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8 to 1 scart switch

Discussion in 'General Electronics Discussion' started by @[email protected]@me, Jan 19, 2018.

  1. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    Hi all,
    I'm thinking about building my own 8 to 1 SCART switch (for the PAL format, in France), supporting both RGB & CBVS, in order to have all my retro game consoles connected to the TV without having to remove / change / re-plug cables each time.
    One of the reasons is that commercial devices are either expensive either not with 8 inputs (and cascading several is ugly to the eye).

    Anyway, I started some research on how to do such a thing, and I found this schematic, using the MAXIM MAX9655 chip :

    https://www.maximintegrated.com/en/images/appnotes/4522/4522Fig03.gif

    I had a look at the datasheet, and all seems to be doing what I want, but as it does only work for 2 inputs, I'm wondering if it would be a safe thing to casacade such ICs (in a way there is always 3 MAX9655 between any input and the output), while keeping a good signal and not to much delay ?

    Thanks in advance for your tips :)
     
  2. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    You would have to 'parallel' the outputs to get eight-into-one but the safest way would be via an 'enable' signal on the MAX device. Clearly it doesn't have one so this would need to be implemented at the output - any device that could 'pass' the signal but be enabled/disabled and driven through an 3-of-8 decoder would work.
     
  3. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    It seems like the MAX 9656 has such an output enable pin (the outputs are as 3 state buffers in the datasheet). The other functionality it offers is not useful for me, but shouldn't prevent me from doing what I need.
    what do you mean by "parallel" the outputs ? Do I really need to be able to hi-Z some outputs ?
    The intended goal is to have 4 MAXs for the 8 inputs, put the 4 outputs to 2 other MAXs, and finally the 2 output to a last one, and towards the TV (like some kind of bintree).
     
  4. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    If you wish to cascade them then the 'parallel' solution proffered is not required and cascading seems perfectly 'do-able'.
     
  5. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    Yeah thanks :) But still, what did you mean by "parallel" solution ?
     
  6. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    The parallel solution was what I initially proposed (but requiring the /EN signals from a 3-of-8 decoder).
     
  7. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    Oh, you meant using tri-state buffers for each input, and selecting which one can pass using the decoder ? Is there no risk for the signal to be altered using these ?
    With something like this for each input :
    http://www.ti.com/product/SN74LS243
     
  8. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    Someone gently reminded me the parts I'm showing here (MAX9655 & MAX9656) are no longer available for sale :( I'll have to find another way of doing what I want
     
  9. @xi@g@me

    @[email protected]@me

    36
    3
    Dec 15, 2016
    I finally managed to break open an old SCART switch, and it seems that simple relays may be largely sufficient :) (they use mechanical switches with 12 contacts at once). So adios the MAX4399 or MAX9655 :) thanks for your help, I'll post the schematics here once I'm done
     
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