The base of each transistor is connected to a bias voltage V.sub.cc through pull-up resistors R.sub.1, R.sub.2, and R.sub.3. Connection 34 is coupled to computer 48 through primary channel 16, while connection 36 is connected to keyboard 47 through data channel 18.sub.2.

Switch 22 includes bi-polar transistors 26 and 28 in which the collector of one is connected to the emitter of the other. Two-stage selector 20 primarily includes two analog switches 22 and 24 which control bi-directional flow of TTL signals between keyboards 46 and 47 and a keyboard controller 49 within computer 48. 2, a two-stage version of multi-stage analog bi-directional selector 10 is shown.

The relationship between the number of channels N and the number of address bits B required may be defined by the equationįor example, two address bits B will support up to four data channels. Addressing logic 14 includes address input 17. N-stage selector 12 provides bi-directional analog communication between a primary channel 16 and one of a number N of data channels 18.sub.N.Īddressing logic 14 determines which one of the data channels 18.sub.N is connected to the primary channel 16 through N-stage selector 12. 1, multi-stage analog bi-directional selector circuit 10 includes N-stage selector 12 and addressing logic 14. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 3 is a circuit diagram of a four-stage analog bi-directional selector. 2 is a circuit diagram of a two-stage analog bi-directional selector within a computer system including first and second keyboards andįIG. 1 is a block diagram of the multi-stage analog bi-directional selector įIG. BRIEF DESCRIPTION OF THE DRAWINGSĪdditional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings, in which:įIG. It is another object of the present invention to provide a multi-stage analog bi-directional selector that is low in cost. It is another object of the present invention to provide a multi-stage analog bi-directional selector that has a low impedance. It is accordingly an object of the present invention to provide a multi-stage analog bi-directional selector. The multi-stage analog bi-directional selector includes a plurality of analog switches including first and second bi-polar transistors coupled together at first and second connection points, a primary channel coupled to the first connection points, a plurality of data channels coupled to the second connection points, and an address circuit which causes a single one of the analog switches to form a bi-directional analog data connection between a corresponding single one of the data channels and the primary channel. In accordance with the teachings of the present invention, a multi-stage analog bi-directional selector is provided. Therefore, it would be desirable to provide a multi-stage analog bi-directional selector with low impedance and with low cost. The invention would be used in the keyboard to control placement of data from the wedged device onto the keyboard's communication channel. The computer may then choose which keyboard to communicate with.Īnother application for this invention is in keyboard wedge devices, in which communication with the wedged device would occur through the keyboard's communication interface. Thus, two keyboards may be more easily connected to a single keyboard controller 49 within a computer. Such a combination would be desirable, particularly for passing bi-directional TTL logic signals on a wire-or bus typical of what is found in the PC-AT keyboard, namely clock and data signals. Presently, there are no low impedance devices which combine the data path selection of a multiplexer with the asynchronous bi-directional communication of an analog switch. This impedance requires a proportional increase in the voltage of the circuit which drives such a switch. They also have input impedances which are too high (about 200 ohms maximum). Multiplexers and de-multiplexers perform data path selection, but do not support asynchronous bi-directional communication.Īnalog switches control bi-directional communication, but do not support data path selection. The present invention relates to communication switching circuitry, and more specifically to a multi-stage analog bi-directional selector.

Transistor keyboard or controller