analyse busload

Measurement accuracy
The bus load is counted directly by the FPGA in CAN-Bus Tester 2, CANtouch and CANobserver. The calculation or counting is performed for one second, including the stuff bits and standardized spaces. The result of this count is highly accurate and reproducible because it is measured directly at the bus and not after the CAN controller on the logical level. Our measuring devices are the only ones with such high accuracy on the market.

Preliminary considerations
Depending on the time window with which you look at the bus, you will notice significant differences there. Of course, the bus load is 100% when sending. After the standardized interval to the following telegram, the bus load drops to 0% until a node sends again (100%). So to get a meaningful value, you have to look at a much more extended period.
There are further limitations when choosing a favorable interval. You don’t want to wait too long for the result or want temporary fluctuations to be suppressed entirely. However, both would be the case if a large counting interval is chosen. If the interval is too small again, strong fluctuations in short intervals of the display are the result, which makes an evaluation or even reading difficult.

A second as a time base, therefore, seems ideal. Although you do not have to wait long for an update, the display remains stable or fluctuates only slightly in many real configurations. Theoretically, fluctuations between 0% and 100% are still conceivable here, depending on how many bits were counted in one second.

Display fluctuates; what could be the reason?
In specific bus configurations, rhythmically changing variations in the bus load and sampling throughout a second can result in a fluctuating display, depending on how the tester’s measuring grid (the second cycle) and differences in the bus load meet.

An example
A master polls some slaves in sync mode at 400 ms intervals. The slaves respond to the rhythm of the requests. At one time, the entire request-response sequence will occur within the measurement second. At another time, the slave responses will be counted for one second or the other.
The following diagram is intended to illustrate this situation. While the same bus load is always counted in the first three seconds, different proportions fall in the fourth and fifth seconds. Then the grids temporarily fit over each other again.

Bus load display of the CAN-Bus Tester 2
The CBT2 displays the busload in the past second with an accuracy of one-tenth of a percent. Minimum and maximum values (since the beginning of the measurement) can be recorded. The measurement runs automatically as soon as valid CAN telegrams are detected.

CANtouch bus load display
The CANtouch provides the bus load of the past second to an accuracy of one-tenth of a percent. In addition to the minimum and maximum values (since the beginning of the measurement), the course over the last minute is also displayed graphically. There is a direct graphical evaluation using our smiley.

CANobserver bus load display
After configuring the baud rate correctly, the CANobserver determines the busload to be within a full percent over the last second. The display is done in the web interface.

CANalarm display Bus load display
In CANalarm, we do not have a physical measuring device available. There the bus load is calculated much less precisely on the firmware side. Here, too, the count is over one second. The exceeding of a freely definable threshold can be signaled at the switching output.