In the manual I can read this:
Modified user data interface 11 Instead of 8 bytes of input data and 8 bytes of output data, 12 bytes of input data and
6 bytes of output data are used, provided the IM 151 supports this.
The following IM 151 modules support this function:
– IM151-1/Standard order no. 6ES7 151-1AA04-0AB0 and higher
– IM151-1/HF order no. 6ES7 151-1BA01-0AB0 and higher
The IM151-8 is not listed.
I would try as test to configure a decentralized periphery with the listed IMs, to see if this explains your constation.
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