Patent GB-1488473-A (GB)
Title: Content-addressable memory arrangements
IPC: G11C15/04
Filed: 1976-02-23
Abstract: 1488473 Transistor circuits INTERNATIONAL BUSINESS MACHINES CORP 23 Feb 1976 [27 March 1975] 06971/76 Heading H3T A content-addressable memory arrangement including a bi-stable T0, T1 whose first and second input/output nodes 23 are connected by circuit devices D11, D12 to first and second search lines D, E has a pair of current switch means one T3, T4 having one input connected to the first node and the second input connected to the first search line D and the second switch T5, T6 having its inputs connected to the second node 23 and the second search line E. The current switches each have an output terminal connected to a common match line F. In use a search bit signal is applied to one of the search lines and a signal is provided on the match line indicating whether the search signal matches the data bit stored in the bi-stable circuit. Standby.-A "1" bit is stored by T1 conducting (or T0 for 0), the driver T13 is held off at its base so the line A is nearly at the potential of terminal 26, - 1À5 v. The two transistors T7, T8 are held off by the base and so the bit lines D, E (which are also used as search lines) are at - 1À5 v. also and the diodes D11, D12 do not conduct. The voltage on line G is about - 1À1 v., the drop in diode D1 subtracting from the V BE  of T1, while H is at - 0-5 v., the same potential as the base of T1. The current switch T3, T4 has a more positive input at G than at D and so T3 conducts, similarly T5 conducts resulting in no current from the match line F which therefore is at zero volts. Search.-The driver T13 is still held off and the potential at B is raised to earth to search for a data bit 0, turning T8 on and bit line D rises to - 0À8 v. If a 1 is stored then line G is at - 1À1, line H at - 0À5 so that the current switch T3, T4 has a more positive input at T4 which conducts drawing current through RM. The potential on match line F falls indicating a non-match condition. On the other hand if a 1 was being searched for T7 would be turned on at C, line E rises to - 0-8 v., but line H is still at - 0À5 and the current switch T5, T6 has a more positive input at T5 which conducts. Similarly T3 conducts in the other switch, there is no current in RM and the absence of charge in the potential of match line F indicates a match condition. During the search condition the diodes D11, D12 remain reverse biased thereby isolating the storage cell bi-stable T0, T1 from the charging potentials on the bit lines. Read.-The driver T13 is turned on and this pulls line A to about - 3À1 volts and this is passed through the conductive transistor of the bi-stable to lower the nodes 23. Transistors T7, T8 are held off and the bit lines D, E lie at - 1À5 volts, consequently one diode, D11 if T1 is on, conducts and the potential of one bit line falls. The differential voltage between the bit lines is read out by a balanced amplifier (not shown). Write.-Again the driver T13 is tuned on and the base of T7 (T8) is raised to store a 1(0). The bit line potential rises also making the diode D11 conduct to raise the potential of the collector of the transistor T1 already on. This causes the other transistor T0 to conduct and regenerative action causes the cell to change state. The load elements L1, L2 of the bi-stable may be resistors or PNP transistors (Fig. 2, not shown). The diodes may be Schottky barrier diodes. A diode D8 reduces the parasitic capacitance of the match line. The diode connected transistor T16 prevents the match line F from discharging to too low a potential when there are multiple mismatches along the match line during a search operation.