Siemens S7 appendix System Memory, Links to Other Systems, Messages in the Diagnostic Buffer

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:

SData blocks (DB)

(When accessing DBs, remember that the highest DB number is 255.)

SBit memory (M, MB, MW, MD)

SProcess image input table (I, IB, IW, ID)

SProcess image output table (Q, QB, QW, QD)

SI/O area inputs (PIW, PID, PIB)

SI/O area outputs (PQW, PQD, PQB)

SCounters (C)

STimers (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