  SRON (Netherlands Institute for Space Research) is developing the Focal Plane
Assembly (FPA) for Athena X-IFU, whose Demonstration Model (DM) will use for
the first time a time domain multiplexing (TDM)-based readout system for the
on-board transition-edge sensors (TES). We report on the characterization
activities on a TDM setup provided by NASA Goddard Space Flight Center (GSFC)
and National Institute for Standards and Technology (NIST) and tested in SRON
cryogenic test facilities. The goal of these activities is to study the impact
of the longer harness, closer to X-IFU specs, in a different EMI environment
and switching from a single-ended to a differential readout scheme. In this
contribution we describe the advancement in the debugging of the system in the
SRON cryostat, which led to the demonstration of the nominal spectral
performance of 2.8 eV at 5.9~keV with 16-row multiplexing, as well as an
outlook for the future endeavours for the TDM readout integration on X-IFU's
FPA-DM at SRON.
