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SN65HVD1780: 3.3V Supply Limitations

Clay Whipp
Prodigy 10 points
Part Number: SN65HVD1780


Hello,

We are using the SN65HVD1780 transceiver with a 3.3V DC power supply and have a few questions about limitations that might occur. 

1) What's the maximum length of wire/cable we can use to reliably transmit a signal? RS485 says 4000 feet, does the 3.3V supply affect this? 

2) In the data sheet, it says the transceiver can handle up to 320 nodes, but I assume this is using the 5V power supply. With a 3.3V power supply can the transceiver still have up to 320 nodes? Our concern is since the transceiver "can operate with a 3.3V supply with reduced driver output voltage for low-power applications" it may not be able to handle as much. 

3) Will the transceiver still meet the RS-485 standard? The datasheet says "these devices are fully compliant with ANSI TIA/EIA 485-A" with a 5V supply but what about a 3.3V supply? I'm fairly sure it can't since RS-485 requires a driver output of at least 1.5V and in the datasheet it says at a 3.3V supply, |V_OD| is typically only 1V.

Thank you in advance!

  • 0
    TI__Guru 74322 points

    Clay,

    Please see this thread for similar questions and response.

    Now then, that thread doesn't specifically address your questions per se, but it does explain how 3.3V supply on an RS485 transceiver can be limiting.

    1) I can't give an exact length, but it will certainly be shorter due to DC voltage attenuation over the length of the cable.

    2) The limitation of nodes is more based on the input impedance of the transceiver, which does not change with supply voltage. You want to make sure the bus impedance doesn't drop below 54Ω, and since two 120 ohm termination resistors already put it at 60 ohms, the total amount of nodes will be a factor of each node's input impedance added in parallel with the termination resistance.

    3) Since the typical VOD for 3.3V is 1V, and the RS485 standard requires at least 1.5V across 54Ω, it technically will not meet the RS485 standard, as you said. It will still transmit and receiver RS485 data, but just at a lower voltage on the bus.

    Regards,

    Eric Hackett