INTERFACES
TRADITIONAL
PC ADAPTER CARDS
Form 473-010131
DATA
SHEET
page 5/6
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Jumper Installation (cont.)
ADDRESS JUMPERS
On the RS-422/485 adapter card, there are seven jumpers labeled A3 through A9. These jumpers are used for selecting the base
address of the adapter card.
The IBM personal computers only use 10 address lines (A0 through A9) for addressing I/O boards. The UART chip on the adapter card
uses the lower three address lines (A0 through A2) directly. This leaves seven address lines (A3 through A9) for decoding the base address
of the adapter card.
The adapter card compares the address lines A3 through A9 on the bus with the address jumpers A3 through A9. When the address
lines match the jumper lines, the adapter card is selected.
When a jumper is present, the jumper value is low (zero). When a jumper is not present, the jumper value is high (one).
With no address jumpers present, the adapter card will be selected from address 3F8 to 3FF hex (addressed as COM1).
You can set the adapter card for a particular base address as follows:
First, write the desired base address as a three-digit, hexadecimal number. The number must be less than 3FF and have a zero
or an eight as the last digit. Also, the number must be chosen so as not to conflict with the base address of any other installed
cards in the computer.
Second, translate the number to binary and discard the first two zeros and the last three zeros.
Finally, install a jumper in each location corresponding to a zero. The most significant bit corresponds to A9, and the least
significant to A3. For example:
Install jumpers at A8, remove A3 through A7, and A9.
Note: If you address the adapter card for an address other than COM1 or COM2, you will need to provide your own interface and initialization routines. The IBM PC or
PC/AT Technical Reference Manual has a good section on the operation f the UART and a listing of the code used in the BIOS to access the communications ports.
Address
Binary
Discard
Ends
2
0010
10
F
1111
1111
8
10
1
