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Connection
Features
Digital interface: The input to the circuit is done as usually with the 74hc573, that is latched in using the strobe control output. The output enable of the 74573 is latched continually low. The number of stepper motors handled by the card can be increased by increasing the number of 74573s, and their latches controlled by the extra lpt control pins. Further still you can use the lpt control pins on a multiplexor and control a larger number of 74573. Each 74573 controls 2 stepper motors. Buffer: The 7407s buffers the output to the transistors. They also serve as protection to the parallel port from surges in the motor circuits. A better choice would have been the use of opto-isolators, where even the ground pins are totally seperated. The buffer feeds in to two resistors that pull the current into the transistor to 0.4mA (0.9V high) or 0mA (0V low). This is sufficient to trigger the transistor on or off, firing the repective motor load. Motor circuit: The transistor open circuits or short circuits the inductive motor load to the power supply. The circuit will be able to handle the current rating of the stepper motor according to the type of transistor used and power supply ratings. Greater motor loads would require greater load transistors and higher rated power supplies. Being an inductive load a power diode is connected parallel to the load so that on switch off the back emf current finds a path way back. Power Supply; The circuit uses two independant power supplies. The reason behind this was two fold. First is that the digital circuit and the motor circuits may use two completely different voltages. The second is to isolate as much as possible the interface and the motor circuit, protection to the former. In my application I was using two stepper motors from IBM proprinters rated at 4.1V and 1.1A.
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