74hct238 vs 74hct138

[Daniel Pitts]

Why does it seem so much harder to find a 74hct238 chip than the
74hct138? I need a 1-of-8 decoder with HIGH output, but it seems like
that is far less common than the one with LOW output.

It's not *impossible* to find, but I'm curious if there is some reason
why, and if I should be looking at another solution to my problem instead.

Thanks,
Daniel.

 

[[email protected]]

Why does it seem so much harder to find a 74hct238 chip than the
74hct138? I need a 1-of-8 decoder with HIGH output, but it seems like
that is far less common than the one with LOW output.

It's not *impossible* to find, but I'm curious if there is some reason
why, and if I should be looking at another solution to my problem instead.

TTL devices historically tended to have low-active enables and strobes due to
the lower impedance of the low state. The 3-8 decoders with low-active
outputs are therefore more common.

 

[Michael Black]

Why does it seem so much harder to find a 74hct238 chip than the 74hct138? I
need a 1-of-8 decoder with HIGH output, but it seems like that is far less
common than the one with LOW output.

Later devices often never had the same success as the earlier set, and
certainly in places where hobbyists might buy things, often the selection
is limited, and hence the earlier devices are what they carry.

So even though later devices might be better than earlier ones, you'd
still see projects using 7490 decade counters (or some later logic type
variation), while the 74390 which was a dual device and for many
applicatins worked fine, was ignored. Remember, a lot of people have
early databooks that dont' cover the later devices.

Michael