Replacement for a DEC 7474 Chip

Paul Koning paulkoning at
Sun May 15 13:55:56 CDT 2022

> On May 15, 2022, at 2:51 PM, Chris Quayle via cctech <cctech at> wrote:
> ...
> Original 74 series TTL can still be found, Ebay often, new old stock
> and other surplus vendors.
> Would need to take care replacing with later ttl series, as the gate
> delay, switching times and fanout probably won't match. Always use the
> original series if you can...

That's sensible advice.  When you can't, faster devices with adequate fanout should work.  The exception would be in circuits with hairy timing, where the design relies on the actual stage delays of the components.  Most people don't build that way, for good reason.  Seymour Cray is the best known exception; for example, the CDC 6600 design is very thoroughly dependent on the circuit and wire delays, and either slower OR faster will break things.  And actually, it isn't clear to me why the thing ever works; the design files clearly demonstrate clashing signals that somehow apparently don't show up in reality.


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