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SN65HVD1781DR questions

Cheok Loon Lee
Intellectual 610 points

Hi, I would like to find out a few things on behalf of my customer whom is working on this chip. Hope you guys can help

1) Is there a limit on the number of nodes this device can handle? Datasheet states that it is suitable for multipoint applications over long cable runs but I'm not sure if there are any limits.

 

2) Are the numbers derived from real testing or calculations and in what sort of environment. i.e. temp & humidity. Would be good if there are any information that can be shared.

 

3) Any recommendation for cable?

Regards,

Lee

  • 0
    Intellectual 805 points

    Hi,

    RS485 Standard says Maximum Devices 32 drivers or receivers can be connected at the single bus.

    if you want to connect more number of devices then you have to connect repeater in the bus and again you can connect further more32 devices. so now total number is 64.

    but repeater itself nothing but the one of the RS485 load in the bus so you have to connect 31 devices + repeater = 32 devices and connect other 32 devices.

    in datasheet, they have mention 320 devices.

  • 0 in reply to milap patel
    TI__Mastermind 28265 points

    No, the RS485 standard says no more than 32 unit loads to connect. 1 unit load is eqquivalent to an input impedance of 12 k ohms.

    in older days one transceiver had an input impedance of one unit load and therefore you could only ocnnect 32 transceivers to the bus.

    Modern transceivers typically have an input impedance of 1/8 or 1/10 unit load and thus allow to connect 256 or 320 transceivers to the bus.

    regards,

    Thomas 

  • 0 in reply to Thomas Kugelstadt
    Intellectual 610 points

    Hi Thomas,

    Thank you for the reply, What about the recommendations of cable and the testing information. Are those allowed to be shared? 
    The kind of cable used to get this test result would be helpful to us. Is it twisted and how big is the size (AWG)?

    Also if you have any data regarding the relationship between IC performance (data rate, cable length) with the environment temperature? Hope you can share too

    Thank you

     

  • 0 in reply to Cheok Loon Lee
    TI__Mastermind 28265 points

    As cable you can either use CAT-5 cable and terminate withh 100 ohms at both cable ends, or use proper RS485-cable (more expensive) and terminate with 120 ohms.

    operation temperature is specified in the data sheet, humidity I don't know. Better ask Dave graham or Clark Kinnaird.

    There is also an RS485 guide for download in the Foles section above. It's the first file listed.

    regards, Thomas

     

  • 0 in reply to Thomas Kugelstadt
    TI__Mastermind 28265 points

    sorry it's the FILES section, no the Foles section.

    regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Intellectual 610 points

    Hi Dave / Clark,

    Any help on this situation? What my customer truly wants to know is this chip can meet RS-485 spec (signal & cable length) under various environments (e.g. temp -40 to 85degC and humidity). In short, its the reliability

    Thanks in advance