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SN65HVD1473: Switching between full and half duplex

Roy Hsu
Genius 9180 points
Part Number: SN65HVD1473
Other Parts Discussed in Thread: CD4503B

Hi there,

My customer plans to use the following configuration to switch between full and half duplex mode with SN65HVD1473.

My question is actually about how to select a suitable mux/switch. 

Since the common mode voltage of the data lines can be up to 12V, I'm looking at the CD405xB family.

However, it's Ron is 125 ohms (typ.) which seemed pretty large and I'm not sure if it affects the design of the termination resistance?

I'm thinking it as adding a lot more resistance on the cables so the termination resistance might also need to be increased?

Please correct me if I'm wrong and help recommend a suitable multiplexer!

Thanks!


Best regards,

Roy Hsu

  • 0
    TI__Guru 62910 points
    Hi Roy,

    Adding this much series resistance could cause issues on the RS-485 bus if it uses termination. Since RS-485 is typically terminated via two 120-Ohm resistances (one at each end of the link), the effective resistance between the two halves of the differential pair is 60 Ohms. Adding 125 Ohms in series on each line would cause quite a bit of attenuation, then (~80%).

    If your bus is able to operate without termination (e.g., if it is shorter point-to-point connection or not operating at very fast rates) then the effects of the series resistance would be less detrimental. It would still cause some high-frequency roll-off due to interaction with the cable capacitance, but at lower rates this may not restrict communication.

    I can check with our switch team to see if they have any other recommendations as well. Do you know what voltage range would need to be supported on the RS-485 bus? Transceiver outputs are always between ground and their VCC rail, but if there is not a common ground on the bus then any ground potential differences between nodes would also cause apparent shifts in the voltage ranges and would have to be supported by the switch.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Guru 62910 points

    Roy,

    I thought about this one a little more, and I think the effects of the series resistance can be minimized if you are careful in your pin assignments.  Say, for example, the device is connected as follows:

    Connector Pin RS-485 Transceiver Pin
    1 Y
    2 Z
    3 A
    4 B

    The switches could then be configured so that there is a switch between Y and A and a switch between Z and B.  When the switches are open the system would be in full-duplex mode and when the switches are closed the system would be in half-duplex mode.  If the half-duplex mode only used pins 1 and 2, then, the signals could propagate directly from the transceiver output to the bus without going through the series resistances.  The signal would only go through the series resistances in half-duplex mode when propagating to the receiver inputs.  Since these inputs are high impedance, there should be minimal attenuation produced by a series resistance in the 125-Ohm range.

    Regards,
    Max

  • 0 in reply to Max Robertson
    TI__Genius 9180 points

    Hi Max,

    I drew a diagram according to your response, is there anything wrong with it?

    Full+Half Duplex Configuration.pdf

    Besides, would the delay caused by the mux (propagation delay) and the transceiver (enabling/disabling the transmitter/receiver) in half-duplex mode be significant?

    Thanks!

    Best regards,

    Roy Hsu

  • 0 in reply to Roy Hsu
    TI__Guru 62910 points
    Roy,

    Thanks for drawing it out, and yes that is the topology that I was trying to describe. I don't see the propagation delay of the switch as being very significant - it should be low, and RS-485 applications generally are able to deal with large propagation delays (since they often transmit over long cables). Let me know if you think I'm misinterpreting your concern though.

    One thing to be careful of is the termination. In this configuration, "closing" the switch would result in the node having two terminators in parallel. This would lower the differential input resistance of the node to 60 Ohms, which would be somewhat mismatched to the cable impedance (120 Ohms differential in most RS-485 applications). If the same scheme were used by the system on the opposite end, too, then the total termination resistance would become quite low (30 Ohms). You may consider instead only including termination on the RX lines. This would still provide impedance matching for the full-duplex case without causing excessive loading in the half-duplex case.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Genius 9180 points

    Hi Max,

    I don't really think having two terminators in parallel would happen in this configuration since CD4503B is a 2-way switch.

    I've changed my drawing a bit so it might seem clearer to you.

    Full+Half Duplex Configuration_Remove mux.pdf

    In this case, do you still think the terminators would be in parallel?

    Thanks!

    Best regards,

    Roy Hsu

  • 0 in reply to Roy Hsu
    TI__Guru 62910 points
    Hi Roy,

    Sorry for the delay, I was traveling and didn't see that you had responded. Looking at this again, I do believe you are correct and the terminations will not end up connected in parallel. I don't have any other concerns with this approach, either. Thanks for taking the time to redraw the circuits to make things so clear.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Genius 9180 points
    Hi Max,

    No worries! Thanks for the continuous support also!

    Best regards,
    Roy Hsu