INTERFACES
TRADITIONAL
PC ADAPTER CARD
Form 468-010131
DATA
SHEET
page 5/6
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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 7 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/AT Technical Reference manual has a good section on the operation of 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
