In the PLC program of the S7-400H, you need to output the data you want to record with the active slave always to the two slaves belonging together. Hereafter, we give an example of how data can be written on the slaves on bus 0 and 1 in the S7-400H in the exemplary PROFIBUS configuration in chapter Operation as active slave in redundancy mode.

For this purpose, you generate in your PLC a data block (DB) for each active slave. In this block, you cyclically write the data you want to record. You can customize the structure of the DB. The length of the DB has to equal the length of the output data range of the active slave (i.e. again a max. of 244 Bytes).

In our example, the interface DB (DB22) looks as follows:

The data is output by calling up SFC15 (DPWR_DAT) several times. When calling up the SFC15 twice in network 3 (figure below), all 244 bytes of the interface DBs DB22 for slave 22 – bus 0 are written. In network 4, with two more SFC15 call ups the same data is put out on slave 22 – bus 1.

Several SFC15 calls are needed as the transferred address range must not overlap the address range of the single modules of the slave in the hardware configuration. The maximum length of a module is 128 Bytes.

For more information about to use of SFC15, please see the SIMATIC Step7 manuals.

Note
	Data should be exclusively given out to the active slave by SFC15 (DPWR_DAT). Data output by load/transfer commands leads to a much higher cyclic load in the CPU.

Caution
	When using active slaves on ibaBM-DP, you have to take measures in the S7-400H CPU program for preventing an eventual downtime of an active slave (e. g. over OB85) for there will be no negative consequences for the rest of the program.