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146 Clock Source The ZX-1281 series devices require a clock source running at 14.7456MHz. The easiest way to provide this clock source is to connect a crystal of that frequency to pins 23 and 24 along with the necessary capacitors. The recommended crystal to use is ECS-147.4-20-4 (Digi-Key #X175-ND). An alternate crystal in a slightly larger package is ECS-147.4-20-1 (Digi-Key #X142-ND). The capacitor value to use depends somewhat on the layout (e.g. stray capacitance). Values in the range of 22pF to 27pF have been used successfully in various situations. ZX-1281 Series Clock Source If you already have a clock source at the required frequency you can feed that signal directly to pin 24. In this case there would be no connection to pin 23. Serial Interface The ZX-1281 series devices require a serial interface for downloading code into Program Memory, performing field updates of the control program, and for your program’s use via Com1. The recommended serial interface circuitry is shown below. This circuit has two sub components. The Max232 chip functions as an RS-232 level converter that translates the 0-5 volt signals of the ZX to the standard RS-232 voltage levels. The second component is the ATN circuitry that is used to signal the ZX to go into download mode. The latter circuitry is that portion connected between pin 4 of the DB-9 serial connector and pin 20 of the ZX- 1281. You may want to include a jumper in the ATN circuitry so that it can be disconnected when downloads are not required. With the ATN circuitry connected, the ZX will receive a reset pulse on every positive transition of pin 4 (DTR) of the serial connector. If alternate component values are chosen you must ensure that the ZX receives a reset pulse of at least 2uS on every positive transition of the DTR signal on pin 4 of the serial connector. One advantage to using this recommended circuit is that the MAX232 chip includes a DC-DC converter that produces nominally +/- 12V from the 5 volt source. This is particularly useful if your application requires one or more of these voltages. For example, some LCD devices require a low-current negative supply for their backlight circuitry. The positive voltage is available on pin 2 of the Max232 while the negative voltage is available on pin 6. Consult the datasheet for the MAX232 device for information on the current capacity of these supplies.