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As 4-20 mA is a wider range than 15-20 mA You can simply attach the signal to standard IO channel and use Scale change (SCAL) Function Block. The exact way of doing this depends on which type of IO modules You are using and if we're talking about input or output signal.
For example if it's S700 AX 722F module and input signal You can simply set channel measunrement range on 4-20 and then scale the signal using SCAL from 15-20 into whatever is Your device's range (in example below it's 0-100).
It's not perfect - for example IO module won't detect underflow of 15-20 mA signal,
so if You want such a function, You would have to add an analog signal monitoring block etc. - it still is the easiest way to do it.