in which case, under no circumstances use a parallel connection.
Note; the adapter does *NOT* appear to provide a 25 pin serial plug only a 25 pin parallel one, which suggest some conversation is happening from the 9 pin serial.
Serial interfaces have to be configured at both ends, e.g.
speed, 6/8 bit, parity, etc.
This is probably what the switches are for.
At the pc end, under dos, the MODE and COM commands were used.
Is there a possiblity the printer also has a parallel interface behind a cover?
If so, use it - it is a much easier connection.
If possible avoid using usb as this is adding a complexity, which might be contributing to your problem.
Good Luck - Keep us posted.