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The design innovations in the IBM 3803 Tape Control Unit and the IBM 3420 Magnetic Tape Unit are discussed. The new design concepts include a full readback parity check, a system of microprogram control that obtains high operating rates through overlap of channel control and motion control functions, a new phase-error detection system, a modified dual-density recording feature and a new interface between the control unit and tape drive. The combination of novel error detection methods, control features and programming support (including an expanded scheme of on-line diagnostics) contributes to greatly improved total system performance over previous models of tape subsystems.
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