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Entry type: FAQ, Entry ID: 24137105, Entry date: 11/06/2006

How can you avoid diagnostics problems when implementing dummy module DM 370?

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Configuration Notes:
When implementing dummy module DM 370, you must distinguish between different reactions with regard to the Online View in HW Config and the module status information that is read out of the system status list (with SSL ID W#16#0D91) with system function SFC 51 "RDSYSST".

The behavior depends on

  • Whether a CPU type with or without Micro Memory Card (MMC) is implemented.

  • Whether the DM 370 has been configured in HW Config.
  • Whether any hardware configuration has been loaded into the CPU at all.
  • Whether the switch on the back of DM 370 is set to A(Address) or NA (No Address).

Note the two different cases when configuring:

  • If you want to use DM 370 as a dummy for a signal module (with reservation of address space), it must be configured in HW Config and the address switch must be set to "A".

  • If you want to use DM 370 to replace an interface module (without reservation of address space) or to simply keep the place free for retrofitting of a wider module, then it must not be configured in HW Config and the address switch must be set to "NA".

If you do not follow these instructions, the signal modules slotted to the right of the DM 370 are diagnosed as defective (I/O access error).

The following table gives an overview of possible practical cases.

Configuration Reaction


HW Config Address switch DM 370 is shown in HW Config ONLINE SSL (W#16#0D91) provides data record for DM 370
1 With MMC loaded;
DM 370 not configured
NA no no
2 With MMC loaded;
DM 370 configured
A yes no*)
3 With MMC not loaded NA no no
4 With MMC not loaded A yes no*)
5 Without MMC loaded;
DM 370 not configured
NA no no
6 Without MMC loaded;
DM 370 configured
A yes yes
7 Without MMC not loaded NA no no
8 Without MMC not loaded A yes yes
*) Modules without user data

The following screenshots show the possible reactions from the table above through the HW Config ONLINE view and the SSL (cases 5 and 6). Here the SM 370 was physically directly to the right of the CPU.

Fig. 01

Fig. 02

Fig. 03

Fig. 04

Modules without user data
In the case of CPUs with MMC only the modules configured with user data are entered in the SSL. But the DM 370 has no user data. So SFC 51 provides no SSL data record for the DM 370. Where applicable take the missing data record into account in the user program so that there is no offset in the data evaluation. Each data record receives a slot number. So each data set can be identified separately. If when using a CPU with MMC your user program doesn't permit you to do without the SSL data record of the slot where the DM 370 is located, then please proceed as follows to remedy the situation.

No. Procedure
1 In the HW Config you configure a signal module with user data instead of the dummy module, preferably the future module that is currently being replaced by the DM 370.
2 In HW Config, you check "Startup if target configuration is not identical to actual configuration" in the "Startup" tab of the CPU object properties.
3 Set the address switch on the back of the DM 370 to A (= address).

HW Config ONLINE and SSL W#16#0D91 then look like this, for example:

Fig. 05

Fig. 06

More information is available in the following manuals:

  • "S7-300 Automation System Module Data", Entry ID 8859629, section 7.3 Dummy Module DM 370.

  • "System Software for S7-300/400 System and Standard Functions", Entry ID 1214574, section 13.3 Reading a System Status List or Partial List with SFC 51 "RDSYSST".

Dummy, Dummy module

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