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SN75ALS1178: Receiver output voltage level

Part Number: SN75ALS1178
Other Parts Discussed in Thread: THVD1451, THVD1424, SN65HVD70,


I am using this transceiver to send signals between a 3.3V microcontroller to a 5V differential bus.

If I provide 5V to VCC and +/-5V signals to the Recieve Inputs (ie. 1A / 1B), what will the voltage level of the Receive output 1R be?

I would like for the output to not exceed 3.3V. If external circuitry is required, can you please share any recommended application circuit examples?

Thank you.

  • This device has TTL outputs, so the high-level output voltage will be about 3.4 V. If in doubt, add a weak pull-down resistor.

    Why are you calling it a "5 V differential bus"? Are there other devices that actually output ±5 V?

    It might be a better idea to use 3.3 V transceivers like the THVD1424, THVD1451/2, SN65HVD347, or SN65HVD70/1/3/4.

  • Thank you for your response.

    I am testing the component and measuring 4V output. In order to have a 3.4V output I need to use a 300 ohm pull down resistor. These voltage levels seem higher than TTL.

    Yes I am using this transceiver to communicate with ±5V node.

    I will look at those other components. The SN75ALS1178 is attractive because it has two sets of receive / transmit ports.

  • Hi Sarah,

    The RS-485 bus isn't typically considered a 5V bus  as a 3.3V RS-485 transceiver can communicate with a 5V RS-485 transceiver on the same bus as long as the transceivers can guarantee an input differential voltage of at least +/-1.5V across a 54 Ohm resistance (which in practice is two termination, one on each extreme end of bus, valued at 120 Ohms +/- 10% (tolerance can be less - but the standard allows up to the 10% tolerance) and they both comply with the rest of the RS-485 standard w.r.t. common mode voltages, minimum input impedance, receiver sensitivity, etc...) these parameters are not dependent on VCC so 3.3V and 5V devices can communicate with one another on the same bus. 

    The VCC of the RS-485 transceiver does set the console side outputs (so in general the "R" pin on most RS-485 devices) so if VCC is 5V than the output will be pretty close to 5V for a logic high (RS-485 standard stipulates any input differential voltage >= 200mV must be a logic high although many devices set this threshold lower than 200mV). So if you are using a 5V Transceiver there would need to be  a level translator between the "R" pin output and microcontroller input. This can be done either with a level translator IC that translates between 5V to 3.3V - some devices to look at can be found here, or a level shifting switch (which is basically just an NFET switch - details can be found here) or you can do it discretely with transistors and/or resistors (there are multiple references found by searching for 5V to 3.3V level translator as it is a very common application circuit and is pretty standardized). I'd advise against just using stronger pull-downs to lower the output voltage as that will be drawing a lot of current from the "R" pin which is going to increase power consumption of the device and may not be the most robust method in level translation. 

    With that being said - if you don't need the dual RS-485 full duplex device,  using a 3.3V device - like one of the device Clemens mentioned  would eliminate the need for voltage translation - if you do need this configuration than the only  options we have are 5V and you'd need to have a level translator between the "R" output and the transceiver input.


    Parker Dodson