The function f(x) takes a string input x and returns the same string if it consists only of numeric characters, and converts the string to uppercase if it contains any non-numeric characters.

[CODE]:
```python
def f(x):
    if x.isdigit():
        return x
    else:
        return x.upper()
```

This code checks if the input string `x` consists only of numeric characters using the `isdigit()` method. If it does, it returns the string `x` unchanged. Otherwise, it converts the string `x` to uppercase using the `upper()` method and returns the resulting uppercase string.

Please let me know if you need further assistance.