As you might already know, the Apple Serial NB cards, as shipped, are
configured with ports 1A and 1B as high-speed V.35/RS-422 types of ports,
and ports 2A and 2B as low-speed RS-232-C ports. If you plan to use port
1A or 1B also as RS-232-C ports, you have to remove two small components,
called R-paks, from the card. Refer to the Apple Serial NB Card
Installation Guide for the location of these R-paks. If you use the V.35
modems, you can use port 1A. Otherwise you need to select ports 2A or 2B
in the SDLC Line Screen. See if the R-paks were removed from your card,
and/or configure the ports correctly, and try again.