  The High Energy Rapid Modular Ensemble of Satellites (HERMES) Technological
and Scientific pathfinder is a space borne mission based on a constellation of
LEO nanosatellites. The payloads of these CubeSats consist of miniaturized
detectors designed for bright high-energy transients such as Gamma-Ray Bursts
(GRBs). This platform aims to impact Gamma Ray Burst (GRB) science and enhance
the detection of Gravitational Wave (GW) electromagnetic counterparts. This
goal will be achieved with a field of view of several steradians, arcmin
precision and state of the art timing accuracy. The localization performance
for the whole constellation is proportional to the number of components and
inversely proportional to the average baseline between them, and therefore is
expected to increase as more. In this paper we describe the Payload Data
Handling Unit (PDHU) for the HERMES-TP and HERMES SP mission. The PDHU is the
main interface between the payload and the satellite bus. The PDHU is also in
charge of the on-board control and monitoring of the scintillating crystal
detectors. We will explain the TM/TC design and the distinct modes of
operation. We also discuss the on-board data processing carried out by the PDHU
and its impact on the output data of the detector.
