Siemens SIMATIC FM 353 Manual page 94

Programming the Technological Functions
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.
As soon as a Write request is detected (also on a signal change in the case of
single functions), it is serviced immediately upon completion of the transfer cur-
rently in progress, if any. Be sure that signals for single commands are not set
cyclically, as this could prevent other job requests from being serviced (priority).
Order/priority of Write requests:
1. Write single commands
2. Write single functions
3. Write requests.
The Write requests are serviced in the order of the Initiate signals,
which is stipulated in user data block AW-DB (from DBX38.0 to 39.7).
Order/priority of Read requests:
1. Read error code, operator/traverse errors or data errors
2. Read requests
The Read requests are serviced in the order of the Initiate signals,
which is stipulated in user data block AW-DB (from DBX42.0 to 43.6).
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FM 353 Stepper Drive Positioning Module
6ES7 353-1AH01-8BG0