The CBM experiment at FAIR will use GBTX and Versatile Link based readout systems for several of its subdetectors. The paper describes the GBT based readout concept in CBM, emphasizing the common features among systems. Particular choices and features of the readout are motivated mainly by the requirements in the readout of the silicon tracking system (STS). Common developments like a common CBM readout board are presented. The prototype board provides full GBT functionality for all systems, can be interfaced to various prototype readout chains and be refined for later detector specific versions.

We report on the design of a 128-channel ASIC named STS-XYTER (Silicon Tracking System - X - Y - Time -Energy Read-out) dedicated for signal detection from doublesided silicon microstrip sensors with high capacitance (C _DET ≈ 30 pF). The STS-XYTER contains: 128 charge processing channels, a calibration unit, a biasing circuitry based on built-in band-gap reference source and a full digital back-end, which provides synchronization, control and sparsified fast data readout through four, 250 MHz DDR LVDS links based on the CBMnet protocol. The single readout channel uses two parallel signal processing paths (fast and slow) to handle an average rate of input pulses equals 150 kHz and provide an information about both interaction time and deposited charge with good noise performance and low power consumption (6.2 mW/channel) at the same time. The fast path, which is dedicated for determining the input charge arrival time, is built of: a fast shaper, a discriminator, a pulse stretcher and a time stamp latch. The slow path, which is optimized for a particle energy measurement, consists of a slow shaper, a 5-bit flash ADC and a digital peak detector.