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TCAN1044V: TCAN loopback mode and connection

Hollis Liu
Intellectual 2555 points
Part Number: TCAN1044V
Other Parts Discussed in Thread: SN65HVD235, TCAN1043-Q1, SN65HVD233

Hi Team,

i have several questions about our TCAN devices, please kindly help to answer. Thanks. 

  1. do we have any TCAN devices which can support loopback mode, MCU-->TXD-->RXD-->MCU OR RXD--> TXD directly? 
  2. any TCAN devices can support AC coupled? any document to introduce that?
  3. Can our TCAN devices be switched to connect to/disconnect from CAN BUS by signal switches? any requirements about the signal switches?
  4. how to do star topology connection in CAN? From 1 point to 6 devices.
  • 0
    TI__Guru 69922 points

    Hollis,

    Thank you for bringing this to E2E. 

    1. All CAN transceivers will support MCU -> TXD ->RXD ->MCU, but they will also transmit to the CAN bus pins, CANH and CANL. We have the SN65HVD235 device which has autobaud loopback mode, which allows for the message to go from MCU to TXD to RXD back to MCU without writing to the CAN bus, but this is a 3.3V CAN transceiver and incapable of data rates higher than 1Mbps.

    2. Do you mean AC coupled on the CANH and CANL pins? There aren't any specific transceivers in TI's portfolio that will do this, and in general since CAN signaling is offset by VCC / 2 (2.5V in a 5V system)., this will make it difficult compared to a standard differential signaling scheme. What kind of application is this that they need AC coupling? The ISO group has isolated CAN transceivers that may be of use.

    3. We've seen some customers do this with success, and in general, yes our TCAN transceivers can do this. There's also the use of the different modes of the device like standby or sleep (on the TCAN1043-Q1) that will effectively disconnect the CAN transceiver from the CAN bus without physically disconnecting it.

    4. Star topology is a very non-ideal network in CAN, especially at higher data rates. The impedance matching with the CAN driver and the termination of the network is difficult, so you tend to see large ringing on the dominant-to-recessive edges of the waveform. This ringing can persist for entire bit widths at higher data rates, which is why a specification was made, CiA 601-4, to address this need. Specific CAN transceivers are being designed to meet this specification, and work in non-ideal topologies like star or ring network. For now though, typical CAN transceivers do not work well in star topologies unless at low data rates like 250 - 500 kbps.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Intellectual 2555 points

    Hi Eric,

    thanks for your detailed feedback. It is helpful. 

    1. we are looking for a CAN FD device which has DIAGNOSTIC LOOPBACK feature in SN65HVD233. Do you have any suggestion?

    3. any recommendation for the signal switches?

    4. Yes, understand. As we know, even in current automotive designs, someone are still using star topology, how do they solve the low data rates problems? If we still want to use star topology, how to do impendence match? How to put the 120ohm resistors for star topology? 

  • 0 in reply to Hollis Liu
    TI__Guru 50795 points

    Hi Hollis,

    1. We currently don't offer any 5V CAN FD transceivers with a diagnostic loopback feature. This is not a common requirement for automotive applications. 

    3. Any switch with a low series resistance when on will be preferable as CAN signals are sensitive so such resistances. Please also consider recommending a Standby or Silent mode device to achieve a high impedance output when not in use. 

    4. There are a few practices that can help with the difficulties or star topologies, but they will still suffer non-ideal impedance matching. One method is to put all or most of the termination resistance (~60-ohms) in the center node. This will ensure the branches have relatively even access to the termination path, but leaves all of the end-nodes susceptible to signal reflections. This can be mitigated by including partial termination (high ohmic termination) at the end nodes to try to dampen the ringing as much as possible. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott