help!!how to share the same input port??

i m using four demultiplexs(A,B,C,D).
Each demultiplex got 10 outputs
(A0,A1,A2,..A9; B0,B1,B2...B9; C0,C1..C9; D0,D1,D2...D9).

i now want to put (A0,B0,C0,D0) in one node; (A1,B1,C1,D1) in one node;
(A2,B2,C2,D2) in one node and so on..

when i "on" 'A' demultiplex and 'off' 'B,C,D' demultiplexs, the output A should be show out on LED...

i control 'on' and 'off' of the demultiplex at their Vcc part..

but i now facing with a problem that since one node is shared by four input, there seen have interference at lighting the LED between the output should be show..

how should i do if i want to share the input port, but it can give one output when either a chip is 'on'..

oobi

i think you have to put the outputs through few more gates - may be one extra ic. sorry, tak berapa ingat pasal ni.

plus, what happen if you turn on C and the rest are off? or B and C are on? or you just want A on only?

Originally posted by oobi at 4-2-2007 11:27 PM

i think you have to put the outputs through few more gates - may be one extra ic. sorry, tak berapa ingat pasal ni.

plus, what happen if you turn on C and the rest are off? or B and C are on ...

actually each Demultipleks will show out different output when their Vcc is 5V,,

for example what i want is ,
when i 5V  'A'  , one LED is ON.
if i 5V  'B' , three LED are 'on';
if i 5V  'C' , two LED are 'ON'..
if i 5V  'D' , five LED are 'ON'..

since i now share the node among four inputs, there will have interference among the LED,
like,when i 5V 'A' , four LED is 'on'
or i 5V 'C' , three LED is 'ON'..

reinloch

The thing is, when using chips in designs, checking the electircal properties are paramount. In certain cases, the chips may apparently function correctly under certain conditions. However, glitches may occur when absolute parameters are surpassed, and more often than not, damage some  i/o buffers which are difficult to trace since the bugs are jittery.

Oobi is correct in that you may require extra buffered tri-state outputs, assuming that the current demux don't have them. In which case, it is certainly best to consult the datasheets on their availability. The fact is, the 4 demux are bus connected, and switching off a chip does not disconnect it from the bus as the low impedance path may still exist, which brings us to the question on whether the demux is driving the leds  or active low demux o/p.

In any case, for your question on sharing the input(output) ports, consult the datasheets on the output high-z state. The chip select  (CS) function will disable the other 3 chips, effectively disconnecting it from the bus so that the output leds will only respond to the active remaining chip.