http://en.wikipedia.org/wiki/Transistor-transistor_logic Well worth reading, www-gth.die.upm.es/~macias/docencia/datasheets/info-familias/intro-to-hct.pd f Nat Semi app note 368 on subbing some CMOS varieties. >-----Original Message----- >On Behalf Of rdrake >anyone know of a source, or modern substitute, for National 8281 TTL >divider IC?