Siemens SINUMERIK 840Di Function Manual page 1207

2.4.4 Machine program
The machine manufacturer creates the machine program using the library routines supplied
with the basic program. The machine program contains the machine's logic links and
machine sequences. The program also handles the interface signals to the NCK. More
complex communication functions with the NCK (e.g., read/write NC data, tool-management
acknowledgments, etc., are activated and executed via basic-program FCs and FBs).
The machine program can be created in various STEP7 creation languages (e.g. STL, LAD,
CSF, FBD, S7 HIGRAPH, S7GRAPH, SCL). The complete machine program must be
generated and compiled in the correct sequence. This means that blocks that are called by
other blocks must generally be compiled before the blocks which call them. If blocks that are
called by other blocks are subsequently modified in the interface (VAR_INPUT,
VAR_OUTPUT, VAR_IN_OUT, VAR) as the program is developed, then the call block and all
blocks associated with it must be compiled again. This procedure is valid in the same way for
instance data blocks for FBs. If you do not follow this sequence, time-stamp conflicts will
occur during recompilation in STEP7. In some cases, therefore, it may not be possible to
recompile blocks, creating problems, for example, with the "Block status" function. It is,
moreover, advisable to generate blocks in ASCIISTL by means of the STEP7 editor when
they have been created in Ladder Diagram or in single statements (incremental mode).
2.4.5 Data backup
The PLC CPU does not store symbolic names, only the data-type descriptions of the block
parameters (VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR) and the data types of the
global data blocks. Without the associated project for this machine, therefore, blocks cannot
be recompiled meaningfully (e.g., for the Block status function or in the event of
modifications subsequently required to PLC CPU programs). It is, therefore, necessary to
keep a backup copy of the STEP7 project located in the PLC CPU on the machine. This is
extremely useful for servicing purposes and saves time and problems. If the STEP7 project
exists and has been created according to the instructions given above, then symbols can be
processed in the PLCCPU on this machine. It may also be advisable to store the machine
source programs as STL files in case they are required for any future upgrade.
The source programs of all organization blocks and all instance data blocks should always
be available.
PLC Basic Program Solution Line (P3 sl)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
Detailed Description
2.4 Starting up the PLC program
2-13