The chassis based Metasys system came in multiple configurations. The most common were 5-slot chassis and 3-slot chassis. The 5-slot system was known as a Network Control Unit because it would support a NCM card as well as modular I/O cards. The 3-slot chassis supported only modular IO cards and therefore was called a Network Expansion Unit, or NEU.
When JCI migrated to the stand-alone NCM many times they simply removed the NCM card from the NCU chassis and turned it into the equivalent of a NEU as a migration strategy. When they did this the logic driving the modular IO cards had to be re-implemented in the NCM.
The modular IO cards, including the DCM, in either a NEU or a converted NCU are accessible to the N2 bus and can be integrated utilizing the S4 Open appliances (OPC-N2 Router and BACnet-N2 Router). However, the logic driving those IO points still resides in the NCM. So, the new head end either has to co-exist with the legacy NCM or the logic in the NCM needs to be moved to the new head end if the NCM is removed. Either approach is technically viable.
In every case that we have encountered so far our integration partners have determined that it is more work to try to salvage the modular IO cards than it is worth and that a better approach is to replace these devices with current technology.