Chapter 1
Understanding I/O Scanner Concepts
1-4
Here is an example layout of the output image table showing the first
8 racks:
Group
word #
(hex)
rack
0
1
2
3
4
5
6
7
00-07
0
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
08-0F
1
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
10-17
2
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
18-1F
3
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
20-27
4
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
28-2F
5
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
30-37
6
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
38-3F
7
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
xxxx
The word numbers above can be used as subscripts. (We’ll look closely at
that in chapter 6, Discrete I/O.) Each 16-bit word corresponds to 16
discrete I/O terminal positions, terminal 17 octal (15 decimal) to the
high-order bit and terminal 00 to the low-order bit.
Just as each I/O group has an address, so does each adapter. Adapter
addresses are used in the scan list (see page
The adapter address is the address of the first I/O group covered by the
adapter, divided by 2. This is numerically the same as (rack x 4) + (starting
group / 2), where the rack and group are both numbered from 0 through 7
as shown above. If you prefer, you can think of 1/4 racks being numbered
from 0 through 3, and then the adapter address is (rack x 4) + (quarter).
A slot is a position in an I/O chassis for one I/O module. In 1-slot
addressing, an I/O group represents a single slot. In 2-slot addressing,
an I/O group represents two slots.
Logical Rack / Starting Quarter and Link Addresses
Adapter link addresses are composed of a rack number concatenated with a
starting quarter for a given rack. The two low-order bits of the address are
the starting quarter of the rack, and the next 5 bits are the rack address.
The high bit is always zero when using the KTx.
Example: rack 7, starting quarter 3 has a link address of 00011111binary,
or 1F hexadecimal. In viewing the binary representation, the address
breaks down as follows: 0 – 00111 – 11. The high order bit is 0, the rack
address is 00111, and the starting quarter is 11.