Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 282

Supplementary information
15.11 Consistent data
Upper limits of the length of consistent user data transmitted to an IO Device
The length of consistent user data that you can transmit to an IO device is limited to
1025 bytes (= 1024 bytes user data + 1 byte secondary value). Irrespective of whether you
can transmit more than 1024 bytes to an IO device, the transmission of consistent data is still
limited to 1024 bytes.
When operating in PN-IO mode, the length of data transmission via CP 443-1 is limited to
240 bytes.
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CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB