Channel Switching Network

1. Field of the Invention

The present invention relates to a channel switching network of balanced type employing PNPN semiconductor elements.

2. Description of the Prior Art

For the selection of a particular cross-point switch in a switch matrix which is the balanced typed channel switching network formed by PNPN elements, there has been required an AND circuit for each cross-point switch. This inevitably increases the number of parts used, and hence is disadvantageous in terms of the packing space, power consumption and the cost.

An object of the present invention is to provide an economical and small-sized channel switching network in which two PNPN elements at each of cross points forming a switching network of balanced type are equipped with the AND function, thereby permitting to omit the provision of an AND circuit at each cross point in addition to the two PNPN elements.

The principle, construction and operation of this invention will be clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit connection diagram illustrating an example of prior art;

FIG. 2 is a circuit connection diagram showing an embodiment of the present invention;

FIG. 3 is a circuit diagram illustrating an example of a trunk circuit used as a power source in the present invention; and

FIG. 4 is a current diagram explanatory of the operation of the present invention.

For the selection of a particular cross-point switch in a conventional switch matrix, there has been required an AND circuit AND.sub.11, . . . AND.sub.lm, . . . AND.sub.nl, . . . or AND.sub.nm for each cross-point switch as shown in FIG. 1 so as to result in the above-mentioned disadvantages.

A detailed description will be given of the present invention.

In FIG. 2 illustrating an embodiment of the present invention, reference characters TRK.sub.1, TRK.sub.2 . . . TRK.sub.m indicate trunks No. 1, No. 2 . . . No. m, each connected to the CO line; and ST.sub.1, ST.sub.2 . . . ST.sub.n designate telephone sets No. 1, No. 2, . . . No. n, each serving as a terminal equipment. The trunks and the telephone sets are interconnected via a cross-point switch matrix 1 constituted by cross-point switches S.sub.11T, S.sub.11R, S.sub.21T, S.sub.21R, . . . . Each cross-point is formed by two PNPN elements S.sub.nmT and S.sub.nmR, which are connected in a forward direction with respect to a direct current flowing to each terminal equipment. Further, a pair of selection control signals for tuning ON and OFF a selected cross-point switch are applied to the cross-point switch matrix 1 as x.sub.1c, x.sub.2c, . . . x.sub.mc and y.sub.1c, y.sub.2c, . . . y.sub.2c after decoded by decoders DEC.X and DEC.Y in the Y- and X-directions, respectively.

In such an arrangement, a so-called speech current is supplied from a trunk circuit, an example of which is shown in FIG. 3. In FIG. 3, reference character P.sub.i indicates an ON-OFF signal (first control signal) for a current source CS; F.sub.i designates an ON-control signal (second control signal) for a switching transistor Q; ZD denotes a Zener diode for clamping the collector potential of the switching transistor Q at a certain voltage V.sub.d ; and C represents a capacitor for a path of an AC signal.

A description will be given, with reference to FIG. 4, of the operation of the above arrangement in a case where the trunk TRK.sub.m and the telephone set ST.sub.n are interconnected or disconnected from each other.

(1) In case of connection:

In this case, the cross-point switches S.sub.nmT and S.sub.nmR are to be turned ON. At first, X-direction and Y-direction select signals x.sub.mc and y.sub.nc are simultaneously applied in the form of pulses to the cross-point switch matrix 1. At the same time, the control current P.sub.m also assumes the ON-state. At this time, the cross-point switches S.sub.1mT, S.sub.2mT, . . . are triggered by the X-direction signal x.sub.mc, but since the Y-direction signal is only y.sub.nc, the cross-point switches S.sub.1mR, S.sub.2mR, . . . are not triggered. Accordingly, the direct current path essential for holding the PNPN element at the ON state is extended to a route, for example, X.sub.mT .fwdarw.S.sub.1mT .fwdarw.ST.sub.1, but the route is cut off at the cross-point switch S.sub.1mR. This means that the cross-point switches S.sub.1mT and S.sub.2mT are turned OFF immediately after being turned ON. After all, only the cross-point switches S.sub.nmT and S.sub.nmR are turned ON by the select signals x.sub.mc and y.sub. nc.

(2) In case of disconnection:

By turning OFF the signal P.sub.m which is the ON state while the telephone set ST.sub.n is connected to the trunk TRK.sub.m, no current flows in a drive line X.sub.mT, turning OFF the cross-point switch S.sub.nmT held in the ON state. Upon turning OFF of the cross-point switch S.sub.nmT, the direct current to the cross-point switch S.sub.nmR is also terminated, turning it OFF. Thus, the trunk TRK.sub.m and the telephone set ST.sub.n are disconnected from each other.

As described above, according to the present invention, it is possible to select one cross-point switch (that is, two PNPN elements), omitting the AND circuit used in FIG. 1. However, the abovesaid arrangement, as it is, has the following defect. Namely, when selected by select signals, the switch selected is once turned ON but thereafter turned OFF since no current route exists. Let it be assumed that signals x.sub.2c, Y.sub.2c and P.sub.2 assume ON-states for interconnecting the trunk TRK.sub.2 and the telephone set ST.sub.2 in the state in which the cross-point switches S.sub.nmT and S.sub.nmR are in the ON state. Upon turning ON of the above three signals, the cross-point switches S.sub.22T and S.sub.22R are turned ON, as described previously, but one of the switches triggered by the signal x.sub.2c is a cross-point switch S.sub.n2T. A cross-point switch S.sub.n2R corresponding to the cross-point switch S.sub.n2T is not triggered, but since the switch S.sub.nmR is in the ON state, there exists a route [X.sub.2T .fwdarw.S.sub.n2T .fwdarw.ST.sub. n .fwdarw.S.sub.nmR .fwdarw.X.sub.mR ], resulting in the turning ON of the cross-point switch S.sub.n2T to be held at the OFF state so that an extra unnecessary connection made in an unbalanced form. This is a serious drawback.

It is the control signal F.sub.i, the transistor Q and the Zener diode ZD in FIG. 3 that overcomes this defect. Generally, in the case of turning ON a PNPN element by applying a trigger signal to its gate, the cathode potential with respect to the trigger signal is important, and the trigger signal must be higher than the cathode potential. In addition, once the element is turned ON, only a direct current is needed for holding it in the ON state. Then, as shown in FIG. 4, when the control signal P.sub.m of the current source SC is switch ON to output the select signals x.sub.mc and y.sub.nc, a control signal F.sub.m is also applied to the trunk. Assume that the signal F.sub.m is the ON state for the same time as the select signals are provided. By this, the potential at a point A in FIG. 3 is held 0 by the conduction of the transistor Q while the selected signals are produced and, in the reminder, it is held at a lower potential V.sub.d determined by the Zener diode ZD. Accordingly, the aforesaid defect is settled in the following manner: Namely, even when the cross-point switches S.sub.nmT and S.sub.nmR are at the ON state, the drive line X.sub.mR is held at the potential V.sub.d ; consequently, even if the signal x.sub.2c is provided, the cross-point switch S.sub.n2T is not turned ON since the cathode of the cross-point switch S.sub.nmR is maintained at the potential V.sub.d.

As described above, according to the present invention, in a channel switching network in which each cross-point of a switch matrix is constituted as a channel switch of balanced type, using two PNPN semiconductor elements, each of these PNPN elements are arranged in a forward direction with respect to a direct current flowing the channel switching network to a corresponding terminal equipment, and a control circuit is provided so that the terminal voltage of a power source for flowing the abovesaid direct current to the PNPN elements is controlled for each selected pair of the incoming line and the outgoing line of the switch matrix to temporarily rise to turn ON the PNPN elements and thereafter drops to such a value as only to hold the PNPN elements in the ON state.

As has been described in the foregoing, according to the present invention, AND circuits need not be provided and the balanced cross-point switch network itself has the AND function; accordingly, the channel switching network of the present invention is economical and small in size. While in the foregoing the current source is provided at the trunk, the current source may also be placed at the telephone set. Further, it is also possible to provide a transformer at the telephone set.