Hello,
Thank you for contacting National Instruments. The BNC 2121 is normally used as an accessory for a PCI/PXI-660x counter timer board. Assuming that you are using one of these boards you will have to write a simple program in LabVIEW or any other programming tool that you are using that generates a pulse every millisecond. You should be able to find examples that will get you started.
Nipun M
Applications Engineer