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128 of the microcontroller chip. The multiple connections allow such pins to serve more than one function. See the descriptions below and the Atmel documentation for more information. Standard ZX-24 Series Pins Pin Description 1 Serial Output 2 Serial Input 3 ATN 4 Ground (common with pin 22) 5 Port C, bit 7 and Port B bit 1 6 Port C, bit 6 and Port D bit 2 7 Port C, bit 5 8 Port C, bit 4 9 Port C, bit 3 10 Port C, bit 2 11 Port C, bit 1 and Port D bit 3 12 Port C, bit 0 and Port D bit 6 13 Port A, bit 7 14 Port A, bit 6 15 Port A, bit 5 16 Port A, bit 4 17 Port A, bit 3 18 Port A, bit 2 and Port B bit 2 19 Port A, bit 1 20 Port A, bit 0 21 +V out or +V in (regulated) 22 Reset (in and out) 23 Ground (common with pin 4) 24 +V in (unregulated) Detailed Pin Descriptions Pin 1, Serial Output This is the output pin for serial channel 1 and Debug.Print. The voltage swing is approximately 0V to the supply voltage (+5V if using the on-board regulator) and can supply a maximum of 25mA of current. Note that the voltage swing does not meet the specifications for the RS-232 standard but it will be properly recognized by most modern serial interfaces. This pin should be connected to pin 2 of a DB-9F serial connector. Pin 2, Serial Input This is the input pin for serial channel 1. It will tolerate a voltage swing from –15V to +15V allowing it to be connected to an RS-232 output from a serial device. This pin should be connected to pin 3 of a DB-9F serial connector. Pin 3, ATN This input is used for signaling the ZX to enter the download mode. If you don’t need to download, no connection is required but you may wish to ground this pin to eliminate the possibility of spurious resets caused by electrical noise.