The CBM heavy ion experiment at the GSI/FAIR facility will employ GBTX and VersatileLink devices in five subdetectors in a free streaming readout system at interaction rates up to 10^7/s starting from 2025.
Currently prototype and medium scale preseries readout systems are used for precursor experiments in the FAIR-Phase-0 program both at GSI (mCBM) and at BNL (eTOF@STAR).
We present the existing readout boards featuring both single and multi-GBTX architectures, together with the underlying concepts for clocking, frontend control and readout, as well as experiences from operation and further developments towards the full CBM DAQ system.