Access control for a plurality of modules to a common bus
TECHNICAL FIELD 
     This invention relates to a system for controlling the order in which individual modules located in a station system are granted access to a common bus. 
     BRIEF SUMMARY OF THE INVENTION 
     A station in a communication loop is usually comprised of a plurality of modules housed, for convenience, in a suitable support structure. As communication between modules is by way of a common bus, it is necessary that a control be provided for the order in which the modules are serially granted access to the bus. It is to such a control that this invention is primarily directed. 
     It is a further object of this invention to provide a control wherein the removal of a module or modules for repair, modification, or replacement does not disrupt the control of access to the bus. 
     These and other objects will be apparent as the description proceeds in connection with the drawing in which: 
    
    
     IN THE DRAWING 
     The drawing is an elementary one-line, block diagram of a bus access control embodying the principles of this invention. 
    
    
     DETAILED DESCRIPTION 
     Illustrated in the drawing are modules, generally identified as a module A and a module B, which are sufficient to explain this invention. However, there may be upwards of a string of 30 or more of substantially identical modules all adapted to receive from and transmit data to a common bus 6. 
     Each module may include, for example, a computer 8 which is adapted to receive data from the bus 6 through line 34 and transmit data to the bus through line 34. As shown, the unit 8 may also receive signals from diverse transducers such as temperature, rate of flow, pressure, liquid level, and the like, generally indicated at 2. 
     To determine the sequence in which modules requesting access to the bus 6 are granted, there is provided a bus busy line 10 and a grant line 12. The bus busy line is normally maintained at a positive voltage as, for example, five volts. The grant line 12 is provided at one end with a constant current source 14 and at the other end with a constant current source 15, each provided with a clamping circuit 16 to prevent saturation of the sources 14 and 15 when, under normal conditions, no current is otherwise being drawn through the grant line 12. 
     Upon a single module, for example module B, receiving a request signal through line 11 for the module to receive from or transmit data to the bus 6, a control and timing logic unit 13 senses through line 18 that the bus busy line is at plus five volts indicating that no other module is using the bus 6. An arbitration circuit, generally designated 17, resolves multiple requests by other modules, if any, and determines which module, for example A or B, may now use bus 6. Unit 13 of the permitted module then drives, through line 20, the bus busy line to zero volts inhibiting access of any other module or modules in the string of modules from gaining access to the bus 6. The control and timing logic unit 13 then transmits, through line 22, the necessary command signal for unit 8 to transmit to or receive from bus 6 the desired data. Following completion of the operation, the unit 13 restores the bus busy line 10 to its normal state of five volts, thus permitting any other single module to request access to the bus 6. 
     Since two or more modules in the string of modules may simultaneously request access to the bus 6, it becomes necessary to establish the order in which the modules will gain access to the bus. Such an order is accomplished by providing in each module a resistance 24 in the grant line 12, across which is connected at point a and point b a polarity detector 26 connected to the unit 13 through lead 28 which, upon a module requesting access to the bus 6, transmits a signal through a lead 30, closing a switch 32, grounding the line 12 at point b, thus causing a current flow through line 12 of one polarity from constant source 14 to the point of grounding. A current of opposite polarity then flows from constant current source 15 to ground. The signal indicative of a current flow of opposite polarity transmitted by the polarity detector 26 then inhibits access of all other modules to the bus 6. Thus, for example, if modules A and B simultaneously request access to the bus 6, module A will be granted access as there is a current flow of the said one polarity through resistor 24. Thus it will be the module in a string of modules, a plurality of which are requesting access to the bus 6, which senses the current flow of said one polarity and which gives access to the bus 6. Since the arbitration circuits 17 do not act instantaneously, the control and timing block 13 includes a time delay which prevents a module from gaining access to the bus 6 until the signal on line 12 has stabilized. This time delay is required to eliminate the possibility of more than one module from gaining access to the bus at the same instant.