I got it working, well, what I have is a parallel interface GNT 3601=20
puncher rather than a serial one. Thanks to chris for the document.
I wouldn't be suprised if that interface is very similar to the one on a
Facit 4070, particularly as it's a DB25 connector. This parallel
interfave seems to have become a de facto stanard for paprr tape punches.
I don't know what the input stages are (I have even seen discrete
transistors used in some punches), but if they're TTL ICs, the one
connected to pin 3 will not have liked the -12V or whatever from a serial