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THVD1410: Termination resistor value - 120 Ohm or 54 Ohm?

Jana Beck
Prodigy 70 points
Part Number: THVD1410

Hello!
I am a bit confused about the termination resistor requirement for THVD1410.

The standard for RS485 is 120Ohm.
In the datasheet though, the recommended value is 54Ohm.



With a 120 Ohm termination resistor I am seeing 4V at the outputs (D and R).



That is an issue since our MCU only supports up to 3.3V.

Should I switch to 54 Ohms?




  • +1
    TI__Guru 50775 points

    Hi Jana,

    You are correct that 120-ohms is typically used for the termination resistance value for RS-485. This is primarily because the common cable type used for such applications has a characteristic impedance of 120-ohms. A termination resistor placed at the receiver or end-point of a network will therefore be most effective at dampening signal reflections. 

    Because RS-485 transceiver can both receive and transmit using the same device, it is common to place 120-ohms of termination at both end-points of the network, resulting in a total differential load of 60-ohms. This differential load is harder for the transceiver to drive, so we also characterise the transceiver characteristics at larger loads (smaller Rdiffs). The 54-ohm load condition also accounts for the relatively small differential load presented by the maximum number of transceivers present on the bus. It is under these conditions we test to show that with this worst-case differential load, the driver can still output a differential voltage that conforms to the physical layer specification of RS-485. 

    If a larger resistance value exists for Rdiff, it will only be easier for the transceiver to drive the differential and this will be no issue. Keep in mind that the Vod is the differential voltage that appears on the RS-485 bus and is only seen by other transceivers. The MCU will only be interfacing with the IO voltages on the D, R, DE, and RE/ pins. For THVD1410, this IO level will be the same as Vcc. 

    If the MCU you are using expects 3.3V, this is the same voltage that should be supplied to Vcc of THVD1410. This transceiver will use the Vcc supply to generate the digital output voltages and input thresholds, as well as the RS-485 main driver. 

    Let me know if you have any more questions. 

    Regards,
    Eric Schott