Printer Electronics

Consistent with the overall system integration philosophy, all of the electronics required, except for the high-current drivers, were integrated into one NMOS integrated circuit—the printer controller. This IC provides internal buffering of up to 32 characters and a 5X7 dot matrix character generator. The information stored in the internal character ROM can be accessed by the system software and thus the character lookup table in the system can be eliminated. Both the character set and the intensity settings are mask programmable.

The printhead consists of eight thin-film resistors arranged in a vertical column. The entire printhead is moved across the paper so that when the correct resistors are activated at specific times, a character is formed.

An eight-position binary switch is connected to the prinler controller chip so the user can select any one ofthe eight preprogrammed print iotensities. with binary zero being the lightest- The chip controls the darkness of the printout by varying the pulse duration of the signals applied to the printhead resistors. To obtain uniform contrast, the controller also adjusts the pulse duration to account for different dot densities. These cover such a wide range that the user can still obtain a printout with acceptable quality under all usual operating conditions.

The controller is capable of printing bidirectional!}1 tor alphanumeric copy and unidirectionally for graphics copy. To maximize the print speed, a look-ahead algorithm is buill into the chip. The controller decides which direction to print by looking al the present line length and the next line length. A block diagram of the printer conlroller IC is given in Fig. 2.

The printhead assembly is moved across the paper by the head drive motor. This motion uses a 7.5 ]4tt steps] step motor that drives the printhead assembly with a toothed bell. The resolution ofthe motor is doubled by driving it in a half-step mode [3.75°,'step), Each step corresponds to one dol column of printout. The motor is driven at 441! sleps pet-second. which is equivalent lo a print speed of sixty-four characters per second. Paper feed is accomplished by use of an additional step motor, it operates in a full-step mode (7.5°,'step) and steps al one-third the rate of the head drive molor. Each step corresponds to one dot row of paper advanced. When the controller finishes printing a line, it automatically advances I he paper, For alphanumeric copy. the paper advances ten dot rows, spacing three dots between lines. For graphics copy, it advances only eight dot rows; thus if is possible to print a continuous vertical line for graphics plots.

There is no home switch designed into the printer. At the first PRINT command after system power-on. the system firmware will send a home command to the printer controller. Knowing that this is the first home command received since power-on, the controller drives the printhead assembly all the way to the left. The assembly moves toward and eventually contacts the left wall of the frame. After the motor has been given pulses, the controller then reverses the direction of the motor and moves the head assembly four steps to the right. Then it turns off the power applied lo the molor and resets all internal position keeping logic. In this manner, a home position is established.

The energy stored in the motor inductance is returned to the unregulated 32Vdc supply through the catch diode [See Fig. 3], This not only increases the power efficiency ol the system, but also lets us drive the step motor at a higher rate. The field built up in the motor can be collapsed faster by returning the current to a high-voltage supply.

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