Time Response; Instruction Run Times; Processing I/O Direct Access; Reaction Time - Siemens SIMATIC S7-400H Manual

System and Operating Modes of the S7-400H
5.5

Time Response

Instruction run times

The run times of the STEP 7 instructions will be found in the instruction list for the
S7-400 CPUs.

Processing I/O direct access

Please note that every I/O access necessitates synchronization of the two
subsystems, thus resulting in a longer scan time.
You should therefore avoid I/O direct accesses in your user program and instead
use access via process images (or process image sections – for example, for
cyclic interrupts). This produces higher performance since a whole set of values
have to be synchronized simultaneously in the case of process images.

Reaction time

Detailed information about calculating the response time is available in the
reference manual Automation System S7-400, CPU Data.
Note that updating the standby CPU extends the interrupt reaction time (see also
Section 6.3.1).
The interrupt reaction time depends on the priority class since a graduated delay of
the interrupts is performed during updating.
5.6 Evaluation of Process Interrupts in the S7-400H System
When you use a module that creates process interrupts in the S7-400H system, it
may be the case that the process values that can be read in the process interrupt
OB by direct access do not correspond to the process values at the time of the
interrupt. Evaluate instead the temporary variables (start information) in the
process interrupt OB.
When you use the SM 321-7BH00 module, which creates process interrupts, it is
therefore not advisable to have different responses to rising and falling edges on
one and the same input, because this would require direct access to the I/O. If you
want to respond differently to the two edge changes in your user program, apply
the signal to two inputs from different channel groups and parameterize one input
for a rising edge and the other input for a falling edge.
5-14
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07