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Here I go again testing more circuits on two computers with each reporting completely different data yet I feed the same data in.

Now this time, the data coming back is all logic highs on my laptop and on my desktop, more accurate data.

I did read somewhere that older parallel ports were TTL and had internal resistors in them that helped defined their default logic level (I assume my desktop), and I also read somewhere that some parallel ports are open-collector (my laptop?)

The status pins of the parallel port are connected to the outputs of a 74HC157 multiplexer through 100-ohm pull-up resistors.

I also heard about using pull-up or pull-down resistors for the status lines, but I'm not really sure since parallel port technology has changed over the years (heck, the internet has too!).

Should I really use pull-ups (I'm thinking 10K?) and assume the output of 74HC157 is too weak for the parallel port to detect?

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    \$\begingroup\$ "modern parallel ports" is already a bit contradictory. \$\endgroup\$ – The Photon Aug 9 '18 at 5:01
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    \$\begingroup\$ It's unclear what you are asking. 100 ohm series resistors between and IC output and another's input are not "pull-up" resistors. A 10K pull-up resistor is far weaker than most IC outputs, and would only be useful if an output were open collector / open drain, or undriven which the 74HC157 is not, unless the enable is inactive. How do your values differ? Do you perhaps need to mask off the unwired bits and only look at the driven ones? Are you sure you are reading from the correct port address in each case? \$\endgroup\$ – Chris Stratton Aug 9 '18 at 5:50
  • \$\begingroup\$ Parallel port technology hasn't really changed all that much. It died off, and was replaced by mostly serial technologies. \$\endgroup\$ – Oliver Aug 9 '18 at 7:15
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    \$\begingroup\$ This question is very vague. You have a laptop with a parallel port? "I read somewhere"..."I also heard"...you need to provide links and details. Your desktop gives "more accurate" results...what does that mean? No details about the OS and drivers? \$\endgroup\$ – Elliot Alderson Aug 9 '18 at 11:22
  • \$\begingroup\$ Ok Laptop has linux with the parport and parport_pc drivers loaded. Desktop is plain DOS with Qbasic and no parallel port drivers \$\endgroup\$ – Mike Aug 9 '18 at 12:34

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