Hi Dieter,
My machine was built in 1987 and is mainly grey in colour.
If you look on YouTube you can find one very similar to mine with a FDD instead of a Tape.
The FDD drive interface is connected to the main CPU board in the same way as the tape drive is except that the 5 volt line is not used.
I understand that the 5 volts are provided by the interface from the 16 volt rail.
The FDD is the type you find in older PC's and is made by TEAC (FD-235HF) Part No 19307344-29.
The interface board is marked 2665 WA85094.
Ther is an Emco EPROM on the interface which is marked P2831155 2.32
The interface board has two connections to the FDD one is a short standard FDD cable and the other is the 5 Volt supply.
Following advice found on various forums I have tried both 720 and 1.44 floppy disks.
I did find some information on one site that said that the disks are only readable by the lathe and cannot be read by another FDD in a PC.
I have tried all the Instructions for the tape and I only get alarms. A10, A5 and A9.
A10 is the common one (write protect) It comes up if you have the disk write protected or not.
My current thinking is that the interface board is faulty.
There is no 5 Volt supply at any of the chips or the FDD.
I have tried everywhere I can think of to find a schematic but no luck up to now.
The rest of the lathe is working OK now so I will keep trying to find further information.

Keith