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147 Recommended ZX-1281 Series Serial Interface Circuit The pullup resistor shown on the ZX’s Com1 transmit output (pin 28) 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. Alternate Serial Interface Circuit #1 The first alternate above is similar to the circuitry used on the ZX-24 series devices. The advantage to this circuit is its simplicity. The disadvantage is that the voltage levels of the serial output signal do not