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Single Control Latch

IP.com Disclosure Number: IPCOM000076964D
Original Publication Date: 1972-May-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 24K

Publishing Venue

IBM

Related People

Varada Rajan, HD: AUTHOR

Abstract

A normal direct-coupled transistor latch needs two lines to change its state. It is often very desirable to be able to change the state using a single control.

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Single Control Latch

A normal direct-coupled transistor latch needs two lines to change its state. It is often very desirable to be able to change the state using a single control.

Described is an arrangement by which it is possible to change the state of a direct-coupled latch with a single-control line. The line necessarily needs three levels, one maintaining the state and the other two initiating a changeover.

The figure shows the circuit of the latch in which D1 is an additional diode. The quiescent voltage at A is 1.0 volt. This enables the latch to exist in either state, i.e. Q1 either conducting or nonconducting.

To write the 1 state (Q1 conducting, say) it is just necessary to drop point A to 0.5V momentarily. This turns off Q2, thereby raising point B to turn Q1 on. When the point A returns to 1.0V, the latch remains in the desired state.

To write the 0 state (Q1 off) point A is raised to 1.6V, which brings Q1 out of saturation (but not demanding an indefinitely large current) and raises its collector potential to such a value to force Q2 to conduct. The current is limited because of the presence of diode D1, which by mirror action limits the current in Q1 equal to that in R2. Again, when point A returns to 1.0V, the latch remains in the set state.

Thus, with one control line attached to A, it is possible to hold either state and change from one to another by properly altering the potential of the point.

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