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There have been reported cases when communication with a device (specifically, Simatic S7-1200) was established, but after sending a read request the device didn't send required data but a packet with ResultCode = 0x8104, that is 33028 decimal.
According to http://stackoverflow.com/questions/23745407/libnodave-error-while-reading-from-siemens-s7-1200-0x8104, the problem is insufficient access rights. The cause is a new security option that was added to TIA Portal 12 and higher, which by default disallows remote access to read/update blocks. Without this option disabled, only Siemens tools have access to the data.
Configuration: in TIA, under the properties for the CPU project, select "Protection"; there is an option for "Permit access with PUT/GET communications from the remote partner" and set also "Access level" according to the following screenshot.
Note: in practice, read/write access has been tested with Simatic S7-1500 even with "Access level" = "HMI access", with "Permit access with PUT/GET communications from remote partner" enabled.
In the case of TIA Portal version 14, the setting "Permit access with PUT/GET communications from the remote partner" is on a dedicated tab "Connection mechanisms" under "Protection & Security":
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