Siemens NCM S7 Manual page 108

Configuring Communication Connections
Notice
Remember that with this configuration on an S7-300, one connection resource
(free connection for S7 functions) of the S7-300 CPU is used. CPU connection
resources are also used, for example, by S7-300 CPs, in the FMS mode or by
PGs and OPs. For more detailed information about the maximum connection
resources, refer to /13/ and /7/.
System Memory
You can access the following address areas in the system memory on the
SIMATIC S7 PLC using FETCH or WRITE:
S Data blocks (DB)
(When accessing DBs, remember that the highest DB number is 255.)
S Bit memory (M, MB, MW, MD)
S Process image input table (I, IB, IW, ID)
S Process image output table (Q, QB, QW, QD)
S I/O area inputs (PIW, PID, PIB)
S I/O area outputs (PQW, PQD, PQB)
S Counters (C)
S Timers (T)
Links to Other Systems
The FETCH and WRITE modes supported on ISO transport connections and
ISO-on-TCP or TCP connections can be used with any other device to access the
S7 system memory areas.
To be able to use this type of access, for example for PC applications, you need to
know the PDU structure of the jobs. The required S7 or S5 headers for request
and response frames are 16 bytes long and their structure is described in the
Appendix.
Messages in the Diagnostic Buffer
As a result of FETCH/WRITE access, negative acknowledgments from the
S7-CPU are possible. This then leads to corresponding connection-oriented entries
in the diagnostic buffer that you can read out with NCM Diagnostics (see Section
6.5).
108
SIMATIC NET NCM S7 for Industrial Ethernet
C79000-G8976-C129–06