Quatech ES-100 user manual Examination of a serial port base address

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, two, and one, hence the maximum value of 8+4+2+1 = 15.

A possible serial port address is 5240 hex. The example below shows how the hex digits are broken down into binary bits.

Binary bits	0	1	0	1	0	0	1	0	0	1	0	0	0	0	0	0
Bit weight	8	4	2	1	8	4	2	1	8	4	2	1	8	4	2	1
Sum of bits	0+4+0+1				0+0+2+0				0+4+0+0				0+0+0+0
Hex digits		5					2			4					0

These address bits are set by the switches.

All other bits are considered to be zero.

0 1 0 1 0 0 1 0 0 1 0 0 0 0 0 0

Figure 4 --- Examination of a serial port base address

	Switch on	Switch off
	bit = 0	bit = 1
Factory default setting ---		0300 hex

A5-A0 are zero for the base address. Position 5 of SW2 is not used.

Position 6 of SW2 is used to enable or disable the interrupt status register.

SW1

	1	2	3	4	5	6
0		0	0	0	0	0
			0
Another Example ---				5AC0 hex

SW1

1	2	3	4	5	6
0	4	0	1	8	0
		5

SW2

		ON
	1		2	3	4		5		6
2			1	0	0			(no digits)
3					0				0
	SW2
		ON
	1		2	3	4		5		6
2			0	8	4			(no digits)
A						C			0

Figure 5 --- Serial port base I/O address selection switches

Quatech ES-100D User's Manual 5