ZBasic Language Reference
192
ZBasic Microcontrollers
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.
Alternate Serial Interface Circuit #1
The first alternate above is similar to the circuitry used on the ZX-24 series. The advantage to this circuit
is its simplicity. The disadvantage is that the voltage levels of the serial output signal do not meet the RS-
232 standard. Even so, most serial receivers are able to properly interpret the output signal. Note that if
the inverters used have protection diodes on their inputs the external diodes shown are not necessary.
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