Detec­ting poten­ti­al differences

Com­mu­ni­ca­ti­on dis­tur­ban­ces attri­bu­ta­ble to poten­ti­al dif­fe­ren­ces in CAN units have often been unde­re­sti­ma­ted. They usual­ly go unno­ti­ced. Such errors can none­theless be detec­ted, mea­su­red, and rectified.


Seri­al bus sys­tems are a decisi­ve fac­tor for deter­mi­ning per­for­mance capa­bi­li­ties of com­plex manu­fac­tu­ring sys­tems in many indus­tries. The who­le elec­tro­nic com­mu­ni­ca­ti­on is rea­li­zed wit­hin com­plex sys­tems, mea­ning that the hig­hest deman­ds must be pla­ced on the reli­able func­tio­n­ing of seri­al bus sys­tems. Mea­su­ring devices for bus ana­ly­sis – both at the time of instal­la­ti­on and for per­ma­nent sta­tus moni­to­ring and ear­ly error detec­tion – are in the mean­ti­me indis­pensable. On the other hand, such devices have to date remai­ned obli­vious to dis­tur­ban­ces in data com­mu­ni­ca­ti­on. The­se dis­tur­ban­ces result from ina­de­qua­te poten­ti­al equalizations.
Until a few years ago, it was assu­med that such pro­blems were cau­sed by sys­tem-inter­nal rea­sons. Today, howe­ver, we know that exter­nal influ­en­ces such as elec­tro­ma­gne­tic inter­fe­rence or ina­de­qua­te poten­ti­al equa­liz­a­ti­on are incre­a­singly the cul­prits whe­re com­mu­ni­ca­ti­on is dis­tur­bed. Out­da­ted or inap­pro­pria­te frame­work con­di­ti­ons (e.g. groun­ding and poten­ti­al equa­liz­a­ti­on) also open the door wider to pre­vious­ly igno­red sources of dis­tur­ban­ce. High-fre­quen­cy cur­r­ents, for examp­le, often use the shiel­ding of a data line as their return path, even when a poten­ti­al equa­liz­a­ti­on con­duc­tor is pro­vi­ded pre­cise­ly for this pur­po­se. That results in cor­re­spon­din­gly error-pro­ne com­mu­ni­ca­ti­on or even loss of the who­le sys­tem func­tio­n­a­li­ty. Gemac has app­lied this know­ledge in its deve­lo­p­ments: the latest CAN­touch dia­gno­sis device now also detects such error sources – in addi­ti­on to the estab­lis­hed mea­su­re­ments of phy­si­cal bus characteristics.

Bus dia­gno­sis

CAN uses a dif­fe­ren­ti­al signal to com­pen­sa­te for the influ­en­ces of exter­nal inter­fe­ren­ces. In other words, the data signal is trans­mit­ted in two lines, which are inver­ted to each other (CAN_​H and CAN_​L). The dif­fe­rence bet­ween the­se two lines gene­ra­tes the signal recei­ved by each CAN trans­cei­ver. Dis­tur­ban­ces on the bus can pre­vent cor­rect detec­tion of the bit stream. Gemac’s dia­gno­sis sys­tems per­mit eva­lua­ti­on of the dif­fe­ren­ti­al signal in the form of a gene­ral qua­li­ty value, of the dis­tur­ban­ce-free vol­ta­ge ran­ge, and of the edge steep­ness. CAN­touch pro­vi­des an abso­lu­te mea­su­re­ment of the indi­vi­du­al signals CAN_​H and CAN_​L against a refe­rence poten­ti­al. This lets it detect an error source which is fre­quent­ly encoun­te­red in sys­tem instal­la­ti­ons: the so-cal­led “com­mon-mode vol­ta­ge”. In a dif­fe­ren­ti­al­ly ope­ra­ting trans­fer sys­tem, such as CAN, the term “com­mon-mode vol­ta­ge” is used for the vol­ta­ge of both signals rela­ti­ve to a com­mon refe­rence poten­ti­al. This is nor­mal­ly CAN_​GND, which in each device is con­nec­ted to CAN_V-. On a CAN net­work, both signal lines (CAN_​H and CAN_​L) should dis­play a com­mon-mode vol­ta­ge of 2,5 V in the reces­si­ve sta­te. For a num­ber of rea­sons, the com­mon-mode vol­ta­ge of the devices may mani­fest an off­set. CAN­touch is able to deter­mi­ne this vol­ta­ge off­set direct­ly. It could also be detec­ted indi­rect­ly via mea­su­re­ment of the shield vol­ta­ge. In prac­ti­ce, two forms of wiring can lead to poten­ti­al dif­fe­ren­ces bet­ween devices. In the first vari­ant, all bus nodes recei­ve their power sup­ply via the CAN cable; in the second, each device pos­ses­ses its own power supply.


Poten­ti­al confusion

Let us first con­si­der the case whe­re all con­nec­ted devices are sup­plied via the four-wire CAN cable (Figu­re 1).
Two of the four wires in the cable are used for the CAN com­mu­ni­ca­ti­on and the remai­ning two for the sup­ply vol­ta­ge. When the wiring is instal­led, the shield initi­al­ly has no low-resis­tance con­nec­tion to a par­ti­cu­lar poten­ti­al, as the con­nec­tion to V- in each device is usual­ly rea­li­zed by way of a par­al­lel resis­tor (1MOhm) and capa­ci­tor (10nF). For a low-resis­tance con­nec­tion, the shield should be con­nec­ted with V- and the pro­tec­ti­ve ground at the cen­tral vol­ta­ge sup­ply. This has the fol­lowing effect: Due to the line resis­tance, the cur­rent load of the indi­vi­du­al CAN devices results in a vol­ta­ge drop (Delta‑U) on the sup­ply lines.
This rai­ses the vol­ta­ge level of CAN_V- at each CAN device and leads to a nega­ti­ve vol­ta­ge off­set of the shield vol­ta­ge mea­su­red against CAN_V-. This “nor­mal” shield vol­ta­ge should lie in the ran­ge from appro­xi­mate­ly 0 V to ‑4 V. CAN­touch reports grea­ter shield vol­ta­ges or a shield which is not con­nec­ted to CAN_V- as errors. The vol­ta­ge drop in the cable will at the same time result in dif­fe­rent GND poten­ti­als for the CAN trans­cei­vers. This is mani­fes­ted as a shift in the levels of the signal vol­ta­ges, which each CAN trans­cei­ver “sees” for its­elf. The CAN sys­tem only per­mits shifts wit­hin the ran­ge of ‑2 V to +7 V. The CAN trans­cei­vers expect the signal vol­ta­ges to lie wit­hin this ran­ge. Even though newer cir­cuits tole­ra­te a wider ran­ge of ‑7 V to +12 V, excee­ding the expec­ted ran­ge may lead to com­mu­ni­ca­ti­on errors and, in an extre­me case, even­tual­ly also to the dest­ruc­tion of the trans­cei­ver (Figu­re 2).commonmodevoltage-e

CAN­touch thus deter­mi­nes the maxi­mum vol­ta­ge off­set among all bus nodes – the so-cal­led “worst-case total com­mon-mode vol­ta­ge” – and issu­es a warning if the limit values are excee­ded (Figu­re 3). In addi­ti­on, a gra­phi­cal visua­liz­a­ti­on shows whe­ther the vol­ta­ge off­set lies abo­ve or below the pre­sent con­nec­tion posi­ti­on. All abso­lu­te mea­su­re­ments are per­for­med rela­ti­ve to V- on the 9‑pin D‑Sub con­nec­tor (Pin 6). CAN­touch even offers the pos­si­bi­li­ty to switch the refe­rence ground to an inte­gra­ted 4 mm socket. This enables
the indi­vi­du­al ground poten­ti­als of the CAN devices to be mea­su­red without nee­ding to recon­nect the test device, and thus poten­ti­al dif­fe­ren­ces to be detec­ted fas­ter. A sim­pli­fied eva­lua­ti­on sys­tem using a com­bi­na­ti­on of traf­fic lights and smi­leys assists the user.
Resis­ting temptation
It is important to resist the tempt­ati­on to con­nect the shield to CAN_V- on all devices becau­se the ope­ra­ting cur­rent of the devices would then flow back via the shield, as it has a lower resis­tance than CAN_V-. The cou­pling of inter­fe­ren­ces into the signal lines is then almost ine­vi­ta­ble. As a reme­dy, the vol­ta­ge sup­ply can be rea­li­zed to the midd­le of
the net­work, or else a sup­ply with several power sup­plies can be pro­vi­ded. The use of CAN cables with a lower loop resis­tance for CAN_​V+ and CAN_V- is ano­t­her pos­si­bi­li­ty. The CAN­touch wiring test is able to mea­su­re the loop resis­tance of the cable.

The right cable

Stan­dard limit values are reached rela­tively quick­ly in prac­ti­ce, as shown by the fol­lowing examp­le: To remain wit­hin the ran­ge of ‑2 V to +7 V as defi­ned by ISO 11898 – 2, a
sym­metri­cal poten­ti­al dif­fe­rence of maxi­mum 4,5 V abo­ve and below 2,5 V is per­mis­si­ble (2,5 V – 4,5 V = ‑2 V and 2,5 V + 4,5 V = +7 V). Taking a typi­cal CAN cable with a cross-sec­tion of 0,22 mm², a line resis­tance of 185 Ohm/​km, and an assu­med total cur­rent load of 100 mA for all devices, the per­mis­si­ble poten­ti­al dif­fe­rence is alrea­dy reached at a
line length of appro­xi­mate­ly 240 m (and at a cur­rent of 1 A alrea­dy after 24 m). An impro­ve­ment can be achie­ved by choo­sing a CAN cable with a lar­ger cross-sec­tion. At a cross-sec­tion of 0,34 mm², the loop resis­tance is redu­ced to 115 Ohm/​km, at 0,50 mm² to 78 Ohm/​km, and at 0,75 mmm2 to just 52 ohm/​km.

A fal­se shield is no solution

In con­junc­tion with more exten­si­ve instal­la­ti­ons, it is not uncom­mon to find cab­ling con­fi­gu­ra­ti­ons which pro­vi­de an indi­vi­du­al power sup­ply to each bus node (Figu­re 4). In most cases, this is rea­li­zed with a two-wire CAN cable.fig2

Here, too, the­re is a risk of poten­ti­al dif­fe­ren­ces if the devices are not inter­con­nec­ted to pro­vi­de poten­ti­al equa­liz­a­ti­on. In prac­ti­ce, the shield is fre­quent­ly abu­sed for the pur­po­ses of poten­ti­al equa­liz­a­ti­on. The equa­li­zing cur­rent which flows through the shield, howe­ver, is its­elf alrea­dy a pos­si­ble source of dis­tur­ban­ces for the CAN com­mu­ni­ca­ti­on and should thus be eli­mi­na­ted as a pos­si­ble solu­ti­on from the very begin­ning. CAN­touch is able to spot such wiring pro­blems by way of the – now pos­si­ble – mea­su­re­ments of the shield and com­mon-mode vol­ta­ges. The hand­held dia­gno­sis device is the indus­try-com­pa­ti­ble equi­va­lent to a smart­pho­ne. For the first time, sys­tem ope­ra­tors, tech­ni­ci­ans and deve­lo­pers are in a posi­ti­on to per­form phy­si­cal and logi­cal bus ana­ly­sis via an intui­ti­ve touch­screen. The device with a 4,3‑inch color dis­play is rea­dy for immedia­te mobi­le use without an addi­tio­nal PC. commonmodevoltage_enSimi­lar to a smart­pho­ne, the user takes his dia­gno­sis device direct­ly to the CAN sys­tem, con­nects it with the cable and immedia­te­ly recei­ves reli­able mea­su­re­ment results – without stop­ping the sys­tem. The indi­vi­du­al mea­su­ring func­tions are ope­ra­ted inter­ac­tively and dyna­mi­cal­ly by way of app­li­ca­ti­ons (“apps”) based on fin­ger ges­tu­re con­trol. A sim­pli­fied eva­lua­ti­on pro­ce­du­re accord­ing to the traf­fic light princip­le and with the help of smi­leys sup­ports the user in the fast eva­lua­ti­on of the mea­su­re­ment results.

CAN­touch this…

The hand­held CAN­touch is the indus­tri­al-sui­ted ans­wer to smart­pho­nes. First-time faci­li­ty ope­ra­tors, engi­neers and deve­lo­pers can ana­ly­ze phy­si­cal and logi­cal values of a bus with an intui­ti­ve touch­screen. It is quick and mobi­le to use without an addi­tio­nal PC. So a user can sim­ply con­nect direct­ly to a CAN sys­tem with a cable and gets reli­able values wit­hin minu­tes without stop­ping the sys­tem. This not only saves time, also it saves also a lot of money if you can get an ear­ly error warning. Fix pro­blems befo­re they stop your bus. The sin­gle mea­su­re­ments are used inter­ac­ti­ve and dyna­mic over apps and fin­ger ges­tu­res. A simp­le rating sys­tem with amp­le colors and smi­leys sup­ports the user to quick­ly rate the mea­su­red values. The 4,3‑inch color dis­play makes it pos­si­ble to show the mea­su­re­ments with appe­aling graphics.

Aut­hors: Hen­drik Ste­pha­ni (Deve­lo­per Fiel­dbus dia­gno­sis), Ant­je Wapp­ler (Mar­ke­ting)

Hendrik Stephani

Hen­drik Stephani

Fiel­dbus dia­gno­sis chief developer

Hen­drik Ste­pha­ni has deve­lo­ped capa­ble tools for fiel­dbus dia­gno­sis for 20 years.