TTL computing

see below, plz. Dick ----- Original Message ----- From: "Peter C. Wallace" &lt;pcw(a)mesanet.comTo: &lt;classiccmp(a)classiccmp.orgSent: Thursday, April 11, 2002 8:04 PM Subject: Re: TTL computing <snipfrom 7474, What was meant here is that you can't tell what function is going to be implemented in a programmable part from simply looking at the part number. You can tell from looking at a discrete device what its functions are. > > That's a dubious definition at best.... > > > If I buy a 7474, then I know it's going to be 2 d-type ff's. But I can > > choose to use those as 2 bits of an register on a CPU I want to build, or > > as a /4 clock divider, or to do the handshake for a centronics input > > port, or... Yes, but it's still a 7474 and is limited to what you can do with a 7474. You can't program it to behave differently than any other 7474 connected externally in the same way. If you can touch it, it's hardware. If you can't alter its internals (except by doing something you shouldn't) so that it functions in a predictable way different from another identical part, when externally connected in the same way, then there's certainly no firmware involved. > Or 1/2 as an inverter for an xtal oscillator and 1/2 for div/2 > (had to do that one to avoid using 1 more package) I'd like to see how that works! I was, just yesterday, looking at a circuit trying to figure out how to use half a 74HC393 as an inverter in order to avoid adding a package. <snip