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THVD1410: RS485 board-board interface and termination requirements?

Part Number: THVD1410

We are using TI RS485 drivers to implement a local half-duplex RS485 communication bus between adjacent boards.  Unfortunately, at ~50C, the RS485 interface stops communicating.  At first we thought the absence of termination might be driving it, so we added a 60 ohm resistor (its a local bus with no distinctive end-end distance, so this in lieu of two 120 ohm termination resistors).  This did not make a difference.  We also removed the TVS diodes shown, and replaced the series termination resistors with shorts, and it still shuts down at around 50C.  Any idea what is happening to the RS485 interface?  Do you agree that for a local bus only a few inches long, then no termination is required?  Do I need to add bias resistors to power and ground too with the termination?  Please Help?  Thanks.

  • Hi Bruce,

    Could you please try posting your image again?  Sometimes with our forum it seems to take a few tries.

    To better debug it would great to take a look at the schematic for the RS-485 interface as well as the observed waveforms on the A/B/D/R/DE/RE pins.  The link should work without termination - you are correct that this is not required for functionality but is recommended to optimize signal integrity in applications where the transmission line is long with respect to the timing of the signals.

    Since this device should operate at relatively high temperatures and won't have very high power dissipation under normal usage, I don't think the issue at 50 C is something like thermal shutdown.  I doubt it is a bus signal integrity issue either given how short the distance is, although the waveform captures I mentioned should be helpful. 

    Something I would check would be how different high-Z states are handled.  For example, if the receiver is disabled, is there a pull-up resistor on the "R" pin to set a valid logic voltage?  If not, the voltage may be dependent on PCB leakage characteristics (which could be temperature dependent).  Similarly, if the driver is disabled and the application is expected to report a "high" logic state, does the differential voltage remain above the "VIT+" threshold for all receivers?  (This threshold is slightly negative in the THVD1410 to simplify issues like this, but if there is noise on the line or if other receivers don't have an offset threshold then some additional biasing via pull-up/pull-down resistances could be useful.)