Method and apparatus for graceful preemption on a digital communications link

A link controller comprising

• receiver means for receiving communications from a communications medium in frames, said receiver means being associated with a set of status variables collectively defining the present status of said receiver means,
• means responsive to a receipt of a start preemption signal after the reception by said receiver means of a portion of a first frame from said communications medium, for saving values of said set of status variables, and
• means responsive to a receipt of an end preemption signal after the reception by said receiver means of a second frame from said communications medium, for returning said receiver means to the status defined by said saved values of said set of status variables,
• wherein said receiver means is responsive to said end preemption signal for receiving a second portion of said first frame after said second frame is received.

In an arrangement comprising means for transmitting a preempt character over a communications medium and receiver means for receiving communications from said communications medium in frame, said receiver means being associated with a set of status variables collectively defining the present status of said receiver means, a method of gracefully preempting the reception from said communications medium of a first frame before a portion of said first frame is received comprising

• detecting said preempt character from said communications medium,
• saving values of said set of status variables,
• receiving a second frame from said communications medium,
• returning, after said second frame is received, said receiver means to the status defined by said saved values of said set of status variables and
• receiving, after said second frame is received, said portion of said first frame.

A communication system for communicating from a plurality of user devices over a communications medium comprising

• coupling means for selectively coupling said user devices to said communications medium for communication, said coupling means being associated with a first set of status variables collectively defining the present status of said coupling means,
• receiver means for receiving communication from said communications medium, said receiver means being associated with a second set of status variables collectively defining the present status of said receiver means,
• selection means for selecting a given one of said user devices while a given other one of said user devices is coupled to said communications medium for communication,
• means responsive to said selection means for interrupting communication from said given other one of said user devices,
• means responsive to said selection means for saving values of said first set of status variables,
• means responsive to said selection means for transmitting a start preemption signal on said communications medium,
• means responsive to said selection means for coupling said given one of said user devices to said communications medium for communication and
• means responsive to said start preemption signal for saving values of said second set of status variables.

A link controller for transmitting data from a plurality of user devices over a communications medium comprising

• transmit data register means for storing data,
• a plurality of formatter interfaces each associated with one of said user devices for receiving data from that user device and transmitting that data to said transmit data register means for storage therein,
• transmit control means for generating control signals,
• transmit shift register means comprising means responsive to said control signals for reading data from said transmit data register means and means responsive to said control signals for serially transmitting bits of data read from said transmit data register means to said communications medium,
• processor means comprising selection means for selecting a given one of said formatter interfaces while a given other one of said formatter interfaces is transmitting data to said transmit data reigster means, means responsive to said selection means for interrupting data transmission by said given other one of said formatter interfaces and means responsive to said selection means for initiating data transmission by said given one of said formatter interfaces and
• stack memory means for saving information and
• wherein said transmit control means further comprises means responsive to said selection means for generating control signals for effecting a saving by said stack memory means of data read by said transmit shift register means but not serially transmitted thereby to said communications medium.

A link controller for transmitting data from a plurality of user devices over a communications medium comprising

• transmit data register means for storing data,
• a plurality of formatter interfaces each associated with one of said user devices for receiving data from that user device and transmitting that data to said transmit data register means for storage therein,
• transmit control means for generating control signals,
• transmit shift register means comprising means responsive to said control signals for reading data from said transmit data register means and means responsive to said control signals for serially transmitting bits of data read from said transmit data register means to said communications medium,
• processor means comprising selection means for selecting a given one of said formatter interfaces while a given other one of said formatter interfaces is transmitting data to said transmit data register means, means responsive to said selection means for interrupting data transmission by said given other one of said formatter interfaces and means responsive to said selection means for initiating data transmission by said given one of said formatter interfaces,
• multiplexer means for transmitting bits having a first predetermined logic value to said communications medium and
• stack memory means for saving information and
• wherein said transmit control means further comprises means for counting the number, N.sub.2, of consecutive bits of data transmitted by said transmit shift register means having a second predetermined logic value, means for generating control signals for effecting the transmission by said multiplexer means of a bit having said first predetermined logic value when said number, N.sub.2, equals a predetermined number and means responsive to said selection means for generating control signals for effecting a saving by said stack memory means of a representation of said number, N.sub.2.

A link controller for transmitting data from a plurality of user devices over a communications medium comprising

• transmit data register means for storing data,
• a plurality of formatter interfaces each associated with one of said user devices for receiving data from that user device and transmitting that data to said transmit data register means for storage therein,
• transmit control means for generating control signals,
• transmit shift register means comprising means responsive to said control signals for reading data from said transmit data register means and means responsive to said control signals for serially transmitting bits of data read from said transmit data register means to said communications medium,
• processor means comprising selection means for selecting a given one of said formatter interfaces while a given other one of said formatter interfaces is transmitting data to said transmit data register means, means responsive to said selection means for interrupting data transmission by said given other one of said formatter interfaces and means responsive to said selection means for initiating data transmission by said given one of said formatter interfaces,
• frame check sequence generator means for generating a frame check sequence of bits from bits transmitted by said transmit shift register means and
• stack memory means for saving information and
• wherein said transmit control means further comprises means responsive to said selection means for generating control signals for effecting a saving by said stack memory means of said frame check sequence.

An arrangement comprising

• means for transmitting a start preemption signal on a communications medium,
• receive data register means for storing data,
• receive control means for generating control signals,
• receive shift register means comprising means responsive to said control signals for serially receiving bits of data from said communications medium and means responsive to said control signals for transmitting data received from said communications medium to said receive data register means for storage therein and
• stack memory means for saving information and
• wherein said receive control means further comprises means responsive to said start preemption signal for generating control signals for effecting a saving by said stack memory means of data received by said receive shift register means but not transmitted thereby to said receive data register means.

An arrangement comprising

• means for transmitting a start preemption signal on a communications medium,
• receive data register means for storing data,
• receive control means for generating control signals,
• receive shift register means comprising means responsive to said control signals for serially receiving bits of data from said communications medium and means responsive to said control signals for transmitting data received from said communications medium to said receive data register means for storage therein and
• stack memory means for saving information and
• wherein said receive control means further comprises means for counting the number, N.sub.4, of consecutive bits received from said communications medium having a predetermined logic value, means for generating control signals for inhibiting the reception by said receive shift register means of a bit from said communications medium when said number, N.sub.4, equals a predetermined number and means responsive to said start preemption signal for generating control signals for effecting a saving by said stack memory means of a representation of said number, N.sub.4.

An arrangement comprising

• means for transmitting a start preemption signal on a communications medium,
• receive data register means for storing data,
• receive control means for generating control signals,
• receive shift register means comprising means responsive to said control signals for serially receiving bits of data from said communications medium and means responsive to said control signals for transmitting data received from said communications medium to said receive data register means for storage therein,
• frame check sequence generator means for generating a frame check sequence of bits from bits received from said communications medium and
• stack memory means for saving information and
• wherein said receive control means further comprises means responsive to said start preemption signal for generating control signals for effecting a saving by said stack memory means of said frame check sequence.

A communication system for communicating data from a plurality of user devices over a communications medium comprising

• transmit data register means for storing data,
• a plurality of formatter interfaces each associated with one of said user devices for receiving data from that user device and transmitting that data to said transmit data register means for storage therein,
• transmit control means for generating first control signals,
• transmit shift register means comprising means responsive to said first control signals for reading data from said transmit data register means and means responsive to said first control signals for serially transmitting bits of data read from said transmit data register means to said communications medium,
• processor means comprising selection means for selecting a given one of said formatter interfaces while a given other one of said formatter interfaces is transmitting data to said transmit data register means, means responsive to said selection means for interrupting data transmission by said given other one of said formatter interfaces and means responsive to said selection means for initiating data transmission by said given one of said formatter interfaces,
• first stack memory means for saving information,
• means responsive to said selection means for transmitting a start preemption signal on said communications medium,
• receive data register means for storing data,
• receive control means for generating second control signals,
• receive shift register means comprising means responsive to said second control signals for serially receiving bits of data from said communications medium and means responsive to said second control signals for transmitting data received from said communications medium to said receive data register means for storage therein and
• second stack memory means for saving information and
• wherein said transmit control means further comprises means responsive to said selection means for generating control signals for effecting a saving by said first stack memory means of data read by said transmit shift register means but not serially transmitted thereby to said communications medium and
• wherein said receive control means further comprises means responsive to said start preemption signal for generating control signals for effecting a saving by said second stack memory means of data received by said receive shift register means but not transmitted thereby to said receive data register means.