Demultiplexer

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Electronics4you

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Hi All

Is it possible to combine more for example 3-to8 line or a 4-to-10 line decoders, to get a 7-to-128 line decoder for 7-bit?

Electronics4you
______________________________________________________________________________
"Be cool"
 
The CD4514 is a 4 to 16 line decoder. You could make a 7 to 128 decoder with additional logic to decode the other 3 bits, but it is more work than I want to tackle right now.
 
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I believe you can do that. did you mean to say an 8-to-256 line decoder instead of 7-to-128 decoder?
it's gonna be very complex circuitry! if you can get it on a single chip the better.
 
Its probably better making your own decoder using logic gates instead, but still, it will be complex circuitry.

and why a 7 to 128 decoder?
 
mstechca said:
and why a 7 to 128 decoder?
Click to expand...

Presumably because 7 bits gives 128 seperate values?.

But as williB asked, knowing what he's trying to do would be helpful?.
 
I have a 128 lenght display, where I can write to the line that I want, without usign a shift register, which runs trough all bit all over again.
 
Electronics4you said:
I have a 128 lenght display, where I can write to the line that I want, without usign a shift register, which runs trough all bit all over again.
Click to expand...

And there's 128 seperate wires to it?, sounds like a poor design?.

The multiplexer solution is simple, these multiplexers were originally designed for memory addressing, so you treat them accordingly.

Connect one 3 to 8 multiplexer to the top three address lines (in your case bits 4, 5 and 6 of your 0-6 bits). Each output will select one lower 16 bit page of addressing.

Connect eight 4 to 16 bit multiplexers all to the lower 4 bits (bits 0, 1, 2 and 3), and the enable bits of each to the outputs of the first multiplexer. You now have 128 (8 x 16) individually addressable pins - and LOT'S of wires 8)

It's standard memory addressing for micro-processors, used to address EPROM, RAM, and I/O.
 
I also have the same question about any type of 7-128 or 8-256 decoder? I need about more than a hundred parallel outputs for logic signal. It is like control the parallel port of a PC but the port have only 8 data pins (D0 - D8), so I have to think about a decoder. Any suggestion please ?
 
hi nobtiba,

Attached a block diagram of a 'Set 1 of 255 high' selector.
NOTE: the Group selector ic is a 4515, the rest are 4514

This should give you the general idea.

If you want to select analog lines, look at the HEF4067 ic.
 
Last edited:
Here is an expander for the parallel port. The circuit can be duplicated and C2, C3 used to get up to 394 outputs. The address of the '573 is put on the lowest 4 bits of the data bus and strobed in. The data for that '573 is put on the data bus and the '4514 enabled.
http://www.geocities.com/gpg212/parport128.2.gif
 
Hi Ericgibb,

What is the difference between HEF4515 and 4514? Why don't we just use one type for both group selector and the rest ?
 
hi,
The 4514s require a /E [pin23] to select the ic, so a 4515 outputs will give a LOW on the selected line.

I understood you wanted to SET a 1 of 255 output HIGH [+V] ,thats why I suggested 4514, as a 4514 will go HIGH when selected.

Is this clear?
 
I want to control the output either on or off. I do not know what does it mean by "with input latches" in HEF4514 datasheet. I just want to enable the IC by an input signal to "enable pin".

Is there any optional for 4-to-16 decoder? I search the whole day at the electric shopping center for 74HCT4514 but could not find it. So I think I should better to have alot of options.

Thanks.
 
hi nobtiba,

>> I want to control the output either on or off. I do not know what does it mean by "with input latches" in HEF4514 datasheet. I just want to enable the IC by an input signal to "enable pin".

The input latch 'EL' pin on the 4514, if connected high, will enable any change on the A0 to A3 pins to immediately 'select' an output pin, the latch is 'transparent'.
If the EL pin is connected low, any change on the A0 to A3 pins will not change the selected output pins.
Whatever the states of A0 to A3 were before EL went low will be be stored in the latch.

When you say "control the output either on or off" do you mean when you have SET the output 'low or high' on an output,
you want it to stay in that state while you control another output pin, high or low ????
If so, this is an important change in the logic required.

>> Is there any optional for 4-to-16 decoder? I search the whole day at the electric shopping center for 74HCT4514 but could not find it. So I think I should better to have alot of options.

What decoder ic's can you get??
 
By saying "either on or off" I mean at first I want to ouput high level then after that turn to low level (or vice versa). What if I want to get the values stored in latch or if I don't care about these values?

Today I checked the stored again with more options but I can not find any 4-to-16 decoder (checked: MM74hc4514, CD4514BE, HEF4514). So at last I have to buy 2 ICs 74LS138 which are the 3-to-8 decoders to check how it work. If everything is ok, then I will buy more. But I have a question, even if I have alot of 74LS138, what is the maximum number of output I can get (control buy parallel port as discussed). I need more than a hundred but I am afraid I could get only about 64 (8x8=64) ?
 
By saying "either on or off" I mean I want the output to be high level after that turn to low level (or vice versa).
What if I do not care about the values stored in the latch or if I want to output it?

Yesterday I searched again all the electronic shops but I could not find any 4-to-16 decoder (searched: HEF4514,HC4514,CD4514). Do you think any type may be easier to find?

Eventually I have to bought 2 ICs 74LS138 which are 3-to-8 decoder. What will be the maximum number I can get (control by parallel port like above discussed) ? I guess only 64 (8x8=64) but I really need about more than a hundred of output. Do you think I can get it by 74LS138 ?
 
hi,
If you decode all 8 output bits on the parallel port you can get 0 to 255 individual lines.
By using latches on the 8bit output and using other pins on the port you could get more than 255, if required.

The 138 as you know is a 3 to 8 decoder, it has other control pins on the input side of the ic, as well as the 3 address pins.
The problem is the 'selected' output is active LOW. Also you will require many 138s to decode 128 or 256 lines.

Its important to remember the 'selected' outputs are NOT latched,
as soon as you select another output [LOW] the previous output will revert to a HIGH state.

Can you post a diagram of the 'simple' version you are going to test, with a description of what you want the outputs for???

Are you able to buy HEF4067's ?
 
But HEF4067 is for analog right? I will post my circuit a bit later, after my dinner and a few more tests, I'm testing it on the whiteboard and it works but I don't understand how could I have 255 output as you said !?
 
hi nobtiba,
Yes, the 4067 is an analog decoder, but if pin #1, 'Z' is connected to '0v' then the 'selected' output is LOW, if pin#1 is +V, then 'selected' output is HIGH. So they can be used as 4514/4515's. [4067 is more expensive IIRC]

With Ref to the LS138 3 to 8, look at Nigels earlier post on how this is done.

>> don't understand how could I have 255 output as you said !?

If you decode ALL 8 output lines, you will get 255.

If you only want 127, decode 7 output lines.
 
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