Programming Isochronous Mode - Siemens SIMATIC S7-1500 Function Manual

Programming isochronous mode

Programming in the isochronous mode interrupt OBs
You program the isochronous section of the program exclusively in the isochronous mode
interrupt OBs Synchronous Cycle (OB 6x) and, for Motion Control applications, in the
OBs MC-PreServo and MC-PostServo.
The isochronous mode interrupt OBs Synchronous Cycle (OB 6x) apply to the isochronous
operation of modules centrally in S7-1500 and in the assigned distributed I/O systems.
Because the isochronous mode interrupts are processed with high priority, only the time-
critical sections of the program should be processed in the isochronous mode interrupt OB.
The isochronous mode interrupt is called with a configured delay time.
Access to isochronous I/O through call of instructions
You access the isochronous I/O via a process image partition. That is, the addresses of the
isochronous modules must be within one process image partition.
The CPU updates the process image partitions of the isochronous I/O in the central and
distributed configurations by calling the instructions SYNC_PI and SYNC_PO .
These instructions are not necessary for Motion Control applications. OB MC-Servo
automatically updates the process image partitions.
Note
Recommendation: To prevent inconsistent data from being returned to OB 6x, do not use the
"DPRD_DAT" and "DPWR_DAT" instructions (direct data access) in the isochronous mode
interrupt OB.
The "SYNC_PI" and "SYNC_PO" instructions update the process image partition only within
the permitted execution window. The execution window stretches from the end of the cyclical
data exchange to the point in time before the end of T_DC at which the outputs can still be
copied in time. The data exchange must be started within this time window. If the execution
window is violated by the processing of the "SYNC_PI" and "SYNC_PO" instructions, the
instructions indicate a corresponding error message.
Program execution models
Depending on your requirements regarding reaction time and execution time of the
isochronous mode interrupt OB, there are two basic models for processing the program:
● IPO model (read Inputs - Processing - write Outputs)
● OIP model (write Outputs - read Inputs - Processing)
You implement the respective model through the different call sequence of SYNC_PI and
SYNC_PO in the user program.
Isochronous mode
Function Manual, 10/2018, A5E43884988-AA
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