CAN bus tranceiver - Should it load the bus when powered down? - Page 1
Forum section: electronics / projects

author: AndersJ
author_rank: Frequent Contributor
timestamp: April 11, 2016, 03:44:19 pm
content: CAN bus withtwomultiple nodes, both terminated, no traffic.Both CAN-H and CAN-L sit idle at approximately Vcc/2.When one node is powered down, CAN-L and CAN-H drop about 0,4V.The tranceivers are NXP PCA82C251.The datasheet states: "An unpowered device does not disturb the bus lines"For me, a 0,4 V drop is a bus disturbance.With a larger system, each unpowered node contribute a voltage drop.More unpowered nodes in a larger system would probably bring the system to a halt.I cannot believe this is right,and cannot figure out what I'm doing wrong or where my reasoning fails.Any ideas?Edited:Multiplenodes, ”not two”

author: AustinEngy
author_rank: Regular Contributor
timestamp: April 11, 2016, 04:26:41 pm
content: Could you give more specifics about the devices and your test conditions? When you say no traffic, are you saying that you aren't seeing any CANH and CANL even though you've got CANTX working properly? I've found that when only one CAN device is hooked up in a bus the CANH/L pins do weird things. Does it work fine when both are hooked up? What is the rest of the circuitry you're using?

author: AndersJ
author_rank: Frequent Contributor
timestamp: April 11, 2016, 04:40:58 pm
content: It works well with CAN traffic on the bus.The voltage drop is the same with or without messages on the bus.I have 2 or more nodes, no difference.A CAN bus will not work with only one node.The rest of the circuitry is a controller on the other sideof isolated logic and power.

author: AustinEngy
author_rank: Regular Contributor
timestamp: April 11, 2016, 05:00:17 pm
content: How is the standby pin wired up? Do you have a schematic for your devices, or even pictures of the PCBs?

author: AndersJ
author_rank: Frequent Contributor
timestamp: April 11, 2016, 05:16:05 pm
content: The Rs pin has 20K to GND.Schematic? It is the tranceiver and a 120 Ohm terminator.

author: Niklas
author_rank: Frequent Contributor
timestamp: April 11, 2016, 05:36:35 pm
content: NXP PCA82C251 is on the black list in some of our customer's product requirements, but usually they don't specify why.A few things that could be the reason:- Skew in drive performace on CANH and CANL, which results in bit timing problems on the differential signal and conducted emissions- Not going off the bus when powered down or during a "loss of GND" event.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 11, 2016, 06:21:47 pm
content: Quote from: AndersJ on April 11, 2016, 03:44:19 pmThere is no CAN Bus left. CAN requiresat leasttwo active nodes to work.

author: kaz911
author_rank: Super Contributor
timestamp: April 12, 2016, 07:35:39 am
content: Quote from: AndersJ on April 11, 2016, 05:16:05 pmI think that is your issue. CAN lines are supposed to be terminated with 2 x 120 Ohm terminators (in parallel) - one at either end of the bus. That gives you an effective 60 ohm resistance. When you only have 1 - you have 120 ohm... So if you have 8 devices only the ones at either end should be terminated - so 6 devices with no termination.You can "simulate" by adding a 120 Ohm resistor in parallel with the one on the board.And CAN bus devices can work fine on their own - as long as you have a bus network with 2 x 120 Ohm - in which case it will just broadcast until someone replies.

author: AndersJ
author_rank: Frequent Contributor
timestamp: April 12, 2016, 08:07:17 am
content: Thanks to everyone for commenting on this issue.However,please allow me to clarify what seems to have been misunderstood by a few.- The CAN bus has MORE than two nodes.- There are 120 Ohm terminators at each end.When a node using the PCA82C251 tranceiver is unpowered,the bus is pulled down by a few 100's of mV for each powered down tranceiver.I believe the tranceiver has issues.Forum user "Niklas" confirms that by saying some of his customers dislike the part for similar reasons.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 12, 2016, 08:58:23 am
content: Does the un-powered node affect the voltage difference between CANH and CANL when the bus is in the dominant state? Or is it just a shift of the DC level, but not actually a change in peak signal amplitude?The DC level on a CAN bus with respect to GND isn't really important, or even particularly well defined.

author: AndersJ
author_rank: Frequent Contributor
timestamp: April 12, 2016, 09:22:23 am
content: It is a DC level shift.Peak signal is not affected.Is DC level with respect to GND not important?No, maybe not, but within reason.Every unpowered node pulls it down a bit more.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 12, 2016, 09:35:40 am
content: Could be as simple as clamping or tvs diode leakage.I have used SN65HVD230 CAN tranceiver since they work on 3v3, they will have an effect on the common mode of the can bus pair.