There is likely insufficient power to the Ports/8em (or Ports/16em) module.
The Xem host adapter can usually supply sufficient power to keep two (2) Ports/Xem modules powered. Modules without power supplies should always be connected in the daisy chain closer to the Digi host adapter than any modules with power supplies. Once any module requiring a power supply is added to the daisy chain, each module added after that requires its own power supply.
Last updated: Aug 08, 2017