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Subject: Re: 74HC259 as a demultiplexer?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 18 Jan 2003 04:30:26 GMT
NNTP-Posting-Date: Sat, 18 Jan 2003 05:30:26 MET
Si Ballenger wrote:
> I'm trying to use a 74HC259 chip as a simple demultiplexer
> connected to the parallel port data pins. What I wuld like to do
> is use the 5v from one data pin to power the chip, and use three
> other data pins as the 3 logic inputs (A0-A2). The chip can
> perform several functions, but I just need the output lines
> (Q0-Q7) on the chip to go to 5v depending on which of the 3 input
> lines is at 5v. Looking at the function table in the below data
> sheet, I would think to set the chip for this type of operation,
> MR and LE lines would need to be connected to ground to keep them
> low. Also, the D line would need to be connected to to Vcc to
> keep it high. In this configuration I expected the output lines
> to go high when combinations of the input lines are made high by
> connecting them to Vcc. This doesn't seem to work (after
> tinkering, I may need to use a "fresh" chip), so does anybody see
> anything obvious in this setup that would not let this work?
> Currently I'm just powering it with a couple of batterys to see
> if it will work. Any info on this would be much appreciated!
If you read the datasheet right, you should have understood that the
latch_enable line has to be toggled before the data can reach the output.
Study a bit about logic functions and then figure out the diagramm on page5.
The truth table clearly states that this decoder is not transparent.
What you want is probably a 74HC138, but there the outputs are inverted
(useful to drive FET switches).
electronic hardware designer
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