Observation Mode; Word - Allen-Bradley SLC 500 Manual

Observation Mode

When you first start a project using Remote I/O, all racks are in OBSERVE mode, which means the
target display can listen to all discrete I/O transfers on the Remote I/O link.
An Allen-Bradley rack corresponds to 128 input bits and 128 output bits of defined PLC data table
memory. In the PLC memory, a 'data table' sets aside 128 bits for input and 128 bits for output. This
data table allows the PLC to communicate with other devices. No matter what I/O scheme you are
using, 1, 1/2, or 1/4 slot addressing, the limit per rack is 128 bits.
When you select 2-slot addressing, each pair of slots is assigned to the corresponding pair of words in
the input and output image tables. You assign one I/O rack number to eight I/O groups. The following
drawing shows a rack of I/O set up using 2-slot addressing.
In a ladder diagram, the input or output instruction address is associated with a particular I/O terminal
and identified by a 6-digit address.
Input (I) or Output (O)
Rack No. (0-32)
Module Group No. (0-7)
Terminal No. (00-07, 10-17)
O:010/12
112
Bit

Word

Address
Address
The discrete I/O address must be translated into tag variable addresses. For example, the discrete
address 0:010/12 becomes the tag variable address O:10/12. You can use the tag variable address to
operate pilot lights. See the tag table in the next section.
You do not have to make any more selections if you just want to listen to the Remote I/O link. If you
want to emulate an unused rack or transfer data using Block Transfers, then you need to change from
observation mode to emulate mode.
54 • • • • Allen-Bradley
010
11
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