Consistent Data
7.2 Consistency for communication blocks and functions
S7-400 Automation System, CPU Specifications
7-2 Manual, 10/2006, 6ES7498-8AA04-8BA0
7.2 Consistency for communication blocks and functions
Overview
Using S7-400, the communication jobs are not processed at the scan cycle checkpoint;
instead, in fixed time slices during the program cycle.
In the system the byte, word and double word data formats can always be processed
consistently, in other words, the transfer or processing of 1 byte, 1 word (= 2 bytes) or 1
double word (= 4 bytes) cannot be interrupted.
If communication blocks (such as SFB^12 "BSEND") are called in the user program, which
are only used in pairs (such as SFB 12 "BSEND" and SFB 13 "BRCV") and which share
access to data, the access to this data area can be coordinated between themselves, for
example, using the "DONE" parameter. Data consistency of the communication areas
transmitted locally with a communication block can thus be ensured in the user program.
S7 communication functions such as SFB 14 "GET", SFB 15 "PUT" react differently because
no block is needed in the user program of the destination device. In this case the size of data
consistency has to be taken into account beforehand during the programming phase.
Access to the Work Memory of the CPU
The communication functions of the operating system access the work memory of the CPU
in fixed field lengths. The field size is a variable length up to a maximum of 462 bytes.
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