Configuring a Board
3-7
board for an address well beyond the 100h to 3FFh range; Keithley
recommends *300h, *700h, *B00h, or *F00h (where * is any number in
the range of 0 to Fh) as valid base I/O addresses. You can also assign your
board an address that is 400h, 800h, or C00h above the upper limit of
3FFh recommended by IBM.
Caution:
Writing to a base I/O address above 400h can sometimes draw
response from a device using an address below 400h. For example, an
address of 01F0h is used by many computers for a fixed disk. If the disk
drive controller board sees only the 10 least significant address lines (A0
to A9), then writing to 1F0h, 5F0h, 9F0h, or DF0h could inadvertently
activate the disk drive controller board.
The base-address switches are preset at the factory for a default base I/O
address of 300h (the address reserved by many computers for a prototype
board). If this address is already assigned to another device in your
computer, reset the switches for a different I/O address. Use the
information shown in Figure 3-2 to determine settings for a new address.
Figure 3-2. Base-Address Switch Values
O
N
1
2
3
4
5
6
7
8
O
N
1
2
3
A15
A14
A13
A12
A11
A10
A9
A8
A7
A6
A5
32
64
128
256
512
1024
2048
4096
8192
16384
32768
20
40
80
100
200
400
800
1000
2000
4000
8000
Computer
Address Line
Decimal Value
Hexadecimal Value
Switch settings indicate a base address of
512 + 256 = 768 Decimal
200h + 100h = 300h
or
Switch 1
Switch 2
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