An update:
I'm able to turn ASCII into FP and back again using Pascal. What I do is converting the integer part and the fractal part of a number into ten words each. The result is converted into a FP with an 80-bits mantissa. To get the ASCII value again, the FP is converted into this 20-word array again. The whole is multiplied with 1 e 16 and then I calculate each digit by subtracting 1e40, 1e39, 1e38 etc. To speed up things I use tables, each record has a precision of nine words. OK, maybe a massive overkill but, as said before, it is easier to scale down then to scale up. This method has at least one advantage: it is fast.
Yet I still do have a problem. The tables only enable me to handle values up to roughly 10^20 / 10^-20. This is about 2^44 / 2^-44. This doesn't come even near handling max exponents of 127 / -127 that I had in mind. So at the moment I run into trouble when handling a value like 1.2345 e 38. OK, just when writing this, an idea popped up that maybe could handle that number but at this moment I have no idea to handle 1.2345 e -38. Yes, I could expand the tables but then I will have routines that are more tables than program.
I'm looking forward to ideas. TIA!