPeriodsTooFast warning while I am configuring a NI-CAN channel to transmit the following messages at 500 kbaud using the extended arbitration Id format:
42 messages at 50 msec
10 messages at 100 msec
16 messages at 1000 msec.
This works out to be 956 messages per second. The data field for each message is 8 bytes. My calculations indicate the bus should be capable of transmitting over 2000 messages per second.
Why do I get a warning? This should be well within the capacity of the card.

[JoshuaP]

Hey Scott,

The problem you are running into is that the CAN devices can only handle a combined transmit rate of 2 msec. If you are transferring at 50 msec then you can only have 25 messages. This is a limitation of the processor and it is a board rate, not a port rate.

In order to get the performance you need, I would suggest getting a second CAN device. Then you could run 25 messages at 50 msec on one device and run 17 messages at 50 msec, 10 messages at 100 msec, and 16 messages at 1000 msec on the second device.

This would give you a 2 msec transmit period on the first device and a 2.19 msec transmit period on the second device.

I hope this helps out.

JoshuaP
National Instruments

[EktaV]

Hi,
Since the combined transmit rate for the card is 2ms , and here for the second card it comes out to be 2.19 ms, what would be the upper limit for this transmit rate?
What i mean to ask is that how much can this limit be stretched?
If the messages to be transferred are more than specified in the example here (with different periods), then it would be our own discretion to decide how many cards would be needed to handle them(depending upon the periodicity) Right?

Regards,
Ekta

[biker2000]

Hey EktaV,

I'm not sure if I understand you right.
Yes, you do need to do the calculation to figure out if all your transmit rates can be handled by a single CAN card.

A (combined) transmit rate of 2.19ms is slower than the 2ms. So, the combined rate can be equal or slower than 2ms (transmit a CAN message every 2ms or at 500Hz).

You might be able to achieve a transmit rate of a little less than 2ms, let's say 1.9ms, however, it depends on the actual number of transmit objects (and ports of the card) when the warning (0x3FF620A1) will turn into an error (0xBFF620A1) and the transmission won't work.

-B2k

[R_J]

Hi

I am sending CAN message and monitoring it using MAX Bus Monitor.

I want to know how we calculate the arbitration ID for the particular message & what is the node id.

Regards

Raman

[Ebele_Obijiofor]

Hi Raman,

I am not sure I understand what you mean by "...calculate the arbitration ID for a particular message..." Do you mean you want to check for a particular ID when you receive a frame? If so, look at the CAN Receive VI from the example finder; the "ncReadNetMult" VI returns a cluster that contains the arbitration ID, amongst other things. Hope this helps.  Please let me knoiw if I misunderstood your question. Thanks!

Regards,

Ebele O.

[R_J]

Hi Belle

Thanxs for reply.

I just wants to know , Is there any formula for calculating the Arbitration ID using Node ID. If Yes then how we find the Node ID without using any VI. May be with the help of MAX.

Please reply as soon as possible.

bye

[DirkW]

This is not related to the "to fast..." message in this thread and confuses other users. Please open your own thread next time,  so we can handle your request seperately.

I assume you are talking about CAnopen, where the node id means the unique number for each device in the network. You calciulate the COB ID using the ID for the task you need to handle(for example SDO 581h, node ID 5) , subtract 1 and add the node ID (581h-1+5 = 585h).

DirkW