The implementation of a 25.6-Gb/s four-level pulse-amplitude-modulation (4-PAM) reduced-state sliding-block Viterbi detector (VD) is presented. The power consumption of the VD is 105mW at a supply voltage of 0.7 V, corresponding to an energy efficiency of 4.1 pJ/b. A data rate of 30.4 Gb/s is achieved with an energy efficiency of 5.3 pJ/b at a supply voltage of 0.8 V. The VD, implemented in an experimental chip fabricated in 14-nm CMOS FINFET, exploits set-partitioning principles and embedded per-survivor decision feedback to reduce implementation complexity and power consumption. The active area of the VD with 12 slices, each operating at one-eighth of the modulation rate, is 0.507×0.717 mm2. Experimental results showing system performance are obtained by using a (215-1)-bit pseudo-random binary sequence. The impact of the synchronization length and survivor path memory length on the detector design and system performance are shown. A new pipelined reduced-state sequence detector algorithm is presented for high-speed implementations. A novel speculative symbol timing recovery scheme is proposed. New simulation results are obtained to compare the performance of the Reed-Solomon (RS)-encoded 4-PAM scheme with that of the concatenated RS 4-D 5-PAM trellis-coded-modulation (TCM) scheme over an ideal band-limited additive-white-Gaussian-noise channel. Drawing on the results achieved for the VD, novel design techniques for a high-speed low-complexity eight-state 4-D 5-PAM TCM decoder is proposed.