Patent GB-793200-A (GB)
Title: Electrical adding device
IPC: G06J1/00
Filed: 1955-09-02
Abstract: 793,200. Digital electric calculating-apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. Sept. 2, 1955 [Sept. 7, 1954], No. 25270/55. Class 106 (1). [Also in Group XXXVIII] A digital electric adding device comprises two shift registers, an analogue adder which produces analogues of the outputs of the registers and an analogue of the sum, and a converter which produces the digital equivalent of the sum and has its output connected to the input of one of the shift registers. In the arrangement described, a number is entered, in series mode as regards the decimal denominations but on four parallel channels in accordance with a binary code, into an input shift register A, Fig. 1. The contents of this register are fed synchronously with the contents of an accumulating register C into the analogue adder B, the output of which is fed into the accumulating register C. The contents of the latter may be read out serially. The registers A and C each comprise four shift registers, operating in parallel and of a known type comprising triggers interconnected by delay-producing single-shot multivibrators. Adder, Figs. 2b and 2d. During each pulse period the output leads of the two registers are each at either of two predetermined potentials according to the corresponding binary bit. The four leads from the register A are respectively connected to four resistances 48-51 which are inversely proportional to the values of the corresponding binary bits, i.e. 1, 2, 4 and 8. Similarly the leads from the register C are connected to four resistances 60-63. All of these resistances are connected to a common lead 52, the potential of which thus varies linearly with the sum of the two decimal digits represented. This potential is compared, by means of a differential amplifier 67, detailed in Fig. 4 (not shown), with that on a lead 71 connected, over resistances 72-76, inversely proportional to the values 1, 2, 4, 8 and 10 respectively, and cathode followers to triggers 136-140. These triggers are normally ON so as to apply a low potential to the resistances 72-76 but they are switched OFF in turn by sequential pulses 10P, 8P, 4P, 2P and 1P; Fig. 12, so as to increase the potential of the lead 71 in accordance with the values to which they correspond. When the potential of the lead 71 becomes greater than that of the lead 52, the potential of a lead 102 connected to the differential amplifier 67 rises and gates 111-115 are opened. The gate 114, e.g., receives the pulse 4P and, when open, produces an output, in response to that pulse, which restores the preceding trigger-the 8 trigger 139-to the ON condition. The other gates are similarly connected, the gate 111, controlling the 1 trigger 136, receiving pulses SP delayed relative to the 1P pulses. Thus when the generated value exceeds the sum value, it is reduced by the switching ON again of the last trigger to be switched OFF. In this way, the potential of the lead 71 is matched with that of the lead 52 and after the pulse SP, those of the 1, 2, 4 and 8 triggers which are OFF represent the binary digits of the sum and the trigger 140, if OFF, represents a decimal carry. The trigger 140 when OFF opens a gate 189 to pass a subsequent pulse CP over a lead 214 to switch OFF a trigger 58 which by means of a resistance 53, increases the potential of the sum lead 52 by one unit. The trigger 58 is switched ON again after the next comparison by the next pulse SP. Each of the triggers 136-139, if OFF; opens a corresponding gate 184-187 and also a corresponding gate 190-193. The gates 184-187 pass a read out pulse, derived from the pulse CP and a control signal on a lead 223, on to the appropriate output leads 200-203, whereas the gates 190-193 pass a pulse, derived from the pulse CP and a control signal on a lead 224, on to appropriate leads 21a-24a connected to the input of the accumulating,register C. Error indicator. When the potentials of the leads 52 and 71 are not matched, one or the other of the leads 93 and 102, connected thereto is at a high potential and a gate 228 is open. If this gate is open at the end of a comparison operation, when the potentials of the leads 52 and 71 should be matched, a pulse CP is passed to turn OFF a trigger 229 which is arranged to operate an error indicator or other control apparatus. Specification 770,487 is referred to.