8-to-256 decoder/demultiplexor possible?

[honayboyz]

um im building a computer interface and im using 1 of the contorl lines to turn the interface into "card select mode" wehre the 8 bits of the data select teh card, i want to be able to have 256 cards so i need a 8-256 demultiplexor to select the card based on the value of the data lines

Does this chip exist, if not how can i rig up a series of chips to do the same

Thanx in adave

 

[nbucska]

e.g. 74ls138 ( 1 IC / 8 lines== 37 IC-s)
If need more, send FAX# <nbucska@pcperipherals.com>

 

[djs03824]

The 74LS138 is a 1-of-8 decoder/demultiplexer. This device is ideally suited for memory chip select address decoding. The multiple input enables allow parallel expansion to a 1-of-24 decoder using just three 138 devices or a 1-of-32 decoder using four 138 devices and one inverter.

Beyond that size it starts to get complicated!

 

[ErKansas]

You can do it with 17 4-to-16 demultiplexer ic's, with the first stage (one demultiplexer) you enable one of the other 16 -second stage-, using the 4 most significant bits of the input bus, and the 4 less significant bits driven to the inputs of the 16 second stage demultiplexers, you can use the 74154 ic to do that thing.

 

[skenn]

Use a shift register. 32 ic's, no spurious logic. Build it up as you test it. 2, or 3 control lines. If you need power, use Texas part TPIC6B595N (LED driver).

 

[Chancy99]

Might be easier to use a PAL. One chip ...
Check Altera or Xilinx.

Dean.

 

[nbucska]

Chancy99: A programmable chip with 264 I/O pins will be
too expensive -- not to speak about software cost and
learning time...

<nbucska@pcperipherals.com>

 

[KD7CDG]

I used a PIC16C55/57 as an 8-bit comparator. 8-position DIP switch tied to port C
with appropriate pullup resistors, and the 8 address bits to port B. The software
compares the two, and when they are equal, it outputs a pulse from port A. I
have a STROBE bit tied to one pin on port A, so it only triggers when I want it to.
Otherwise, it could be decoding while bits are still flipping on the bus.

With this method, I am able to decode three different addresses per chip, so I can
have multiple, separate functions on the same card. I use the pulse from port A
to trigger a data latch, or a DAC with internal latching bus interface, or the enable
line on a 4-16 decoder.