Data Consistency - Siemens SIMATIC S7 System Manual

8.5.1.4

Data consistency

PROFIBUS supports three types of data consistency:
● Byte: Ensures that bytes are transferred as whole units
● Word: Ensures that word transfers cannot be interrupted by other processes in the CPU.
● Buffer: Ensures that the entire buffer of data is transferred as a single unit, uninterrupted
by any other process in the CPU.
The EM DP01 always utilizes buffer consistency in its data handling.
S7-200 SMART CPU and EM DP01 data buffer consistency
The EM DP01 and S7-200 SMART CPU offer buffer consistency for the entire transfer:
● The EM DP01 receives the outputs from the DP master in one message.
● The EM DP01 transfers all outputs to the S7-200 SMART CPU in one message that
cannot be interrupted.
● The S7-200 SMART CPU transfers all outputs to the V memory area at one time. The
transfer cannot be interrupted by a user interrupt.
The same consistency is true for the inputs to the DP master:
● The S7-200 SMART CPU transfers all inputs from the V memory at one time. The
transfer cannot be interrupted by a user interrupt.
● The S7-200 SMART CPU transfers all the inputs to the EM DP01 in one message. This
transfer cannot be interrupted.
● The EM DP01 sends the inputs to the DP master in one message.
DP master consistency
Consistency in the DP master CPU is not always buffer consistent. The DP master CPUs do
not handle the entire DP message as one indivisible object unless it is very small. The
DP master CPUs usually move the PROFIBUS data in smaller pieces. They can either move
the data to the I/O area or the user can control the movement with DPRD_DAT (Read
consistent data for DP devices) and DPWR_DAT (Write consistent data for DP devices)
instructions. Using the DPRD_DAT and DPWR_DAT instructions, you obtain the information
for one configuration "slot" at a time. Since we allow two configuration slots, it can take two
DPRD_DAT instructions to obtain all of the data. Consistency is only guaranteed for each
DPRD_DAT instruction.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Communication
8.5 PROFIBUS
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