Isochronous Mode - Siemens SIMATIC S7-300 Manual

6.5 Analog input module SM 331; AI 8 x 14 Bit High Speed; synchronous; (6ES7331-7HF0x-0AB0)
6.5.3

Isochronous mode

Properties
Reproducible reaction times (i.e. of the same length) are achieved in a SIMATIC system by
means of a constant DP bus cycle, and synchronization of the single cyclic processes
outlined below:
● Cyclic user program execution. The length of the cycle time may vary due to acyclic
● Independent and variable DP cycle on the PROFIBUS subnet
● Cyclic operation of the backplane bus of the DP slave.
● Cyclic signal conditioning and conversion at the electronic modules of the DP slave.
The constant DP cycle runs in synchronism and at the same length. The CPU run levels
(OB61 to OB64) and isochronous IO are synchronized with this cycle. I/O data are therefore
transferred at defined and constant intervals (isochronous mode.)
Requirements
● The DP master and slave must support isochronous mode. You require
Mode of operation: Isochronous mode
Table 6-16 Conditions of isochronous mode:
Default mode
Filtering and processing time T
transfer buffer
(the value defined for TWE
including an input delay time of
T
DPmin
Diagnostic interrupt
Fast Mode (only possible with 6ES7331-7HF01-0AB0)
Filter and processing time T
transfer buffer (diagnosis not selectable)
including an input delay time of
T
DPmin
Note
You can accelerate the cycle of your DP system by setting "Fast Mode." However, this is at
the expense of diagnostics: Diagnostics functions will be diabled in this operating mode.
S7-300 Automation System Module data
Manual, 08/2006, A5E00105505-04
program branching.
higher.
between reading actual values and writing these to the
WE
applies, irrespective of the enable status of diagnostics)
between reading actual value and writing the results to the
WE
Analog modules
STEP 7
V5.2 or
max. 625 μs
10 µs
3.5 ms
max. 4 x T
DP
max. 625 μs
10 µs
1 ms
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