Compdigitec Labs

Scanning with Brother DCP-7065DN in Ubuntu

By admin | September 12, 2020

First, make sure the printer is detected in lsusb:

$ lsusb | grep Brother
Bus 001 Device 011: ID 04f9:024a Brother Industries, Ltd 

Then, either add a udev rule to ensure that we have permission to access it, or use the following one-time hack:

sudo chmod 777 /dev/bus/usb/001/011

Now, install the correct driver for the printer from support.brother.com. For the DCP-7065DN on Ubuntu 64-bit, the following package worked:

5937473842d95571b0d8ff50d6095198 brscan4-0.4.9-1.amd64.deb

Check that sane-find-scanner can see it:

$ sane-find-scanner
# sane-find-scanner will now attempt to detect your scanner. If the
# result is different from what you expected, first make sure your
# scanner is powered up and properly connected to your computer.

# No SCSI scanners found. If you expected something different, make sure that
# you have loaded a kernel SCSI driver for your SCSI adapter.

found USB scanner (vendor=0x04f9, product=0x024a) at libusb:001:011
# Your USB scanner was (probably) detected. It may or may not be supported by
# SANE. Try scanimage -L and read the backend's manpage.

# Not checking for parallel port scanners.

# Most Scanners connected to the parallel port or other proprietary ports
# can't be detected by this program.

# You may want to run this program as root to find all devices. Once you
# found the scanner devices, be sure to adjust access permissions as
# necessary.

Finally, check that scanimage can see it. If this succeeds, then the scanner is operational (and can be used from Simple Scan or any other scan software).

$ scanimage -L
device `brother4:bus1;dev4' is a Brother DCP-7065DN USB scanner

Topics: Linux | No Comments »

Simple recursive DNS server with Unbound DNS

By admin | July 30, 2020

This is a simple configuration for running a recursive DNS server (passes DNS requests to another server and caches responses) with the Unbound DNS server.

Installation (Ubuntu):

sudo apt-get install -y unbound

Open the config

sudo vim /etc/unbound/unbound.conf

Configuration

Replace 8.8.8.8 below with the desired upstream DNS server.

# The following line includes additional configuration files from the
# /etc/unbound/unbound.conf.d directory.
#include: "/etc/unbound/unbound.conf.d/*.conf"
# NOTE: needed to comment out the above line avoid a "status: SERVFAIL" response

server:
    # Enable verbose debugging messages
    verbosity: 1000

    # Run on all interfaces
    interface: 0.0.0.0

    # Hide the server name and version
    hide-identity: yes
    hide-version: yes

    # Who should be able to query the server
    access-control: 0.0.0.0/0 allow

    do-ip4: yes
    do-ip6: no

    do-udp: yes
    # Enable this to support TCP DNS which is required in some applications
    do-tcp: yes

    # Allow forwarding to another 127.0.0.0/8 DNS server (e.g. another local dnsmasq or systemd-resolve)
    do-not-query-localhost: no

forward-zone:
    name: "."
    # Replace 8.8.8.8 with your desired upstream DNS server
    # You can have multiple forward-addr lines
    forward-addr: 8.8.8.8@53

Starting the server

sudo systemctl restart unbound

Debugging / Troubleshooting

Query the server

dig @your_server_here example.com

Read the DNS server log

sudo systemctl status unbound -n 50

References:

Topics: Linux | No Comments »

Generating POT files for WordPress plugins

By admin | July 6, 2020

The easiest non-GUI way appears to be using WP CLI.

Run the following in the plugin folder:

wp i18n make-pot . output.pot

Topics: HTTP (Internet) | No Comments »

Make fails silently

By admin | February 28, 2017

Run make <...> -d &> log.txt and examine the log. Likely there is some prerequisite which is missing, in the form of “File `foo’ does not exist.”

Topics: Linux | No Comments »

LaTeX macros ending in numbers

By admin | February 13, 2016

Only letters are directly permitted in macro names, but by using csname to create definitions along with a variable command, we can simulate them.

First, define the macros with numbers using \csname:

\expandafter\def\csname xR1\endcsname{-3}
\expandafter\def\csname xR2\endcsname{9}

Now, create a command which takes in one variable argument, and simply expands it to call the appropriate macro (“xR” + variable) defined above.

\newcommand{\xR}[1]{\expandafter\csname xR#1\endcsname}

And to use the macros:

The first is \xR1 and the second is \xR2.

Topics: Linux | No Comments »

Phabricator LDAP (and other) account integration table

By admin | January 17, 2016

phabricator_user.user_externalaccount

See also https://secure.phabricator.com/T4279

For LDAP, the following structure is used:

Topics: PHP | No Comments »

Possible crash in method_exists() with PHP 5.3

By admin | August 9, 2014

Apparently in some versions of PHP 5.3, calling method_exists() on a class which doesn’t exist seems to cause a crash. The solution, seen here, apparently involves checking class_exists() to prevent a potentially crashing call to method_exists().

This particular bug was causing the administration page of the MathJax-LaTeX WordPress plugin to be blank in the admin interface.

The trick is to replace the two lines in mathjax-latex-admin.php lines 91-92:

$wp_latex_disabled = method_exists('WP_LaTeX','init') ? "disabled='true'" : "";
$wp_latex_disabled_warning = method_exists('WP_LaTeX','init') ? "Disable wp-latex to use this syntax." : "";

with:

$wp_latex_disabled = (class_exists('WP_LaTeX', false) && method_exists('WP_LaTeX','init')) ? "disabled='true'" : "";
$wp_latex_disabled_warning = (class_exists('WP_LaTeX', false) && method_exists('WP_LaTeX','init')) ? "Disable wp-latex to use this syntax." : "";

Topics: PHP | No Comments »

Disable dragging of an element in JSXGraph

By admin | July 20, 2014

Set the attribute ‘fixed‘ to true in JXG.GeometryElement:

var line = board.create('line', [ [0,0], [1,1] ], {dash:0, strokeColor:"#FF0000", strokeWidth:3, fixed: true});

or

var line = board.create('line', [ [0,0], [1,1] ], {dash:0, strokeColor:"#FF0000", strokeWidth:3});
line.setAttribute({ fixed: true });
board.update();

Topics: (X)HTML, Windows | No Comments »

Reverse engineering the Fischertechnik blinker

By admin | July 13, 2014

The Fischertechnik blinker comes in a enclosed case with no other sensory inputs with two wires, a yellow wire and a blue wire.

The Fischertechnik blinker

The Fischertechnik blinker

As such, I reverse engineered the Fischertechnik blinker to understand how it works through desoldering, a camera, and a lot of drawing on the GIMP.

It turns out that the blinker is intended to be used like a switch in a series circuit before a load, with the yellow lead being the positive end and the blue lead being the negative end.

Series circuit with FT blinker on mySTEM board

Series circuit with FT blinker on mySTEM board

The main idea here is that the board uses a 555 timer (555C EZ606 with STMicroelectronics logo, surface-mounted) with fixed frequency (fixed R1, R2 and C) in astable mode to control a BSP 75N MOSFET transistor.

The power supply for the ICs is controlled in parallel with a diode and a 470Ω resistor to protect the ICs, while the MOSFET controls the connection to the other lead.

It features an unknown-model fuse (VW UG4 GP613) on the emitter of the transistor, presumably to protect the circuit from excess.

A diode and 100 µF capacitor cap are present, possibly to smooth out the power input for the ICs during temporary dislocations. Considering that it is a Fischertechnik component, its purpose is probably to ensure continued reliability when it is handled roughly, shaken, and dropped by students.

After complete disassembly:

Fischertechnik blinker, disassembled

Fischertechnik blinker, disassembled

With traces and PCB features:

Front view, with traces and PCB features

Front view, with traces and PCB features

Back view, with traces and PCB features

Back view, with traces and PCB features

Schematic:

Fischertechnik blinker schematic

Fischertechnik blinker schematic

Since C is a generic-looking ceramic capacitor with no markings, its capacitance was estimated by using the period.

The light was measured to blink 100 times in 41.21 seconds, giving:
$$
T = 0.4121 \text{s} \\
f = 2.426 \text{s}^{-1}
$$

Given R1 = 1 MΩ and R2 = 1.2 MΩ from above and f = 2.426, we can calculate C using the following equation:
$$
f = \dfrac{1}{\ln 2 * C * (R_1 + 2*R_2)}
$$

C then works out to about 0.175 µF.

Topics: Other | No Comments »

Reverse proxy with HTML rewriting using Apache

By admin | May 25, 2014

<VirtualHost *:80>
 ServerName foo.example.com

 ... other directives ...

 ProxyHTMLEnable On (comment in some cases)
 SetOutputFilter proxy-html
 SetOutputFilter inflate;proxy-html;deflate  (for compression issues - e.g. The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression)

 ProxyHTMLURLMap http://192.168.12.34/myfolders/ /
 ProxyHTMLURLMap /myfolders /
</VirtualHost>

Topics: Linux | No Comments »

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