[BBC-Micro] New 6809 TUBE, was: TUBE chip, accessing 'Parasite' side

Phil Blundell philb at gnu.org
Tue Mar 18 23:13:06 GMT 2008

On Tue, 2008-03-18 at 21:12 +0100, Johan Heuseveldt wrote:
> I think the GALs are also fully TTL compatible, so considerations are:
>  * the proper IC family is possibly HCT anyway?
>  * it's probably not good to mix HC and HCT?
>  * availability could force me to use a more standard chip like the 174?

Mixing HC and HCT is generally fine.  The only difference between these
two families is that HCT has modified input thresholds to make it
compatible with the drive characteristics of TTL outputs (at the expense
of worsened noise immunity).  The output stages are identical: both have
rail-to-rail swing, so there is no problem with feeding an HCT output
into an HC input -- or vice versa, though it's relatively rare that
you'd want to do this.

I must admit I haven't really been following the details of what you're
planning to do.  But, as a general rule and assuming 5V logic, you
probably want to use HC or AC devices whenever the input is coming from
another CMOS device, and HCT or ACT devices whenever the input is coming
from a TTL device.


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