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Using Parallel Port for Input

Posted on 1998-08-03
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Last Modified: 2010-04-26
Hi,
I'm trying to use the parallel port for inputing a couple of switches into a laptop computer.  I have a requirement to be completely standalone, running just on the batteries of the laptop.  That leaves me with a problem of providing a 5V source to use to hold the input bits high to allow the switch to ground them out and provide the input (this is how I think I'll have to do it).  My question is, "Can I use the data bits safely to provide a constant 5V source by holding a logic 1 (or zero as the case may be) on them?"  I am planning a load resister of 10k.  My plan will be to put the resistor between pins 2(D0) and 11(BSY) and the switch between pin 11 and 18(GND).  I'll read the switch input in the BSY bit of the status register.  Will this work safely?  Thanks.
Mark
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Question by:mustang070597
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by:mustang070597
ID: 1122121
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warmcat earned 100 total points
ID: 1122122
Mustie -

Most implementations already have a pullup resistor on BUSY, which is usually something between 1K and 10K.  Your additional resistor will cause no problems but will usually be unnecessary.  The 10K value is playing it very safe: there are no issues if you chose a 1K resistor.  Your whole plan will work fine.

-Andy
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