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 calculation over one second

CBT2 Bus load displayBus 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 displayCAN­touch 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

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.

CAN busload displayCANalarm 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.

Nie wie­der den Bus verpassen.

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