There can be many reasons for this, and indeed this is the title of one segment of the ASTi training course, however there are a few common errors that should be checked as a starting point.

An ASTi DACS is networked using Ethernet, and a first check would be to ensure the correct hardware connections and indications are being seen. It is recommended that the DACS is connected to a 10/100Mbs Ethernet hub or switch. The DACS should be connected to the hub/switch using a straight 'patch' cable (NOT a crossover cable - this will NOT work correctly). If the DACS is connected correctly the 'link' light should illuminate on the DACS network card. If not there could be a problem with the hub/switch, cable, or the DACS Ethernet card. Check the possible causes by elimination - substitute a replacement hub/switch, and/or cable.

Next there are certain software settings that must be made correctly to set the system up such that the DACS is visible on your network. First, check the IP address(es) for the network interface cards installed in your DACS. Make sure they're valid IP addresses for your network, and that you have the proper subnet mask. There are extensive diagnostic screens available within the Model Builder software running on the DACS that will assist you in determining the operation of the network interfaces. For details of how to do this refer to the supplied ASTi documentation.

Alternatively if you are in the fortunate position of having a valid and funded annual support contract in place you may call ASTi and obtain the assistance of one of ASTi's knowledgeable support engineers who will be able to walk you through the set-up and diagnostic procedure.