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two, and one, hence the maximum value of 8+4+2+1 = 15.
A serial port's address is a 16-bit quantity that is most often expressed
in four hexadecimal (base 16) digits. A hex digit can hold a value from 0 to
15 (decimal), and is made up of four binary bits given weights of eight, four,
A possible serial port address is 5220 hex. The example below shows
how the hex digits are broken down into binary bits.
Bit weight
Binary bits
Sum of bits
Hex digits
2
2
0
5
8 4 2 1 8 4 2 1 8 4 2 1 8 4 2 1
0+4+0+1 0+0+2+0 0+0+2+0 0+0+0+0
0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 0
0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 0
These address bits are set by the switches.
All other bits are considered to be zero.
Figure 5 --- Examination of a serial port base address
Switch on
bit = 0
Switch off
bit = 1
Factory default setting --- 0300 hex
ON
0200
0
1
123456
SW2
0000
0
0
3
123456
SW1
0
Position 6 of SW2 is used to enable or
disable the interrupt status register.
0
(no digits)
Another Example --- 54A0 hex
ON
20 0 8 0
A
123456
SW2
0001
5
4
4
123456
SW1
4
0
(no digits)
A4 - A0 are zero for the base address.
ON
ON
Figure 5 --- Serial Port base I/O address selection switches
QS-100D User's Manual 2-3