Siemens SIMATIC S7-1500 Function Manual page 174

Programming
7.2 Technology data block
– Effective after restart:
Note
Save changes with "WRIT_DBL"
Changes to immediately effective tags are lost on POWER OFF of the CPU, or restart of the
technology object.
If changes in the technology data block should also be retained after POWER OFF of the
CPU, or restart of the technology object, you must write the changes to the starting value in
load memory using the extended instruction "WRIT_DBL".
Note
Using the "READ_DBL" and "WRIT_DBL" data block functions
The "READ_DBL" and "WRIT_DBL" data block functions can only be used on a single tag in
conjunction with the tags of the technology object. The "READ_DBL" and "WRIT_DBL" data
block functions cannot be applied to data structures of the technology object.
Evaluation of data in the isochronous interrupt OB
If you want to write the technology data block's data isochronously from a Motion Control
application cycle, there is the option of evaluating these data in the isochronous interrupt OB.
The MC-Servo [OB91] is called synchronously with a bus system.
Assign the same bus cycle (Page 78) to the isochronous interrupt OB as to the MC_Servo
[OB91]. By this means, the isochronous interrupt OB is called in each Motion Control
application cycle.
In the isochronous interrupt OB, read the necessary data into global tags. Use these global
tags for further programming.
174
DINT/BOOL data types
(e.g. <TO>.Senso[n].ActiveHoming.Direction)
Changes are only permitted in the defined value range. Value changes outside the
value range are not applied. If you enter invalid values, the programming error OB
(OB 121) is started. You write changes using direct assignments. The changes are
retained until the next POWER OFF of the CPU or restart of the technology object.
(e.g. <TO>.Homing.AutoReversal)
Since restart-relevant tags have dependencies on other tags, value changes cannot
be applied at any arbitrary time. The changes are only used after reinitialization
(restart) of the technology object. During a restart the technology object is reinitialized
with the data in load memory. You therefore write changes to the starting value in load
memory, using the extended instruction "WRIT_DBL" (write to data block in load
memory).
You trigger the restart in your user program using the Motion Control instruction
"MC_Reset" with parameter "Restart" = TRUE. Additional information on a restart can
be found in the Restarting Technology Objects (Page 197) section.
S7-1500 Motion Control V13 Update 3
Function Manual, 07/2014, A5E03879256-AC