RepRap Project

The RepRap project is an initiative to develop a 3D printer that can print most of its own components. RepRap (short for replicating rapid prototyper) uses a variant of fused deposition modeling, an additive manufacturing technique. 

As an open design, all of the designs produced by the project are released under a free software license, the GNU General Public License.

http://en.wikipedia.org/wiki/RepRap_Project

Resonance in Stepper motors

Stepper motors have a natural resonant frequency as a result of the motor being a spring-mass system. When the step rate equals the motor’s natural frequency, there may be an audible change in noise made by the motor, as well as an increase in vibration. The resonant point will vary with the application and load, but typically occurs somewhere between 70 and 120 steps per second. In severe cases the motor may lose steps at the resonant frequency. Changing the step rate is the simplest means of avoiding many problems related to resonance in a system. Also, half stepping or micro stepping usually reduces resonance problems. When accelerating to speed, the resonance zone should be passed through as quickly as possible.

http://www.haydonkerk.com/Resources/StepperMotorTheory/tabid/192/Default.aspx

Stepper motor basics and control

http://www.youtube.com/watch?v=bngx2dKl5jU

A very good you tube link that explains stepper motor funda in 8 minutes.

Stepper wiring

For any stepper motor to be wired up properly, we'll need to determine which wires are "pairs" or connected to the ends of each coil.

Using an ohm-meter: On six-wire motors you'll find two groups of three wires in which all three wires are electrically connected to each other. These wires are connected to the same coil, one in the center of the coil, two at their ends. Accordingly, two of the possible pairings have a lower resistance, one pair has a higher one. Take the pair with the higher resistance and ignore the third wire.

http://reprap.org/wiki/Stepper_wiring
http://www.youtube.com/watch?v=fTtwWJZlGt4

8051 microcontroller - 13 bit timer (mode 0) a relic from the Intel 8048 microcontroller

The 13 bit timer (ie, timer mode 0) is rarely used these days in 8051 uc based applications. The only reason  there is such a timer mode is to maintain compatibility with its predecessor the Intel 8048  microcontroller (Intel MCS 8048 family).

The 8048 had a divide by 32 prescaler, so the Intel guys set up the mode 0 of 8051 timer with lower five bits of TLx register acting as divide by 32 element which is then cascaded to the 8 bits of THx to give a 13 bit timer.

Microcontroller: Features and Applications  By D. S. Yadav, Arun Kumar Singh 

http://goo.gl/AZ94XG

http://en.wikipedia.org/wiki/Intel_MCS-48

The Computer History Museum

The Computer History Museum offers many online exhibits on a variety of topics related to the history of computing. Some online exhibits like Visible Storage and Mastering the Game complement physical exhibits you can also experience when you visit the Museum in person. Other online exhibits are available only through the Internet and extend the reach of the Museum to virtual visitors around the world.

http://www.computerhistory.org/exhibits/

http://www.computerhistory.org/