Rockwell Automation T80015 Application Note Trusted SC300E Migration Process - Manual (Page 8)

Trusted TM
AN-T80015
Issue 01 Apr 08 AN-T80015 Page 8 of 16
An acceptable alternative is to use Structured Text, which allows complex program flow using
IF, CASE, REPEAT and other flow control statements. The CASE statement allows the
creation of a state machine
In SC300E, only one network is run at a time, so the programmer may concentrate on the
operation of that network without worrying about its influence on other networks. In
Trusted
TM , the need to use a state machine to allow I/O to be synchronised between steps
means that all other networks will also run throughout the operation of a state machine.
Therefore, the effect of the state machine’s actions on other networks must always be
considered. For example:-
Event logs included many points which were exercised during the logic scan but provided no
useful information. These internal points were removed from SOE unless required, in which
case efforts were made to mask unwanted transitions
2.4.3. Execution Speed
SC300E logic scans can take several hundred milliseconds to operate, probably due to the
inline I/O calls and system calls. However, Trusted
TM operates the SC300E I/O scanning
asynchronously from the application scan, since the I/O scanning is operated by the 8161
bridge modules and reported to the processor as requested. The Trusted
TM application scan
was measured at the minimum 32 milliseconds, which is due to the ‘sleep period’ set in the
system INI configuration. The actual scan overhead in the example system consisted of the
following elements:
I/O: 10-13ms Transactions with bridge modules, communications
modules, standby processor
Input: 3-4ms Writing input data to input boards in the application, running
conversion tables, updating variables
Application: 3-4ms Actual application processing time, rarely more than 10% of
total
Communications: 0-1ms INI changes, active/standby signals
Output: 3-4ms Transfer of output data from application to bridge modules
and communications modules.
It can be seen that the processor is only using about two thirds of the 32ms scan; the rest is
free for diagnostics and background functions.
The faster application scan time in Trusted TM and the asynchronous I/O handling means that
Trusted TM programs must account for the delays expected in the SC300E.
2.4.4. Printed messages, reports and events
SC300E is able to print reports and events, controlled by the firmware. Trusted TM has no
printer output. However, it has two event logs. The processor holds an event log with 200
entries, which automatically records all system faults and actions without any intervention
from the application. This reports all module faults and swaps as well as program downloads
and Reset pushbutton presses. There is also a Sequence of Events log (SOE). The SOE log
holds records changes to Boolean (digital) variables, complete with timestamps. These
timestamps are to the nearest millisecond but due to the disconnection between the
Trusted
TM application and the real I/O in a hybrid system, are not absolutely accurate. Native
Trusted TM I/O modules can record SOE timestamps on arrival to the system. There is also an
analogue event logger called Process Historian if required, but usually analogues are trended
on the DCS or other graphics station.
In order to make an entry in the SOE log, a Boolean variable is required, which must be set
up as an output and wired to an SOE board in the I/O connection table. It should have an
appropriate tagname (max 32 characters) and description (max 80 characters), which will
appear in the SOE log. It is possible to name the states True and False as appropriate, e.g.