Siemens SINUMERIK 840Di Function Manual page 1215

NCK/PLC interface
As illustrated in the figure, NCK/PLC data exchange is organized by the basic program in the
PLC. At the beginning of the cycle (OB1), the status information (e.g. "Program running")
written to the internal DPR by the NCK is copied to data blocks by the basic program, which
the user can then access (user interface). The control signals for the NCK (e.g. NC start)
entered in the interface data blocks by the user are also written to the internal DPR and
transmitted to the NCK at the start of the cycle.
Workpiece-program-specific auxiliary functions transferred to the PLC are first evaluated by
the basic program (interrupt-driven) and then transferred to the user interface at the start of
OB 1. If the relevant NC block contains auxiliary functions that require the interruption of the
NCK machining process (e.g. M 06 for tool change), the basic program halts the decoding of
the NCK block initially for one PLC cycle. The user can then use the "read disable" interface
signal to halt the block execution until the tool change has been completed. If, on the other
hand, the relevant NC block does not contain auxiliary functions requiring the interruption of
the decoding process (e.g. M 08 for cooling on), the transfer of these "rapid" auxiliary
functions is enabled directly in OB 40, so that decoding is only marginally affected by the
transfer to the PLC.
The evaluation and enabling of the G functions transferred from the NCK are also alarm-
driven, however they are transferred directly to the user interface. Where a G function is
evaluated at several points in the PLC program, differences in the information of the G
function within one PLC cycle may arise.
In the case of NC actions triggered and assigned with parameters by the PLC (e.g. traverse
concurrent axes), triggering and parameter assignment is performed using FCs and FBs, not
interface data blocks. The FCs and FBs belonging to the actions are supplied together with
the basic program. The FCs and FBs required must be loaded by the user and called in the
PLC program of the machine manufacturer (machine program). For an overview of FC, FB
and data blocks, sorted according to basic and extended functions, please refer to:
Chapter: Starting Up the PLC Program
Interface HMI/PLC
HMI/PLC data exchange takes place via the software C bus or via the Ethernet, internal CP,
control-internal C bus link. The CP transfers the data intact from one bus segment to
another. The HMI is always the active partner (client) and the PLC is always the passive
partner (server). Data transmitted or requested by the HMI are read from and written to the
HMI/PLC interface area by the PLC operating system (timing: cycle control point). From the
viewpoint of the PLC application, the data are identical to I/O signals.
PLC Basic Program Solution Line (P3 sl)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
Detailed Description
2.5 Linking PLC CPUs to 840D
2-21