GFK-0467K Chapter 2 System Operation 2-21
2
Table 2-4. Discrete References - Continued
Type Description
%T The %T prefix represents temporary references. These references are never checked for
multiple coil use and can, therefore, be used many times in the same program even when coil
use checking is enabled. %T may be used to prevent coil use conflicts while using the cut/paste
and file write/include functions.
Because this memory is intended for temporary use, it is never retained through power loss or
RUN-TO-STOP-TO-RUN transitions and cannot be used with retentive coils.
%S The %S prefix represents system status references. These references are used to access special
PLC data, such as timers, scan information, and fault information. System references include
%S, %SA, %SB, and %SC references.
%S, %SA, %SB, and %SC can be used on any contacts.
%SA, %SB, and %SC can be used on retentive coils –(M)–.
%S can be used as word or bit-string input arguments to functions or function blocks.
%SA, %SB, and %SC can be used as word or bit-string input or output arguments to functions
and function blocks.
%G The %G prefix represents global data references. These references are used to access
data shared among several PLCs. %G references can be used on contacts and retentive coils
because %G memory is always retentive. %G cannot be used on non-retentive coils.
Transitions and Overrides The %I, %Q, %M, and %G user references have associated transition and override bits. %T, %S,
%SA, %SB, and %SC references have transition bits, but not override bits. The CPU uses
transition bits for counters and transitional coils. Note that counters do not use the same kind of
transition bits as coils. Transition bits for counters are stored within the locating reference.
In the Model 331 and higher CPUs, override bits can be set. When override bits are set, the
associated references cannot be changed from the program or the input device; they can only be
changed on command from the programmer. CPU Models 323, 321, 313 311, 211, and the Micro
CPUs do not support overriding discrete references.
Retentiveness of Data Data is said to be retentive if it is saved by the PLC when the PLC is stopped. The Series 90 PLC
preserves program logic, fault tables and diagnostics, overrides and output forces, word data (%R,
%AI, %AQ), bit data (%I, %SC, %G, fault bits and reserved bits), %Q and %M data (unless used
with non-retentive coils), and word data stored in %Q and %M. %T data is not saved. Although,
as stated above, %SC bit data is retentive, the defaults for %S, %SA, and %SB are non-retentive.
%Q and %M references are non-retentive (that is, cleared at power-up when the PLC switches
from STOP to RUN) whenever they are used with non-retentive coils. Non-retentive coils include
coils —( )—, negated coils —(/)—, SET coils —(S)—, and RESET coils —(R)—.