Rockwell Automation T80015 Application Note Trusted SC300E Migration Process - Manual (Page 7)

Trusted TM
AN-T80015
Issue 01 Apr 08 AN-T80015 Page 7 of 16
2.4. Application logic
It was decided to use ladder logic to mirror the existing system as much as possible so as to
minimise the change for the users, although there are some things to bear in mind.
Each I/O module is mapped to blocks of discretes (Booleans) in groups of eight, which are
not necessarily in order. Discretes above 8000 are also mapped to registers: to find the
relevant register, subtract 8000 from the discrete address and divide by 16 (or vice versa,
multiply by 16 and add 8000 to find the LSB discrete for a particular register).
Some logic uses registers instead of individual discretes, especially where data is gathered
from a whole module or diagnostic function block.
2.4.1. Ladder Logic Replication
Trusted TM ladder logic, using the FBD/LD editor, can be made to look very similar to SC300E
ladder logic. There are minor exceptions to the appearance.
Trusted TM requires a left-hand power rail to be drawn to ‘energise’ each branch of ladder
logic, and several power rails may be drawn for one network. Elements may be placed
anywhere. SC300E ladder logic is fitted on a grid with the left hand edge acting as a power
rail.
Vertical lines in either system act as OR gates, however in Trusted TM if an OR gate is wired
to a power rail, the two will be connected at the top. In SC300E, horizontal connections may
be wired anywhere, so the visual appearance may be different.
In SC300E, ladder elements are executed strictly in order through the grid. The left column is
executed from top to bottom, and the results passed to the next column, again executed
downwards. In Trusted
TM , execution order is based on the inputs of each element. A tree of
execution hierarchy is built so that each element is only executed when all its inputs have
been resolved by other executions. An option is available in the editor to ‘Show Execution
Order’, which highlights the calculated order.
In Trusted TM , ladder and function block diagram may be mixed, but follow different visual
rules in terms of shape, which will influence the layout (especially for tags in FBD, which are
very long horizontally). In SC300E, functions may be placed on the grid as part of the ladder.
2.4.2. Logic operation
There are two major differences between Trusted TM and SC300E logic operation.
In SC300E, ladder logic may include calls to synchronise I/O within the network, so that later
ladder elements may use fresh inputs, possibly based on recent outputs. This allows
sequential programming to be built using the ladder execution order; complete sequences
that have several interactions with I/O may be built in one network.
In Trusted TM , inputs are only read before the application scan and outputs are only written
after the application scan, so that in order to create a sequential program, the application
must suspend the logic until the next scan to deliver outputs and receive fresh inputs. This
means that the sequential ladder networks in SC300E must be replicated in Trusted
TM using
state machine logic, where the logic performs a different operation each scan depending on a
remembered state setting.
An ideal language for state machine logic is Sequential Function Chart, but this is not
recommended for safety systems for two main reasons: processor hot swaps are not
possible with SFC programs, and SFC programs may be (rarely) liable to enter indeterminate
states, either hanging or performing two steps simultaneously. For the latter reason, SFCs
are not permitted by TUV for SIL3 approved systems.