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136 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 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. The second alternate serial interface is shown in the diagram below. This circuit produces a serial output signal that meets the RS-232 standard by using the negative voltage on the receive line to provide a reference for the output level. Although the parts count is higher, the circuit uses inexpensive and non- critical components. Alternate Serial Interface Circuit #2