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256 outputs from 8 bit

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I am new to electronics but understand many principles. Very fascinating stuff.

In my mind I would think it possible to control up to 256  outputs from an 8 bit input such as a parallel port controlling home automation relays/devices.

I will not need all the outputs but would like to learn how to create such a project.

Could someone help me with the basics and logic and get me pointed in the right direction of thinking here.

Thanks very much.


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Hi Scott

start small and build up to it...

the previous post will get you initially 4 bits, building up from there (16 possibilities and beyond) There are things like the 8255 chip which will expand an 8 bit interface.  Lots of PICs allow multiple I/O ports which can then be used to drive the outside world.  There are many processors like the ST10 range which, if memory serves me correctly, will drive directly 114 I/O ports.  There are also 8035/8041 derivatives with different ranges of I/O.

I know it's not easy but you have to try to define your requirement and then work backwards to try to find a minimal/low cost solution whicih will satisfy the original problem

Best of Luck


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One method is to use a 4 port microcontroller and connect the port pins in a matrix. 16 ground pins and 16 control pins in a matrix will give you 16x16 combinations, or 256. You would change the data direction in the code to allow for 8 of the port pins to be inputs when needed. You could insure the matrix was not effected by this with a few diodes.

If using the parallel port of a pc, you could add a 8255 and associated hardware to expand it to a 24 bit input/output port.


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Did alot of research on this topic. With all the help you gave, it steered me in the right direction.

I ended up working up a simple 64 output using
9 - 74xx138 3 to 8 decoders

Using one as an address bus and the last 8 for outputting.

Now I just need to research latching the outputs. I think maybe a D-latch flip-flop (if that sounds right) is the way to go.

Again thanks for the brain fuel.


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