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Physical Connection of Active Low vs Active High signals to Differential Drivers/Receivers

Anthony Campos
Intellectual 400 points

I am questioning the following convention to see if it is common amongst the community. 

For differential receive signals:

H of the signal is connected to the + of the receiver, and L of the signal is connected to the - of the receiver, results in an active HIGH signal at receiver output

L of the signal is connected to the + of the receiver, and H of the signal is connected to the - of the receiver, results in an active LOW signal at receiver output

For differential drive signals:

If active High signal is desired, present a logic 1 at the differential driver input. The differential driver + output will represent the H of the signal, and the driver - output will represent the L of the signal

If active Low signal is desired, present a logic 0 at the differential driver input. The differential driver + output will represent the L of the signal, and the driver - output will represent the H of the signal

Is this convention consistent with what is intended in industry for Active High and Active Low signals? Does it really matter in the end, since it's all interpretation of signal levels, especially when an end target is an FPGA?

 

  • Mastermind 7395 points

    I agree with the convention you present, and I think that it really matters, either that you connect and FPGA or any other device to the driver, you have to know in advance what are you going to receive and transmit.

  • in reply to Avi Chami1
    Intellectual 400 points

    I think it really depends on what's driving the signal.  I can have a driver that I present a logic 1 to and it will provide a logic 1 on the + output, and logic 0 on the - output and connect these outputs to the + input and - input of the receiver. This would provide Active High logic both the driver and receiver. 

    However if I reverse the wiring, meaning I connect the + output of the driver to the - input of the receiver, and the  - output of the driver to the +input of the receiver, I get an Active Low signal in the receiver but still provide Active High logic out of the driver. 

    But couldn't this be all avoided if I just change the convention of what I am driving? Say, I have a driver and I want to provide an active low signal. Therefore, I ensure the default state of this driver is logic 1, and when I want to transmit I provide the driver with a logic 0.