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termination for TTL driver

Discussion in 'Electronic Design' started by john, Sep 27, 2003.

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  1. john

    john Guest

    I have a ckt board outputing, thru a TTL driver, 8 bit data in parallel
    to another board loacted 3 feet away. I was wondering is it necessary to
    add some termination on the data line? If so, what type of termination
    should be used and how to connect it ? Resistor connecting to V+ is ok ?
    any help is very much appreciated
     
  2. Henry

    Henry Guest

    Series termination. Value is Z(0) of line minus Z(out) of TTL Driver. On the
    receiving end a resistor is normally not required.
    -Henry

    john schrieb in Nachricht ...
     
  3. TTL was never made for transmission lines.
    You should consider conversion to something else.
    Eg LVDS, ECL.

    Rene
     
  4. Norm Dresner

    Norm Dresner Guest

    There are TTL parts that were designed to drive lines of specific
    impedances. IIRC, 74S140 is a (dual)? 50 ohm driver and 74128 is a (triple)
    75 ohm driver. The important thing about the termination is that it has to
    be matched to the characteristics of the lines as much as to the
    characteristics of the drivers.

    Here are some important considerations:

    What kind of cable are you using?

    What frequencies are the lines switching at?

    What, if any, transition-time requirements do you have at the receivers?

    What logic family are the receivers?

    If I were doing this and had not-to-fast signals, I'd consider using some
    differential drivers and receivers which are TTL-compatible. 26LS31/26LS32
    pairs are probably reliable to 10 MHz or so, but only if you terminate the
    lines properly. For twisted pairs, my first guess would be to assume the
    line impedance was on the order of 100-150 ohms and I'd try to put two
    resistors at each receiving point, one to ground and one to Vcc, each twice
    the value of the cable impedance.

    Norm
     
  5. As long as the data rate isn't too high, this is the right
    answer. 30 ohms (one can play) is about right for normal TTL
    drivers and .100" IDF cable with a ground inbetween each signal.
     
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