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Receiving RS485 signal and TTL signal with the same RS485 receiver

Attila Kalinka
Prodigy 230 points

Hi All !

I have to test two types of encoder chips with a microcontroller. One has RS485 outputs, other just 3.3V standard single ended TTL outputs.

Is it possible to connect both types of signals to an RS485 receiver input and get a correct data on the receiving side ( mocrocontroller input ) ?

Obviously the only problem is how to get the single ended encoder signal trough the rs485 receiver...

I have no spare pins on the external connector to use separate I/O lines for the different encoders, so I have to use the same lines for TTL as for the differential inputs.

I was thinking..in case of TTL I can connect 3.3v output to line B, and tie line A to GND...

It will be up to user to use the correct connector for different encoders.

So...>

if B = 0 (LOW) : A-B = 0  ...output should be 1

if B = 3.3v (HI) : A-B = -3.3 ...output should be 0

The output would be inverted but that's not an issue.

All the encoders and the microcontroller are using 3.3v supply. The micro will be actually a Tiva launchpad.

Am I missing something or it really should be that easy ?

Is there any RS485 receiver/transciever that should suit this more than others?

Thanks,

Attila

  • 0
    Guru 243820 points

    A differential receiver is a comparator; you can tie one input to half the high-level voltage (1.65 V), then the single-ended signal will result in a positive or negative difference compared to that.

    (You should not rely on a difference of 0V to result in a valid output; while your receiver might have a fail-safe feature that guarantees this, you'd have almost no noise margin left.)

  • 0 in reply to Clemens Ladisch
    Prodigy 230 points
    Thanks Clemens !

    The 0V fail safe ability has blinded me obviously. The half-level solution just didn't crossed my mind, but it's quite obvious. It does not take anything away from your great help and fast answer. It brought me out of this dead-end thinking !

    Attila