ISOBUS mea­su­re­ment and analysis

The ISOBUS has under­go­ne a resound­ing suc­cess in agriculture.
The fact that you can and must mea­su­re in the ISOBUS is new ter­ri­to­ry for many technicians.


Bosch laid the foun­da­ti­on for ISOBUS back in the 1980s, with the deve­lo­p­ment of the CAN bus. They quick­ly dis­co­ver­ed the fan­ta­stic pos­si­bi­li­ties that exist when ECUs can exch­an­ge data quick­ly and secu­re­ly. In the mean­ti­me, many vehic­les now have far more than fif­ty indi­vi­du­al ECUs instal­led in them that com­mu­ni­ca­te via CAN. We are now used to having our car radio lower the volu­me when we slow down. Hard­ly anyo­ne thinks about how the radio actual­ly knows how fast the car is going.

ISOBUS in use - Field sprayer, ISOBUS measurement

Why is the­re ISOBUS?

For a long time, the­re were not so much elec­tro­nics in agri­cul­tu­re and muni­ci­pal ser­vice vehic­les. A trac­tor was a trac­tor and elec­tro­nics were not instal­led the­re. Too sus­cep­ti­ble to faults. But soon the con­trols, which are now being used by all engi­ne manu­fac­tu­r­ers and pro­vi­de dri­ving plea­su­re while at the same time ensu­ring envi­ron­men­tal­ly fri­end­ly ope­ra­ti­on of the machi­ne, also arri­ved here. The sus­cep­ti­bi­li­ty to faults is now low and the trac­tors are run­ning lon­ger and more relia­bly than ever before.

Espe­ci­al­ly here in agri­cul­tu­re, the pos­si­bi­li­ties of net­work­ed ECUs are fan­ta­stic. Seeds and fer­ti­li­zers are spread accor­ding to the cha­rac­te­ristics of the soil. The com­ple­xi­ty of the situa­ti­on can be seen when a new seed drill is atta­ched to the trac­tor and the appro­pria­te con­trol ele­ments appear on the dis­play in the driver’s cab. Data from the trac­tor, such as PTO shaft rpm and speed, are trans­mit­ted to the seed drill, from whe­re con­trol ele­ments and indi­ca­tors are trans­mit­ted to the trac­tor. Young far­mers are not the only ones who enjoy this and the work is done quick­ly and with unbe­lie­va­ble pre­cis­i­on and efficiency.

Sin­ce 2007, seve­ral com­pa­nies have been working inten­si­ve­ly tog­e­ther in the Com­pe­tence-Cen­ter-ISO­BUS to ensu­re that machi­nes from dif­fe­rent manu­fac­tu­r­ers are com­pa­ti­ble with each other. The ISOBUS is now real­ly taking off after the big­gest hurd­les have been over­co­me. Of cour­se, the sys­tems are still under con­s­truc­tion, some manu­fac­tu­r­ers have misun­ders­tood the stan­dar­diza­ti­on or are deli­bera­te­ly moving away from it. But this can no lon­ger stop far­mers, they are well on their way to digi­tal agri­cul­tu­re and the jour­ney is get­ting faster.

What is ISOBUS?

ISOBUS = CAN + SAE J1939 + NMEA2000 + x, ISOBUS measurement

The basis of ISOBUS is the CAN bus. If you want to mea­su­re in ISOBUS, all the basics that app­ly to the CAN bus are also valid. A two-wire line con­nects all con­trol units, sen­sors as well as actua­tors. If one par­ti­ci­pant sends some­thing, ever­yo­ne else lis­tens. It can hap­pen that seve­ral par­ti­ci­pan­ts want to send at the same time. Prio­ri­tiza­ti­on ensu­res that important mes­sa­ges can be sent first.

can bus voltage rangesThe CAN stan­dard first estab­lishes a phy­si­cal basis for com­mu­ni­ca­ti­on. One vol­ta­ge level means a logi­cal one, ano­ther a zero. Com­plex para­me­ters have been descri­bed, e. g. bit timing. Cle­ver frame­work con­di­ti­ons were crea­ted to enable robust communication.

The second level, descri­bed in the CAN stan­dard, is the logi­cal basis, which also pro­vi­des for robust com­mu­ni­ca­ti­on in a uni­que way. For exam­p­le, par­ti­ci­pan­ts can make non-unders­tood mes­sa­ges inva­lid for the enti­re bus and thus trig­ger a repe­ti­ti­on of the message.

SAE J1939 PGNs Measure in ISOBUSNow we come to the third level, which regu­la­tes the pro­to­col, i. e. deter­mi­nes how com­mu­ni­ca­ti­on has to be done. This level was taken from the SAE J1939 stan­dard, which had alre­a­dy estab­lished its­elf in power­train, truck & trai­ler com­mu­ni­ca­ti­on and con­s­truc­tion machi­nery. Signals and para­me­ter groups are used here, com­mu­ni­ca­ti­on is effec­ti­ve, which enables low baud rates with fewer phy­si­cal pro­blems and lon­ger cable lengths. Based on SAE J1939, the NMEA 2000 pro­to­col was deve­lo­ped for mari­ne appli­ca­ti­ons, which defi­nes spe­ci­fic para­me­ters and signals. In defi­ning ISOBUS, the best ingre­di­ents from all the­se worlds have been used and the basis for cross-manu­fac­tu­rer digi­tal com­mu­ni­ca­ti­on in agri­cul­tu­re has been laid. The unbe­lie­va­ble func­tions that can be rea­li­zed with it are alre­a­dy visi­ble in the manu­fac­tu­r­ers’ show­rooms and at trade fairs. The end of the road is far from being reached.

The pro­blems and how to mea­su­re them in ISOBUS?

no digital signal, ISOBUS measurementThis brings a new field of respon­si­bi­li­ty to far­mers and repair shops, main­ten­an­ce and repair of ISOBUS devices and sys­tems. That’s not real­ly easy. Of cour­se, the manu­fac­tu­r­ers must deli­ver first. The sys­tems must be pro­per­ly desi­gned and all elec­tro­nics must be pro­tec­ted from influen­ces. But the repair shop also has to be able to deter­mi­ne whe­ther a bus is OK or not. This is a ques­ti­on that has cau­sed exten­si­ve dis­cus­sions in the auto­mo­ti­ve sec­tor for deca­des. The forums of han­dy­men are full of requests for help.

Digi­tal com­mu­ni­ca­ti­on has a spe­cial cha­rac­te­ristic: it sud­den­ly fails in the event of an error. Wit­hout war­ning, the DAB+ car radio sud­den­ly beco­mes silent. In the ana­log FM car radio it beca­me clear by incre­asing back­ground noi­se that one moves out of the covera­ge area of a radio trans­mit­ter. Digi­tal radio remains crys­tal clear until it fails. One or zero. When it’s run­ning, it’s run­ning – trou­ble­shoo­ting is impossible.
It is the­r­e­fo­re dif­fi­cult to judge whe­ther a digi­tal com­mu­ni­ca­ti­on is fail-safe or not. Becau­se even if it bare­ly works, the con­tent of the mes­sa­ge is crys­tal clear. The break­down is not announ­ced by incre­asing noi­se. We have alre­a­dy descri­bed in this artic­le why it is important to mea­su­re on the phy­si­cal, ana­log level. Only the­re, in the ana­log envi­ron­ment, can I judge how good the signal qua­li­ty on a digi­tal bus is and whe­ther the­re is still an amount of inter­fe­rence reser­ve or whe­ther the com­mu­ni­ca­ti­on is just bare­ly possible.

If the brand-new trac­tor sud­den­ly stops under the high-vol­ta­ge line, but runs smooth­ly in the work­shop and ever­y­whe­re else, then the thought of a fun­da­men­tal pro­blem can come up. It is the­r­e­fo­re also important for the manu­fac­tu­rer of ISOBUS sys­tems and for the ISOBUS user to have a sui­ta­ble mea­su­ring device in his hands in order to be able to eva­lua­te the cur­rent qua­li­ty of his bus.

GEMAC CANtouch Measurement tool for ISOBUSWith its CAN bus mea­su­ring and ana­ly­sis devices, GEMAC pro­vi­des the tools for eva­lua­ting the important phy­si­cal basis of the bus. An engi­neer might now men­ti­on the oscil­lo­scope as a mea­su­ring device. Howe­ver, it is not only important to be able to mea­su­re at all, it is also important to be able to compa­re mea­su­re­ments. Here, the Qua­li­ty Level pro­vi­des a com­pre­hen­si­ve infor­ma­ti­ve value that is easi­ly com­pa­ra­ble. The uni­que gene­ra­ti­on of this value by 64x sam­pling of the bits allows repeata­ble and com­pa­ra­ble state­ments. Sin­ce the phy­si­cal basics are the same as in CAN, it is also pos­si­ble to mea­su­re in ISOBUS.
stuffbit in oscilloscope view, ISOBUS measurementThings like easy ope­ra­bi­li­ty for non-elec­tro­nic tech­ni­ci­ans and a simp­le mea­su­re­ment set­up are not neces­s­a­ri­ly given with an oscil­lo­scope. Alt­hough DSO’s beco­me more and more intel­li­gent when it comes to trig­ge­ring an event, the eva­lua­ti­on of the oscil­lo­gram remains in the hands of the user. The CAN­touch, on the other hand, allows every tech­ni­ci­an to ope­ra­te and check the bus. The oscil­lo­gram can also be dis­play­ed for the expert.

PC soft­ware is also available for data recei­ving and deco­ding, deco­ding spe­ci­fic ISOBUS mes­sa­ges as well as stan­dard J1939 mes­sa­ges and NMEA2000 spe­ci­fic con­tent. Even manu­fac­tu­rer-spe­ci­fic PGN’s and SPN’s can be defi­ned. This makes it pos­si­ble to decode the­se spe­cial messages.