24 May
2007
24 May
'07
10:27 p.m.
Tony Duell wrote:
More seriosuly, I was once sort-of involved in a piece
of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off?
If all you want to do is select a single combination of three
bits and enable based on that, a 74138 will do it. If you want
a more complex set of enable conditions, a multiplexer like the
74151 would be better. Connect the three bits to the three
'select' inputs, and wire up the eight 'data' inputs to
generate the truth table you need. It's a bit like an 8-bit
ROM, in a way.
You can use it with four input bits, too, but you might need
an inverter as well.
--
John Honniball
coredump at gifford.co.uk