Canbus without ground. - Page 1
Forum section: electronics / projects

author: Sived
author_rank: Newbie
timestamp: July 16, 2020, 07:45:48 pm
content: Dear all,I need to connect 2 devices with canbus interface.I have no ground reference. There are available only the CanH, CanL and Power+ pins.How can I connect them in order to exchange can messages?The positive pin of the first device is at 42 Vdc from its internal ground.The positive pin of the second device is at 30 Vdc from its internal ground.Can I connect the positive line and choose a canbus driver +/-15V common mode tollerant?See the enclosed sketch.Any your help is very appreciated. Thanks a lot. Best regards.Sived

author: rounin
author_rank: Regular Contributor
timestamp: July 16, 2020, 07:57:57 pm
content: Electrically isolated can drivers like ADM3053 can be happy with no ground, sometimes. The moderate input impedance floats it to a somewhere safe common mode voltage.In a super noisy environment this would still be bad, and there are some EMI considerations that are beyond me.https://e2e.ti.com/support/interface/f/138/t/128310?Is-CAN-communication-working-between-two-different-ground-suppling-cards-https://ez.analog.com/interface-isolation/f/q-a/84965/isolated-can-with-adm3053-and-ground-wire

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2020, 07:58:33 pm
content: How has it come to this point and no one thought to ask: why don't we have a signal ground here?Use an isolated transceiver.Tim

author: Dave
author_rank: Super Contributor
timestamp: July 16, 2020, 08:03:26 pm
content: You could use an isolated interface on one side.There are three common ways to implement this:1. Power isolator, signal isolator and regular CAN transciever.2. Power isolator and isolated CAN transciever.3. Isolated CAN transciever with integrated power isolator.The former tends to be cheaper, the latter tends to have a smaller footprint/BOM length.

author: dietert1
author_rank: Super Contributor
timestamp: July 16, 2020, 08:15:12 pm
content: Maybe Sived wants to find a way to use the CANL and CANH lines as a power return. Sometimes there is an existing bus with a shielded twisted pair and it would be nice to use the shield for powering a remote node. This is not possible. You need something like a USB cable with four wires to do this, so you need to upgrade the bus.Regards, Dieter

author: nctnico
author_rank: Super Contributor
timestamp: July 16, 2020, 08:39:59 pm
content: Quote from: T3sl4co1l on July 16, 2020, 07:58:33 pmThis is quite common in battery packs. Fortunately CAN transceivers are very tolerant to overvoltage. An isolated transceiver is the best option but in some cases you'll see there is still no ground. Since CAN is differential and has termination at some point it will work.

author: Sived
author_rank: Newbie
timestamp: July 17, 2020, 09:02:49 am
content: First of all, many thanks to all for your suggestion.I enclosed here a sketch to better understand the matter.As nctnico stated, this concern arise because we have 2 batteries without a common ground.Thanks.Sived

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2020, 10:40:24 am
content: Treat it as positive ground, or more properly, positive common. There is no reason ground has to be negative.Yes, it is possible, but with a bidirectional level shift to positive common.

author: max_torque
author_rank: Super Contributor
timestamp: July 17, 2020, 11:28:40 am
content: Quote from: Dave on July 16, 2020, 08:03:26 pmI've been round this loop a few times in the past year (i do a lot of CAN stuff being automotive based mostly) and when i sat down and worked it out, unless you were making tens of thousands of devices, then the cost to do each option was actually withing a few pennies/cents of each other! It surprised me too, because i had always considered the rather nice all-in-one isolated CAN transcieivers with built in DCDCs to be "expensive" but in all cases what you have to achieve is the same, so the costs tend to level out.The way to actually save money is to include a common CAN bus power feed as well as a ground, with a single master module being responsible for that power, which means your slaves can avoid having any DC/DC in them

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 18, 2020, 12:00:56 am
content: Using the cable to distribute power to multiple attached devices is common, but in such a case I still recommend to use DC-DC converters.* One reason is you can use a higher bus cable, which lowers current and voltage drop across the cable. ( 1A is sort of maximum for std. Ethernet cable, less is safer)* Power loss / heating / fire hazard is less with low current limited power supply on the cable.* The DC-DC compensate for the voltage drop over the cable. (With 5V over the cable you have very small margins).* Long cables are antenna's, which both receive and transmit. the Inductor in a DC-DC converter filters that noise significantly. Without a DC-DC converter you will need more filtering. Just adding more (ceramic) capacitors does not help much. You need inductors for good filtering.When there is a potential for high DC voltages over a cable, then protection of the electronics is a real concern. Especially for wiring faults etc.You can use dual polarity TVS diodes to short overvoltage on the CAN bus, and also add PPTC fuses in the CAN wires to protect the TVS diodes from too much DC current. Be carefull with the PPTC fuses, these have a maximum voltage rating which can be quite low (30 Volt for some).

author: dietert1
author_rank: Super Contributor
timestamp: July 18, 2020, 05:48:48 am
content: As far as i understand DC-DC converters always add noise and cost.And as far as i know a canbus node must not add > 10 pF onto CANL and CANH, so over-voltage protection isn't as simple as a TVS limiter. Over-voltage protection should be considered part of the bus, not part of the bus node.Power over the bus will always be limited and should first serve as a supply to the isolated bus interfaces, while the canbus controllers in the nodes should get their supply elsewhere. That will reduce the cost of isolation to two opto-couplers per node - no need to design in special single chip isolation interfaces nor DC-DC isolators.Regards, Dieter

author: Sived
author_rank: Newbie
timestamp: July 18, 2020, 07:44:37 am
content: I cannot use a cable to distribute the power to the nodes.I prefer adopt a canbus driver with isolation (as rounin, Dave, T3sl4co1l, nctnico and David Hess suggestions).I will try to see the ADM3053 driver.If you have any other driver to prompt me, thanks a lot.Regards.Sived