Quatech DS-100 user manual Addressing Ports, Setting the address

5. Addressing Ports

5.1 Setting the address

The base address of each port of the DS-100 is set using two DIP switch packs. When setting the address selection switches, a switch in the "ON" position specifies that the corresponding address line must be a logic 0 for the port to be selected. Similarly, a switch in the "OFF" position forces the corresponding address line to be a logic 1 for the port to be selected.

A full sixteen bit address decode is implemented to reduce the chance of address conflicts with other adapters in the system. Each serial port on the DS-100 uses 8 consecutive I/O locations. The two ports reside in their own independent block of I/O space in eight byte increments. For example, if the base address for port 0 were set to 3F8, the registers associated with port 0 would occupy I/O spaces from 3F8 to 3FF.

Switches SW1 and SW2 select address lines A15 through A3 for Serial 0 and switches SW3 and SW4 select address lines A15 through A3 for Serial 1. The remaining address lines (A2, A1 and A0) are used by the UART to select the register being accessed. The sixth position on SW2 is used to enable or disable Serial 0 and the sixth position on SW4 is used to enable or disable Serial 1.

Figure 5 shows how the switches on the DS-100 represent the address values for serial ports. This figure can be used to explain the examples shown in Figure 6.

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 5220 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	0	1	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+0+2+0				0+0+0+0
Hex digits			5				2				2				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 0 1 0 0 0 0 0

Figure 5 --- Examination of a serial port base address

4-1 Quatech Inc.