Siemens SIMATIC FM 353 Manual page 112

Programming the Technological Functions
Function description
The block operates with an AW-DB user data block. The DB number is determined
when calling the FB using a transferred instance DB.
S Start-up
POS_CTRL acknowledges start-up of the module. During this time, the
RETVAL parameter and the "Write/Read job in progress" signals (AW-DB,
DBX68.0 and DBX68.2) are TRUE.
S Control and checkback signals
When POS_CTRL is called, the checkback signals are read from the FM 353
immediately (via direct access). Since the control signals and job requests are
processed after these signals have been read, the checkback signals reflect the
status of the module before the block was called. The control signals are also
written to the FM 353 using direct access.
Depending on the chosen mode, the control signals "Start, "Negative direction"
and "Positive direction" (AW-DB, DBX15.0, 15.2 and 15.3) are reset once
start-up has actually taken place (edge formation of the signals for the FM).
For information on the generation of the checkback signals "Process" (AW-DB,
DBX13.6) and "Position" (AW-DB, DBX13.7), see "Mode control".
S Job requests
Data interchange with the module that goes beyond control and checkback sig-
nals is handled using job requests. Simultaneously pending Write or Read re-
quests, however, can only be executed in succession, whereby one Read and
one Write request are processed in one call.
To issue a request, set the relevant initiation signal in user data block AW-DB
(DBB38 to DBB43). In the case of Write requests, you must also make the ap-
propriate data available.
The request is serviced when POS_CTRL is called. When the FM is used
centralized, a Read request is performed during the call. A Write request re-
quires at least three calls (or OB cycles) due to the acknowledgements required
from the module. The time interval between the calls should exceed the length
of one FM cycle.
When a request has been serviced, the Initiate signal is removed (does not ap-
ply to single functions).
The next job request is not determined or executed until the next block call has
been made.
For each job request, there is a Ready signal (AW-DB, DBX44.0 to 53.7) and
an Error signal (AW-DB, DBX54.0 to 63.7) in addition to the Initiate signal.
You should reset the Ready and Error signals for a job request following evalua-
tion or prior to issuing the request.
S Order in which job requests are serviced/priority
You may submit several job requests simultaneously, even together with Write
requests for single commands and single functions.
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FM 353 Stepper Drive Positioning Module
6ES7 353-1AH01-8BG0