The building blocks of the upgraded ATLAS Strip Tracker for HL-LHC are modules that host silicon sensors and front-end ASICs. The modules are mounted on carbon-fibre substructures hosting up to 14 modules per side. An End-of-Substructure (EoS) card on each substructure side connects up to 28 differential data lines at 640 Mbit/s to lpGBT and VTRx+ ASICs that provide data serialisation and 10 Gbit/s optical data transmission to the off-detector systems respectively. Prototype EoS cards have been designed and extensively tested using lpGBT and VTRx+ prototypes. The status of the electronics design and recent test results are presented.