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03.96
7.1 PLC start-up
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Siemens AG 2000 All Rights Reserved
SINUMERIK 840D Installation and Start-Up Guide (IAD) – 04.00 Edition
Note
By default, the STEP7 project manager (S7 TOP) does not display the SDBs.
The SDB display is activated in the View / Set filter menu “All modules with
SDBs”.
The PLC status is displayed under the “Diagnosis” menu to permit control and
monitoring of PLC inputs, outputs, flags etc.
The PLC always powers up in RESTART mode, i.e. the PLC operating system
runs through OB100 after initialization and then commences cyclic operation at
the beginning of OB1. It does not return to the point of interruption (e.g. on a
power failure).
Bit memories, timers and counters are stored in modal and non-modal memory
areas. Both area types are contiguous, but are separated by a parameterizable
limit, the area with the higher-order address being designated as the non-reten-
tive area. Data blocks are always retentive.
If the retentive area is not buffered (backup battery empty), then start-up is
blocked. The following operations are performed during a restart:
S Delete IStack, BStack and non-retentive flags, timers and counters
S Delete process image of outputs (PIO)
S Reject process and diagnostic alarms
S Update system status list
S Evaluate parameterization objects of modules (from SD100 onwards) or
output defaults parameters to all modules in single-processor mode
S Process restart OB (OB100)
S Read in process image of inputs (PII)
S Cancel command output disable (OD)
In chronological terms, the basic program is executed before the PLC user pro-
gram. In cyclic operation, the NC/PLC interface is fully processed. The current
G functions are transferred to the PLC (provided function is activated) on the
process alarm level.
A cyclic monitoring function is activated between the PLC and NCK once pow-
er-up and the first OB1 cycle have been completed. When the PLC fails, alarm
“2000 sign of life monitoring PLC” is displayed.
References: /FB/, P3, “Basic PLC Program”
/S7H/, SIMATIC S7–300
PLC status
Start-up behavior
of the PLC
RESTART
Cyclic operation
Sign-of-life
monitoring
7 PLC Start-Up
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