e signals seen on the right-hand side of Fig. 2.1 are for external world interfacing. ese are called
input/output (I/O) ports, or simply ports. ere are a total of four such ports designated 0, 1, 2 and
3. Each of these four ports has eight I/O lines, which are bit-programmable. Being bit-programmable
means that individually each port bit may be programmed as either input or output, irrespective of the
condition of any other bits of that port.
Along the remaining three sides of XX51 of Fig. 2.1, all system input signals, necessary for running
the device, are shown. ese are power input [common-collector voltage ...
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