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1/2 Unit load or 1/8 unit load

Paul PV
Prodigy 70 points
Other Parts Discussed in Thread: SN65HVD23, SN65HVD24, SN65HVD82, SN75176B, SN65HVD05, SN65HVD07, SN65HVD22, SN65HVD3082E

I have a half duplex rs485 bus system in a heavily noise environment with contractors, vfds and mechanical brakes.The system is having

1) 40 nodes

2) 200m Non shielded Twisted Pair

3) 19200 Baud-rate

Question: Is there any added advantage of using 1/2 unit load transceivers instead1/8 unit load ones since I only needed 40 nodes?

  • 0
    TI__Expert 5400 points

    Paul,

    No.  The 1/8 UL will present a lighter load to the driver, and consume less power.

  • 0 in reply to Bart Stiller
    Prodigy 70 points

    So you prefer SN65HVD24 instead of SN65HVD23 since I require only low signal rate?

  • 0 in reply to Paul PV
    TI__Expert 5400 points

    Hi Paul,

    Both the SN65HVD23 and SN65HVD24 are quite fast considering your modest data rate and distance.  You may want to consider a lower speed device, such as the SN65HVD82, which would offer better ESD protection and lower EMI emissions due to its longer rise and fall times.

    Are there other reasons you are considering the SN65HVD2x family of devices?

  • 0 in reply to Bart Stiller
    Prodigy 70 points

    HI Bart Stiller,

    Currently I am using sn75176B which is giving me occasional errors. So I am looking for a replacement. That means PDIP package and 5V are also required. After some research if found that SN65HVD23 and SN65HVD05 are better transceiver in all aspects. Now considering the low signal rate as well, my new choices are SN65HVD07 and SN65HVD22. So I believe it is like "higher Vod and output current" vs "larger Common mode voltage and Hysteresis"....So help me choose between them(Or do i have any other option?).

    Regards

    Paul

  • 0 in reply to Paul PV
    TI__Expert 5400 points

    Paul,

    You're right.  There are a number of parts that sound like they would work in your system.  It seems like the best input to the decision would be to determine why exactly your current system is producing errors.  For only 200m, it seems unlikely that you have a common mode issue, so the larger VOD of the SN65HVD07 may be the best place to start with limited knowledge of the cause of the problem you are experiencing.

    The other possibility I would mention is the SN65HVD3082E, which offers a higher level of ESD/EFT protection than the other devices.

  • 0 in reply to Bart Stiller
    Prodigy 70 points

    Hi Bart Stiller,

                     OK, SN65HVD3082E seams to be a good choice for me since my 40 Receiver nodes will consume around 4ma steady state current. And from graph both SN65HVD3082E and SN65HVD07 give comparable Vod at 4ma.

    The bus output slew rate of SN65HVD3082E makes it more suitable for my 20kbps.

    So my question is, does it's receiver have a low pass filter for high frequencies above 200kbps?


    Regards

    Paul

  • 0 in reply to Paul PV
    TI__Mastermind 21779 points

    Hi Paul,

    Bart is out of the office, so I wanted to chime in. There is not a low pass filter in the receiver of the device.

    Thanks,

    John

  • 0 in reply to John Griffith
    TI__Mastermind 21779 points

    Hi Paul,

    I spoke with a designer and got a little more information that might be helpful. There is a differential receiver that has some amount of parasitic capacitance on the inputs. in addition there is a resistive attenuator on the front end of the receiver. Therefore, there is an inherent low-pass RC single pole filter created. This is not implemented on purpose, but exists nonetheless.

    Hope that helps,

    John