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  1. #1
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    Default Cheat on driving long OC loads?

    I'm working on a design that involves driving 5V 220/330 ohm terminated lines, TTL levels. The spec calls for OC drivers, but I'm wondering if I can get away with driving 1-2 m lines at about 1.4 MHz using a 74HCT244 totem-pole output ather than the more expensive 74HCT760. Same pinout, so I could swap the ICs if one didn't work well enough.

    Any advice--is it worth trying?

  2. #2
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    Chuck,
    Google isn't locating the 74HCT760 Datasheet for me. Was there a typo?
    If you have the 74HCT760 Datasheet will you upload it?

    Thanks.

    Larry

  3. #3
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    I don't see any reason not to give it a try. A 1.4MHz signal has a wavelength of 214 meters, and your lines are a tiny fraction of that. As long as the capacitance of your lines is small I'd guess you'll have no issues, but AC signal transmission is *way* outside my zone, so much so I considered not replying at all.

    Also 1.4MHz is a 714ns period (assuming I didn't lose track of the decimal point) or 357ns for a half-period, so the enable/disable/transition times for the 74HCT244 look like they will easily handle that.

    Anyway, as long as you can manage line capacitance to prevent instantaneous output current from exceeding 35mA I don't see why using this part would cause any problems. And since it's easily swapped, why not?

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    Quote Originally Posted by kgober View Post
    I don't see why using this part would cause any problems.
    The issue is actively driving the line when it is high, instead of letting it be pulled up to 2/3 vcc by the terminator network.
    I assume it would be used on a unidirectional signal line?

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    The only thing I can think of that might be a problem is if the device you are talking to isn't powered on.
    We burned up a ribbon cable once where the device being driven was off, and the cmos interface drivers
    tried to supply power through the terminated signal lines.

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    Quote Originally Posted by Al Kossow View Post
    The issue is actively driving the line when it is high, instead of letting it be pulled up to 2/3 vcc by the terminator network.
    I assume it would be used on a unidirectional signal line?
    Unidirectional--as I said, terminated through 220/330 ohms on the far end, which uses bog-standard LSTTL as receivers.

  7. #7
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    Quote Originally Posted by Chuck(G) View Post
    Unidirectional--as I said, terminated through 220/330 ohms on the far end, which uses bog-standard LSTTL as receivers.
    If you use OC drivers do you terminate at both ends?
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

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    Quote Originally Posted by ldkraemer View Post
    Chuck,
    Google isn't locating the 74HCT760 Datasheet for me. Was there a typo?
    If you have the 74HCT760 Datasheet will you upload it?
    Sloppy fingers--it's the 74BCT760

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