RE: (slightly OT:) Parallel port question

From: Rainer Buchty (
Date: 2003-10-14 00:24:25

> If data is sent to the LPT port, STROBE/ will go active low. So if you
> monitor that, what's the problem? Unless you find a way to hack the
> hardware of your PC, there's no way that a "print command" can send data
> withut changing the state of STROBE/ and some other control pins.

Why would you need a hardware hack to do so? The STROBE line is bit0 of
the control register and nothing hardwired which automatically indicates
that someone is doing something with the parallel port. According to "my"
documents it is indeed allowed to change the direction of strobe, line
feed, error, init and select in.

So you would rather need to monitor both sides of the strobe signal, the
PC side and the LPT side, to determine who is sending data right now. By
just tapping the strobe line you can only detect if someone is sending
data but not who. But then you still can't be sure that some software
fiddles with the port bits as the above monitoring scheme will only detect
valid transports.


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