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.

 

[krw@att.bizzzzzzzzzzzz]

On Fri, 13 Jul 2012 11:20:48 -0700, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

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.

 

[WangoTango]

In article <6GZLr.23414$cE7.16191@newsfe13.iad>,
newsgroup.nospam@virtualinfinity.net says...

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.

Because active low enables are so much more common that active high ones
and there is a reason for that.

 

[Phil Hobbs]

Daniel Pitts wrote:

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.

Chip enable inputs are usually active-low.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Michael Black]

On Fri, 13 Jul 2012, Daniel Pitts wrote:

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

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.