> The requirements were to both transmit and receive:
>
> > I need both to drive 5V TTL devices as well as get input
> > from 5V TTL devices
Another thing to check; if you do use a buffer powered
from 3.3V, then check that your destination device will
be happy with the Voh(min) of the buffer.
If the 5V logic is TTL, then it will be. If its some
ancient CMOS family (HC I think), then it might not
be TTL compatible, and in that case you will want to
use an open-collector (or tri-state) and a pull-up
to 5V. For the tri-state, your 3.3V logic level goes
to the enable pin (active low), and a 0V control
signal will enable the buffer and it'll drive low,
and a 3.3V control signal will tri-state the buffer
and pull high. This means you'll get an RC rise
for 0V to 5V transitions. You'll have to decide if
that is acceptable too.
Lots of 'design' for a simple I/O, eh :)
Cheers,
Dave
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