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2024-07-13 - 12:58

HOWTO: The OSADL Parport Monitor

It sometimes is required to directly signal a computer state at an output port without using a driver, e.g. in a crashed system that is no longer able to handle interrupts but still has some kind of life in it. The parallel port is ideally suitable for this purpose. A simple DB25 plug with LEDs to monitor its output bits is described here.

Ordering information

If you don't have the time to build your own device using the below manufacturing data, order one that is assembled and ready for immediate use. It is available at Amazon, and we always have a certain number of them in stock. Click here to place your order.

A simple schematic

The eight output pins of the parallel port are connected to LEDs via 330 Ohm resistors; and four input pins are pulled to logical 1 with 3.3 kOhm resistors, but can temporarily be set to logical 0 using a push button. This is intended to signal particular input to the kernel. The schematic at the left and a printed circuit board were created using the Open Source KiCad EDA Software Suite.

Below is a link to the schematics in PDF format; in addition, a screen hardcopy and some intermediate images of the KiCAD application are shown. Finally, all Excellion and Gerber files are provided that may be sent to a professional PCB manufacturer. Alternatively, ready-to-use assembled Parallel Port Monitors can be obtained from OSADL.

Related kernel patch

Many systems of the OSADL QA Farm are equipped with this device; here is a kernel patch that exemplifies how the parallel port may be used to monitor NMI activity. This feature may help to determine whether there still are some embers in the ashes of a crashed system. Another functionality allows to output the number of the most recent system call, hardware IRQ or software IRQ vector as additional hint in a post-mortem diagnosis of a system crash.

Download the schematics of the OSADL Parport Monitor


Schematics of the OSADL Parport Monitor (PDF format)

20 K

From schematics to PCB with KiCAD

Screen hardcopy of the KiCAD PCB layout window
The PCB layout
The OSADL Parport Monitor in KiCAD's 3D viewer

3D image of PCB only


3D image of fully populated board


KiCAD board file


This file can be loaded directly into KiCAD

28 K


Some additional components used in the parport project

119 K

Excellon and Gerber files to submit to a PCB manufacturer


Drill commands

1.5 K


Front circuit

3.7 K


Back circuit

3.7 K


Front mask

2.0 K


Back mask

1.7 K


Silk Front

71 K


PCB edges


Related kernel patch of setparport module


Setparport module

18.8 K