© ebm-papst

The formula for the data transfer time

In a Local Inter­con­nect Network (LIN) bus, the data transfer time is an impor­tant para­meter for the commu­ni­ca­tion between master and slave devices.

Markus Wein­gart, Head of Depart­ment Elec­tronics, ebm-papst Land­shut

In the auto­mo­tive sector, the LIN bus is a long-estab­lished commu­ni­ca­tions solu­tion for networking sensors and actu­a­tors and their control devices. In view of the success of smart home appli­ca­tions, the serial fieldbus system is gaining rapidly in impor­tance in the heating sector as LIN bus commu­ni­ca­tion provides a lean solu­tion for connecting compo­nents such as pumps or gas blowers and thus making smart condensing boilers possible.

The LIN bus is a single-master, multi-slave system. The ­master  — in a condensing boiler, the elec­tronic controller — deter­mines when the slave — for example, the blower — should provide data or carry out commands. This takes place at cyclical inter­vals according to a time­table kept in the master. This clear schedule defines which infor­ma­tion is to flow and when. For our gas blowers, we recom­mend retrieving the data rele­vant to control systems, such as the speed, every fifty millisec­onds.

Infor­ma­tion as key

For data that do not change so often, such as temper­a­ture or power consump­tion, longer inter­vals are suffi­cient. For defining the schedule, that makes it impor­tant to know the data transfer time for the various infor­ma­tion packets. This is similar to plan­ning in produc­tion, where it is also neces­sary to take timing into consid­er­a­tion when sequencing the indi­vidual steps in a process.

Example of a LIN message for a gas blower

How long does it take to transmit a data packet? To answer this ques­tion, it helps to take a look at the typical struc­ture of a LIN message (see example). In the LIN bus, it may include a maximum of 64 bits. But this infor­ma­tion is not enough to calcu­late the transfer time. A so-called “over­head” is also needed, because before data retrieval starts, the master sends synchro­niza­tion pulses to fore­warn the slave so it can adjust to the master’s clock rate.

All in order

In addi­tion, a packet iden­ti­fier (PID) is sent with a checksum for every transfer to announce the data packet’s contents and verify whether the data are trans­ferred correctly. On top of that, a safety buffer of 40 percent is added since, as in produc­tion, some­times a step can take longer than usual. But the produc­tion plan must not be allowed to get mixed up because of that.

The maximum data transfer time is calcu­lated from the formula above. The term t Frame_Nom is the time required to transfer the data packet with the content infor­ma­tion including over­head. It is calcu­lated from the number of bytes multi­plied by ten. This factor is comprised of the number of bits per byte (eight) and a start and a stop bit per byte. The number of bits in the over­head is given as 44. The para­meter t bit = baud rate-1 desig­nates the baud rate, a unit that expresses trans­mis­sion speed. In a LIN bus, bit rates of 19.2 kbit per second are typical. For this value, a bit transfer time of t bit = (19.2 kbit/s)-1 = 52 μs results. So a LIN message with 64 bits has a maximum data transfer time of about nine millisec­onds. 

Required fields: Comment, Name & Mail (Mail will not be published). Please also take note of our Privacy protection.