• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 35 subscribers
  • Views 13084 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CAN bus without CAN Ground. Could be working with isolated node ?

damien unknown
Prodigy 30 points
Other Parts Discussed in Thread: SN65HVD230, TPS60205, TPS60204, TPS60100

Hi all,

Recently, i'm arrived on a project with a CAN bus and 5 nodes exchanging data using a 250 Kbit/Sec. speed. The context is an off-road vehicule.

The CAN Bus wiring is, today, done without "CAN Gnd" and sometime, we have communication error (sorry for the lack of details, but i didn't have possibility to plug a CAN analyser or a Scope)

One of my node is powered by a batterie, and sometime, this node is powered using a DC-DC converter. I don't know (yet) if the DC-DC is isolated or not. Doesn't matter because when working on battery, this is the case. The transceiver is a SN65HVD230.

This nodes seems to be root of the problem because, when disconnected, problem disapears.
When this battery node is installed, sometime communication crash  - sometime not.


I suspect the communication failure to be connected with a Gnd problem - Commun Mode Voltage (TBC but seems more failure when machine preheater is engaged):
I agree about the idea of a CAN Ground wire is not necessary if all nodes share a same ground, but is it true when you have floating node ?
Is somebody as already experience by integrating multiple node, with isolated powering, on a CAN Bus without CAN Ground.

Sometimes, my battery could drop until 2,0 volt under specific condition (and stay for a while at this voltage):
Is the SN65HVD230 is guaranted to not write on CAN Bus (HighZ) below its nominal 3 V. VCC ?

Thanks in advance to share your experience.

Regards.

  • 0
    TI__Mastermind 20590 points

    If the GNDs of various CAN nodes pull the Common Mode range of the CAN spec then the network may stop working. If the pre-heater pulls a lot of current, a large GND shift is possible which could pull the node below the CM range of the CAN standard.  Even if the CAN nodes are isolated you must provide a reference GND for the CAN network to make sure it stays within spec. 

    Any device, including the HVD230, that is operated outside of datasheet limits generally is not "guaranted".   In this case if you supply the HVD230 with 2V it is clearly undervoltage and will not function within spec and will not be able to drive CANH to the differential levels needed for CAN.  As outlined on page 5 of the datasheet, the recommended supply, Vcc, range of the HVD230 is 3V to 3.6V and the header for each table in the electrical specification following outlines that the values are "over the recommended operating conditions (unless otherwise noted)".   In this case with Vcc at 2V there would be no way to meet the CANH VOH output spec of 2.45V.  Also CAN normally has a differential output of ~2V between CANH and CANL but if the transceiver is only supplied with 2V there would be no way to make the necessary differential voltage.  You could address this via a buck/boost power supply that could ensure the HVD230 supply is in the 3V to 3.6V range regardless of the battery voltage. 

    Best Regards,

    Scott

  • 0 in reply to Scott Monroe
    TI__Mastermind 28265 points

    For a boost regulator you can use an ultra-low-current charge pump such as TPS60204 or TPS60205. They have 90% efficiency and come in a 10-pin MSOP package. operating temperature is from -40 to +85 degC.

    If you require synch-pins use TPS60100. But this one comes in the larger 20-pin TSSOP package.

    Regards,

    Thomas