WARNING: Spontaneous LPT output changes when using input lines on XP

[Paul Gr]

This message contains a solution for unexpected printer port triggering on
PC’s configured with Windows XP.

Behavior:
In our lab we recently discovered spontaneous state changes off the parallel
(LPT) output pins (d0-d7; pins 2-9) on *ALL* our XP machines. This behavior
occurred when pulling one or more of the input lines to ground (0 Volt):
Acq, pin 10; Busy, pin 11; PE, pin 12; Select, pin 13; Error, pin 15. After
a state change only d2 (pin 4) will be active.
We often use those lines to connect button boxes for accurate response time
measurement, while at the same time the output pins of the same printer port
are used to send stimulus codes to acquisition hardware (such as digital EEG
recorders.)

Cause:
Newer XP releases run an autodetect sequence on the parallel port where
every 5 seconds for about a minute or so. It seems that this autodetect may
be triggered when the parallel port is first accessed.

Solution:
The problem can be solved by creating a specific key in the registry
database:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Parport\Parameters\DisableWarmPoll

This registry entry should be of type DWORD and have a value of 1
(0x00000001).

WARNING: Editing the registry improperly can cause damage to the operating
system. Modifications should only be performed by ‘skilled’ users. The
registry entry can be created by running regedit.exe. Note that you should
have administrator privileges to be able to do so.

Another possibility to add the entry is to create a text file with extension
.reg, and run it: Open notepad and enter or copy the following text:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Parport\Parameters]
"DisableWarmPoll"=dword:00000001

Then save this file as 'DisableWarmPoll.reg' on your desktop, close notepad
and double click the file, accept the modifications and reboot. The file is
not required after the modification and can be deleted.


More info: http://www.lvr.com/jansfaq.htm


Paul Groot
Vrije Universiteit van Amsterdam
The Netherlands

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