HP 13255 Technical Information

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1j2~5Cartridge Tape MOdule 13255-91032/26Rev AUG-Ol-7b3.1.3.2 Refer to the truth table in section 3.1.1.2 for a summary of the fol-lowing. When RE is high (read) and USO is low (DATA is a don't care),Q4 and Q5 are tied together and go low. This drives U17, Pin 12 highturning switches 1 and 2 on and Head 1 is connected to the preamp.~hen RE and usa are high (DATA is a don't care), 06 and Q7 are tiedtogether and go low. This drives Ul1, Pin 6 high turning switches 3and 4 on and Head 0 is connected to the preamp.3.1.4 HEAD AMPLJFI~R CIRCUIT - READ/WRITE PCA.3.1.4.1 The read amplIfier circuit amplifies the input signal from either head,differentiates the signal so that the peaKS of the flux transitions be-come zero crossing, and then detects these zero crossings to produce aTTL signal with transitions at eaCh zero crossings (flux transitionspeak).3.1.4.2 the preamp (U5) is a differential input/differential output op-amp. Itsgain is determined by R39 and R40 plus the MOS switCh on resistance inone leg, and by R38 and R41 plus the MOS switCh on resistance in theother leg. The differential gain is 40dB. R38 and C36 on one side,and R39 and C39 on the other side, roll off the response starting at30 kHZ. C40 resonates the head at approximately 50 kHz, effectivelycompensating for some loss in the head output.3.1.4.3 Differentiator (U4) is also a differential input/differential outputop-amp. Its differentiating characteristics are determined by C21,R46, C16, and R34 in one leg, and by R41, C22, C19, and R35 in theother leg. Its gain at the two frequencies of interest (4 kHZ and 8KHz) is -2 d~ and +4 dB, respectively. C21 with R46, and C22 with R47stop its gain rise at +20 CB at 80 KHZ; and R34 with C16, R34 with C16,R34 with C16, and R35 with C19 start its roll oft at 160 kHZ. The pre-amp outputs are connected to this ditferentiator Which translates eachamplitude peak of the preamp outputs to zero crossings.3.1.4.4 The differentiator outputs are ac coupled at Pins 8 and 9 to comparatorUl. The two inputs to U1 are lifted otf ground and reference to +5Vthrough RtOtF and Rl0tG. This is necessary because this particularcomparator must not have its inp~ts allowed to go below -0.3 Vdc. Thiscomparator detects the zero crossings of the difterentiator outputsand changes state each time, effectively squaring the ditferentiatoroutput waveform. A small amount of hysteresis is used to assure that