If Auto Line Feed is enabled, a line feed is inserted after each carriage return by enabling pin 14 on the parallel port to go active high. If Auto Line Feed is turned off and you are still having this problem, then there is probably an issue with pin 14. The threshold of pin 14 is such that the printer recognizes it as an active high and adds the line feed.
Solution Try a different printer cable.
If that doesn't work, then you will have to prevent connectivity of pin 14. This can be done by removing pin 14 from the parallel cable or putting finger nail polish on pin 14 to prevent connectivity. Update the NetportExpress XL print server firmware and RBL code to the latest firmware release. These files contain the latest version of NetPort Manager. The readme file contains update instructions.