147 of 158
141 Serial Interface The ZX-44 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 9 of the ZX. 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. Recommended ZX-44 Series Serial Interface Circuit The pullup resistor shown on the ZX’s transmit output (pin 10) is needed to ensure that the serial output line stays in the idle state during reset cycles. Its value is non-critical – anything from 4.7K to 22K should work fine. The pullup resistor on the reset input isn’t technically required since the chip itself has an internal pullup. The external pullup could be eliminated or could be made larger if desired. There are other serial interface circuits that would provide the required functionality, two of which are shown below. Both alternate circuits use the same ATN circuitry as the recommended circuit above.