Patent GB-844846-A (GB)
Title: Improvements in or relating to digital computers
Inventor: WILLIS DONALD WESLEY
IPC: G11C11/06
Filed: 1955-12-20
Abstract: 844,846. Electric digital-data-storage apparatus. DECCA RECORD CO. Ltd. Dec. 19, 1956 [Dec. 20, 1955], No. 36548/55. Class 106 (1). To produce a pulse to be recorded on a magnetic recording medium in a digital computer, a magnetic core having two stable magnetic states is switched from one state to the other at the beginning of the required pulse period, and returned to its original state at the end of the required pulse period, the core being coupled to a recording-head by a circuit having a time constant which is long compared with the duration of the required pulse period. A magnetic core 10 of a material with a substantially rectangular hysteresis loop, Fig. 1, has a drive winding 15 thereon to which is applied a driving pulse sufficient to change the magnetization of the core from a normal to a set state. The resulting flux change induces a current in the output winding 11 which passes to the recording- head 12. In order that there should be little change in the current through the recording- head during the required pulse period, the time constant of the recording head circuit is made long compared with the pulse period duration by ensuring that the total resistance R2 of the recording head circuit shall be sufficiently small compared with the inductance L of the head. At the end of the required pulse period, a reset current is applied to the drive winding of the core to return the core to its normal state, thereby terminating the current through the head 12. Alternatively, a steady bias current may be applied to a bias winding on the core to hold it in its normal state in the absence of any drive current. This arrangement enables selection of a driving core from a plurality of driving cores to be achieved by removing the bias only from the selected core. To provide pulses of both polarities in the recording head two cores may be connected thereto, each core having a separate drive winding to be switched by drive and reset currents at appropriate times. If current pulses of opposite polarities are required to be passed through the head successively, the drive current for switching the core producing the second component of current in the head may be used to reset the other core (Fig. 3, not shown). If each core has similar drive and reset winding, a single transformerdriven push-pull operated winding for both driving and resetting may be employed. Alternatively there may be a single bias circuit linking the two cores in opposite senses. In operation, the bias is removed and application of a drive pulse to a drive winding linking both cores will switch one core only. When the driving current is removed, the residual secondary current begins to switch both cores back, and after the cores have been switched, the restoration of the bias will take the cores back to their original states, switching one core in the process. To obviate short-circuiting of the head output if a head is used for reading as well as writing, a circuit comprising two rectifiers 37, 38, Fig. 5, connected reversely in parallel, is included. These rectifiers will present a low-impedance at high-power levels and a high impedance at low-power levels. The reading output may thus be taken from terminals 39, 40 across the rectifiers. For selecting a particular head from a plurality of heads such as might be associated with a magnetic drum a selection matrix may be provided. The four heads 50-53 are shown in a 2 Î 2 matrix, Fig. 6, two cores 54, 55 being associated with each head and linked thereto by a series circuit including two rectifiers as in Fig. 5. Each core also has biasing and back biasing windings thereon, the back biasing windings 57 for all the cores being connected in series and the biasing windings 58 for all the cores of each column being series-connected to current-supplying biasing valves 59. The current drawn by any selected biasing valve passes also through all the back-biasing windings. By making the number of turns in each winding equal, no effective bias is applied to the cores of the selected column only. A column is selected by applying positive to a write terminal 60 and to a selected one of the column selection terminals 61 and negative to the unwanted column terminals 61 so that the selected valve only conducts. Each core also has two drive windings such as 70, 71 thereon, the drive windings of all the cores in a row being driven in a push-pull circuit from an output transformer 72, the first drive windings and the second drive windings of the cores in each row being connected through a load impedance 77. These drive windings can be used both for switching and resetting. A row is selected for writing by applying a positive voltage to a write terminal 80 and to a selected one of the row terminals 81 and a negative voltage to the unwanted terminals. When the selected valve 82 conducts, the associated push-pull valves 74, 75 will supply the necessary drive current to the cores of the selected row in accordance with an input waveform applied via input terminals 83 to the control grids of the push-pull valves 74, 75. Reading from the matrix is effected by applying positive to the grids of two reading valves 85, 86 which conduct, thereby cutting off the writing circuits. The signals read by each head are developed across the associated rectifier pair and the required signal may be selected by a separate reading-head selection matrix as described in Specification 844,852. When the reading valve conducts, it draws a current through the back-biasing circuit, thereby ensuring that all the cores are returned to the biasing condition.