Abstract:
An access circuit for use in a computer input-output channel. Retriggerable mono-stable multivibrators are used to provide access timing periods during which an input-output device can access a computer channel to the exclusion of other input-output devices.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to computer channel access circuits for input-output devices and more particularly to a circuit which controls computer channel access time of input-output devices. 
     (2) Description of the Prior Art 
     Input-output devices access a computer through an input-output device controller or channel. This controller transmits access requests and data from the device to the computer and it transmits data to the device from the computer. Multiple devices, such as local and remote teletypewriters, can be connected to one controller to receive data since they will both print the same data. However, only one teletypewriter can transmit data to a controller at a time since the probability that they will both be transmitting the same data at the same time is infinitesimally small. 
     If two teletypewriters with input capability were connected to one controller, garbled data would be transmitted to the computer if both teletypewriters were transmitting at the same time. A lockout circuit could be provided such that the teletypewriters accessing the controller first, would lockout the other teletypewriter until its communication with the computer was complete. However, this could result in the other teletypewriter being locked out indefinitely. 
     This problem has been solved in the prior art by connecting an additional controller to the computer for each additional teletypewriter with input capability. However, such a technique is expensive and unnecessary since it requires an additional controller for a remote teletypewriter which will only occasionally require access to the computer. 
     Accordingly, it is an object of the present invention to provide a novel, low cost technique of providing computer access capability to multiple input-output devices connected to a single device controller. 
     SUMMARY OF THE INVENTION 
     The present invention is a circuit which provides computer channel access capability to multiple input-output devices, such as teletypewriters, through a single device controller on a time limit basis. 
     The circuit consists of a retriggerable mono-stable multivibrator and a first gate circuit connected to each other and to an associated teletypewriter. The outputs of each first gate circuit are connected to a second gate circuit whose output is connected to the device controller. 
     When a teletypewriter initiates a date transmission, its associated multivibrator enables its associated first gate circuit for five seconds and simultaneously disables the multivibrator associated with other teletypewriters for five seconds. The enabled gate circuit then transmits data from its associated teletypewriter through the second gate circuit to the device controller. At the end of five seconds, the multivibrator times out, disables its associated gate circuit and removes the disable signal applied to the other multivibrators. Whichever teletypewriter is first to initiate the next data transmission will then gain control of the device controller for the next five seconds. 
     Thus multiple teletypewriters are provided with computer channel access capability through a single device controller. 
    
    
     DESCRIPTION OF THE DRAWING 
     The single FIGURE of the accompanying drawing is a circuit diagram of a computer channel access circuit for multiple input-output devices in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the accompanying drawing the computer channel access circuit of the present invention is shown. Multivibrator 10 is shown connected to an associated teletypewriter and its output is connected to NAND gate 12. Inverter 11 is also connected to multivibrator 10 and to NAND gate 12. Similarly multivibrator 20 is connected to an associated teletypewriter and its output is connected to NAND gate 22. Inverter 21 is also connected to multivibrator 20 and to NAND gate 22. The outputs of NAND gate 12 and NAND gate 22 are connected to AND gate 30 and its output is connected to an input-output device controller. 
     Initially all teletypewriters have an equal opportunity to enter data. This circuit operates to allow only one teletypewriter to input data at a time, on a first come first serve basis as determined by the leading edge of the start bit of a data transmission from a teletypewriter. Multivibrator 10 operates in response to the leading edge of a start bit (transition to logic φ) from teletypewriter 1 to generate a five second timing period. Multivibrator 10 generates a five second enable signal (logic 1) on its Q lead which enables NAND gate 12 to receive data from teletypewriter 1 via inverter 11. Multivibrator 10 also generates a disable signal (logic φ) on its Q lead. Multivibrator 20 detects this disable signal on its clear input and operates to generate an inhibit signal (logic φ) on its Q lead. NAND gate 22 responds to this inhibit signal to prevent transmission of data from teletypewriters via inverter 21 and to generate a logic 1 signal. AND gate 30 operates in response to the logic 1 signal from NAND gate 22 and the data from NAND gate 12 to gate this data to the input-output device controller. AND gate 30 will continue to gate data for five seconds at which time multivibrator 10 will time out and turn off the enable signal on its Q lead. NAND gate 12 responds to the absence of the enable signal to prevent transmission of data from teletypewriter 1 via inverter 11. When multivibrator 10 times out it also removes the disable signal on its Q lead. Multivibrator 20 operates in response to the absence of the disable signal to allow the leading edge of the start bit of data from teletypewriter 2 to trigger multivibrator 20, should it occur before the start bit of data from teletypewriter 1 triggers multivibrator 10. 
     Monostable multivibrators 10 and 20 are retriggerable thus allowing extension of the five second timing period. If teletypewriter 1 initiates a subsequent data transmission before termination of its prior five second timing period, retriggerable monostable multivibrator 10 will extend the timing period for an additional five seconds from receipt of the start bit of this new data transmission. Thus once a teletypewriter gains access to a device controller it can retain access to the controller beyond five seconds in order to complete its data transmission uninterrupted. 
     Thus retriggerable monostable multivibrators provide access timing for multiple teletypewriters connected to a single device controller. While the preferred embodiment shows a circuit using five second timing periods and allowing only two input-output devices to gain access to a device controller, the capability of additional input-output devices to gain access to a device controller can be provided by connecting an additional multivibrator, NAND gate and inverter for each additional input-output device. 
     It will be obvious to those skilled in the art that numerous modifications of the present invention can be made without departing from the spirit of the invention which shall be limited only by the scope of the claims appended hereto.