Entry type: FAQ, Entry ID: 8751062, Entry date: 03/22/2011
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How can you access consistent data without SFC14/15 as part of the process image?

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Description:
Consistent data access > 4 bytes is now possible without the system functions SFC14/15. This option of being able to consistently access data > 4 bytes using download/transfer commands provides a particularly convenient and high-performance method of access (low runtime load).
The data area of a DP slave or IO device that is to be transferred consistently is transferred to a process image or process image partition. The data in this area is then always consistent. Afterwards you can use download/transfer commands (e.g. L IW 1) to access the process image or process image partition. There is no limitation of the area with respect to addressing.

  • The operating system automatically controls updating of the process image.
  • Updating of the process image partition is done either on the user side using SFCs or automatically on the system side through a link to an OB.
  • When you make a direct access (e.g. L PEW or T PAW) then there is no I/O access error.

Note:
Refer to the technical data to see whether your CPU supports process image partitions.

Overview:
You can use the following CPUs for configuring without "SFC14/15":
 

MLFB

S7 CPU 

Minimum firmware version

6ES7 31..

CPU 31x

2.5

6ES7 41.. CPU 41x 3.0
6ES7671-0RC03-0YA0 WinAC1 V4.0
1Only possible when using Hardnet CPs (CP 5613 / CP 5603 / CP 5623) and not Softnet CPs (CP 5611 / CP 5621).

Limits:
Transfer of consistent user data to a DP slave:

Upper limits for the transfer of consistent data to a DP slave are defined by the PROFIBUS DP standard. This is why only a maximum of 64 words = 128 bytes of user data can be transferred consistently in one block to a DP standard slave.

When configuring, you define the size of the consistent area. For this, in the special identification format (SKF) you can set a maximum length of 64 words = 128 bytes for the consistent data (128 bytes for inputs and 128 bytes for outputs); anything beyond that is not possible.

This upper limit applies only for pure user data. Diagnostics and parameter data is grouped into whole data records and is therefore always transferred consistently.

For this, in the general identification format (AKF) you can set a maximum length of 16 words = 32 bytes for the consistent data (32 bytes for inputs and 32 bytes for outputs); anything beyond that is not possible.

Please also note here that a CPU 41x as DP slave must be able to be configured on a third-party master (connection via GSD) via the general identification format (AKF). For this reason the transfer memory per virtual slot of a CPU 41x as DP slave to the PROFIBUS DP has a maximum size of 16 words = 32 bytes. You can configure up to 32 such virtual slots in the i-slave. The highest slot number is 35.

Transfer of consistent user data to an IO device:
The upper limit for the transfer of consistent data to an IO device is 255 bytes (254 bytes user data + 1 byte associated value). Even if more than 255 bytes can be transferred to an IO device, the upper limit is 255 bytes of consistent user data.

Important:
Do not use the system functions and the process image simultaneously. The consistency between process image values and the values of system function SFC14 is not guaranteed, because the process image is not tracked when reading with system function SFC14. In principle the process image is tracked when writing with system function SFC15, but not when reading. This means that the consistency between process image values and the values of system function SFC14 is not guaranteed.

Example:
The following example (for the process image partition 3 "TPA 3" of an S7-400 CPU) shows one possible configuration in the HW Config:

Requirements: The image process partition has already been updated via the SFC 26/27 or the process image updating has been linked to an OB.

  • TPA 3 under Output: These 50 bytes are consistent in the process image partition 3 and can thus be read with normal "load input xy" commands.
  • "---" under Input means: no storage in a process image. Only handling with system functions SFC14/15 is possible.

Keywords:
Hardware configuration, Consistency, Updating