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SN65HVD231 CAN transceiver issue

Sky Lee
Prodigy 200 points
Other Parts Discussed in Thread: SN65HVD231

1)      Is it possible to recommend us any low capacitance cable suited for CAN bus applications?

Our system details are:

  1. Target bitrate: 250kbps
  2. Max cable length = 200m
  3. Max nodes = 16
  4. Transceiver= TI sn65hvd231

We tested above system using unshielded twisted pair but communication struggles when tested on 100m cable.

2)      For TI sn65hvd231, can you help us check if TI provides a table of cable distance v.s. terminating resistor values v.s. number of nodes? but we could not find it from TI website.

3)      With the stated setup and target speed above, what is the recommended slope control resistor? Is there any technical guide on this subject besides that already in the datasheet?

4)      Any info on max number of nodes that the transceiver can support? How does this impact the bus? Is there any technical guide on this subject besides that already in the datasheet?

  • 0
    TI__Mastermind 20580 points

    1) A couple of reference cables could be Belden 9842, 3086A and 3087A  (with power pair), Belden DeviceNet cables http://www.belden.com/products/browse/industrial/DeviceNet.cfm, Belden CANopen http://www.belden.com/products/browse/industrial/Can-Open.cfm   or simliar.   As for your system you have to keep many things in balance and remember that CAN bit timing relys on the 2 way loop time for aribtration (addressing) to work.  See article:  http://www.eetimes.com/design/industrial-control/4014279/Signaling-rate-versus-cable-length-the-CAN-bus-timing-trade-off?pageNumber=0   for more info this and the length of the cable and the 2x ~5ns/m impact to bit timing and data rates.  You also need to ensure that as cable lengths increase the ground references are not causing ground loops and large common mode shifts between the nodes.  If you are using the SN65HVD231 make sure the Rs pin is set to high speed and not limiting the slope or you may see additional timing issues.   Try to avoid stub wiring which can cause signal integrity issues.   If you are having ground loop / common mode issues isolating the CAN nodes and using a CAN ground may be a way to go.  We could approach that with using ISO7xxx digial isolators with the HVD23x or you could look at the ISO1050DW.

    2)    HVD231 is compatible to the ISO11898 CAN standard.  Cables as above with 120 ohm impedance should be used along with 120ohm termination at both ends of the cable.  These are all system level trade offs of the network so they may not be specified in the datasheet.  Most CAN protocols and systems (ie CANopen, DeviceNet, etc) outline their specific cabling, lenghts, connectors, termination, topology etc.   Section 4 of the TI application note SLLA270   http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slla270&fileType=pdf   gives some basic guidelines for this type of information.  You may also want to consider split termination as outlined in section 4.5.2.  

    3)  You should use the slope control at the fastest setting (pin 8 to GND) with this combination since you are on the edge of getting the desired data rate out of the length of cable.  Using split termiation per 2) above should help with filtering the common mode.   If you are in a very small network for example and only need to go 10kbps then you can slow the device down. 

    4)  The number of nodes depends on a large number of factors.  Differential loading, length and other parasitics in the network all add up.  Some of those are directly from the transceiver but most are network dependent more than transceiver dependent (like connector capacitance, cabling impedence, etc).  From a purely resistive loading on the network the HVD231 should be able to have over 100 nodes since it has a 40k ohm input, but you must take signal loss down the cable and capacitances into account for network timing.    Many of the system level specs for CAN (CANopen, DeviceNet, etc) have references of this system level discussion of number of nodes, topology, cabling, connectors, lenght, data rate, etc. 

    -- Scott