Patent GB-894209-A (GB)
Title: Improved method and system for magnetic recording
Inventor: O'BRIEN HUGH ALLAN
IPC: G11B20/18
Filed: 1959-03-06
Abstract: 894,209. Electrical digital data-storage apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. March 6, 1959, No. 7862/59. Class 106 (1). In magnetic recording, as described of digital data on to tape, the signals are reproduced and checked substantially concurrently with the recording, an indication is given at the end of a group of signals that an error has occurred therein, the tape is reversed until the beginning of the group is detected then restarted in the forward direction for a predetermined time while the signals are erased after which the group is re-recorded. The groups of signals are of variable length and should an error arise due to a defect in the tape towards the end of a long group two or more reversals and signal erasures may be necessary until the signal group is shifted along the tape past the defect (Figs. 7 to 15, not shown). The method of recording is non-return-to-zero and a character is recorded and read parallel by bit by the two gap head described in Specification 850,322. Recording and error detection.-In the drawings, all triggers are initially assumed to be off with the left-hand output positive. A write pulse from a sequence controller is applied to trigger 62 switching it on, the resultant signal on line W switching read/write trigger 120 and conditioning one input of AND unit 70, while the negative signal on line W deconditions AND unit 110. The positive signal from the read/ write trigger 120 conditions AND unit 114 and is applied to the write control circuits 140. Each write head comprises a dual triode inverter Iw having the cathodes commonly connected to the anode of a single triode inverter Ig while the anodes are connected each to an end of the write windings. The grid Ig is connected to the line from read/write trigger 120 rendering Ig conductive when there is a positive signal on the line. The grids of Iw are each connected to a side of a trigger Tw which thus determines the direction of flow of current in the write windings. A 1 bit is recorded by reversing the flow of current in the windings, i.e. by switching Tw, a 0 bit by maintaining the direction of the current constant. Trigger Tw is switched under the control of a diode gate DG one input to which is from a clock pulse generator 112 through AND unit 114 and the other from character register 54. If a 1 bit is in the register 54a pulse from generator 112 is gated to switch Tw. With the error trigger 77 off and single shot multivibrator 136 inoperative the positive signal from write trigger 62 is gated through AND unit 70 to the coil contacts of the tape motor and through AND unit 98 to a write delay multivibrator 100 which blocks the signal to start clock trigger 102 until the motor has reached its operating speed. Additionally, the right-hand output of BKWD trigger 104 maintains the magnets in the forward/reverse circuits 106 conditioned for forward movement. As soon as the first character recorded passes over the read portion of the magnetic heads it is sensed by a read amplifier 142 and transferred to a read register 144. The first bit stored in read register 144 causes a signal to be applied to the clock pulse generator 112 to produce a character gate output signal which is applied to deconditioned AND unit 130 and to the read register which it clears to a data receiver, parallel by bit, which during a write operation ignores it. The delay caused by the first bit signal in transferring information to the data receiver is to compensate for tape skew since under the worst conditions of skew the register effectively receives bit signals from the read amplifier in series. The vertical check unit 148 makes an even parity check of the 1 bits stored in the read register and if an error is detected actuates error trigger 77 which deconditions AND unit 98 and emits an error signal. Writing continues until a specially coded character indicates end of record to a detector 56 which switches write trigger 62 and start clock trigger 102 off. The positive signal from trigger 62 conditions AND unit 110 to gate a clock pulse to the stop clock line, while the negative signal deconditions AND unit 70 as a result of which the tape motor stops. Error correction.-As a result of the error signal at the end of record the sequence controller pulses BKSP switching BKSP/CTRL trigger 90 on, conditioning AND unit 92, and BKWD trigger 104. The negative signal from trigger 104 deconditions AND unit 110 and activates multivibrator 116 blocking the line while the reverse solenoid in the forward/ reverse cicuits 106 is operated by the positive signal from the trigger. Additionally, trigger 104 starts the clock pulse generator. When multivibrator 116 stops DEL BKSP CALL trigger 118 is switched on switching-off trigger 120 and conditioning AND units 122 and 130. Trigger 120 starts the tape motor by switching contacts 128 and deconditions AND unit 114. The first character sensed by the read heads, i.e. the last character written causes a first bit signal to issue and consequently a character gate signal from the generator 112. This is gated to BOR multivibrator 134 which operates for 500 microseconds. When the multivibrator switches off it applies a negative signal to BKWD to FWD DEL and BKWD RESET DEL multivibrators 136, 138 and switches DEL BKSP trigger 118. Multivibrator 136, deconditions AND unit 70, while multivibrator 138 permits the tape to come to a complete stop as a result of the deconditioning of AND unit 122 by trigger 118 and a consequent switching of contacts 128. On stopping multivibrator 138 switches off trigger 104 conditioning AND unit 110 to stop the clock pulse generator and energizing a forward solenoid preparatory to the next action. Multivibrator 136 stops and a write pulse is signalled which drives the tape forward with saturating current in the write heads from the W-status signal somewhat as described above. However, since the trigger 90 is still on and erase forward multivibrator 96 is activated by the W signal and operates for 40 milliseconds, this activates a reset multivibrator 88 which after 10 milliseconds switches triggers 77 and 90. At the conclusion of the action of multivibrator 96 the circuit is conditioned for normal writing which then proceeds. Specification 892,433 also is referred to.