LS TTL & HC CMOS ICs make compatible

Discussion in 'General Electronics Chat' started by vinodquilon, Mar 3, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    I am using both LS & HC ICs in a single application.
    As Rogs (member-'all about circuits') pointed out, the output is not getting due to type mismatch.
    But in our local market only 74126, 7474,7447 LS versions are only avail.
    Does there exist any methods to make compatible both versions without any problems ?
  2. Papabravo


    Feb 24, 2006
    There are two basic problems: drive levels and input thresholds. Putting a pullup resistor on a TTL output can help get that TTL output up closer to Vcc so that it is above the Vcc/2 threshold of the CMOS HC part. When connecting an HC output to a TTL input be careful of the fanout. With a 6 mA output stage, 2 or 3 TTL inputs @ 1.6 mA each would be the limit.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Can you get 74HCT parts? They are compatible with 74LS.