Priority token protocol circuit for a token ring local area network adaptor

A fairness protocol circuit for a token ring local area network adaptor having a data-in line for serially carrying tokens/frames, comprising:

• a shift register delay of at least 8 bauds coupled to said data-in line;
• a serial bidirectional shift register TTT stack coupled to said shift register delay for serially storing values of old token priority indicators;
• a serial bidirectional shift register III stack for storing values of old token priority indicators;
• a 3 baud TTT shift register coupled to said TTT stack so that a parallel transfer of data in the top 3 baud positions of said TTT stack can be made to said TTT shift register in response to a LOADIT control signal and so a serial shift of data out of said TTT register is in continuous progress except when transferring data from said TTT stack;
• III retention means coupled to said III stack for outputting bit by bit the value of a last new token priority indicator "I" stored on said III stack while at the same time maintaining the value of said "I" on said III stack immediately following the outputting and for replacing said "I" with a reservation indicator "R" of the token and discarding said "I";
• a PvsI comparator having inputs coupled to a second baud position of said shift register and to said III retention means and operative to provide outputs if the value of the last new token priority indicator "I" is less than or greater than the value of the token priority indicator "P";
• an RGTP comparator having inputs coupled to said shift register first and sixth bit positions and operative to provide an output if the value of the token reservation "R" is greater than the token priority indicator "P";
• an RGTT comparator having inputs coupled to said shift register delay first baud position and to said TTT means and operative to provide an output if the value of the token reservation "R" is greater than the value of the last old token priority indicator "T";
• an output multiplexer having a first input coupled to an output of said shift register, a second input coupled to an output of said TTT retention means and a third input coupled to a fourth baud position of said shift register delay and operative to transmit a selected one of its inputs to its output in response to control signals applied thereto;
• and
• a state machine having inputs coupled to outputs of said Pvs I, RGTP, RGTT comparators and to an IEQZ output from said III stack and operative in respone to selected combinations of its inputs to issue control signals to said stacks, said III means, said TTT means, and to said PvsI, RGTP, RGTT comparators.

A method of controlling fairness protocol on a token ring local area network system on which circulate from station to station tokens and frames having an access priority field which includes a 3 bit priority field, a token indicator bit, a monitor count bit and a 3 bit priority reservation field, comprising:

• detecting the arrival of an incoming or internally generated token/frame;
• comparing the priority indicator "P" of a token/frame arriving at a given station with a new priority indicator "I" of the last token whose priority was modified by that station;
• comparing the value of the reservation of the token/frame "R" with the value of the old priority indicator "T" of the last token whose priority was modified by that station;
• comparing the value of "R" to that of "P";
• replacing "P" of the token/frame by "R" if "R" is greater than "P" and storing the value of "R" as "I" and the value of "P" as "T" and clearing the "R" and monitor count of the outgoing token/frame;
• replacing the priority indicator "P" of the token by the value of "T" if "P" equals "I" and discarding the last stored values of "I" and "T" and clearing the monitor count;
• replacing "P" of the token by "R" if "P" equals "I" and "R" is greater than "T" and substituting "R" for the last stored "I" value and clearing the reservation and monitor count of the token;
• discarding the values of "I" and "T" if "P" is less than "I";
• removing fairness from being active if the value of "I" "T" and Rare Zero.

A local area network system, comprising:

• a token/frame conducting ring;
• a plurality of host stations;
• and
• a plurality of adaptors coupled between said ring and respective host stations for interfacing associated host stations to said ring, wherein each adaptor has a token/frame priority protocol circuit for reading priority protocol values of circulating tokens/frames, and in response to a comparison of such values with priority values stored therein, either re-transmitting such tokens/frames or copying such tokens/frames and generating and transmitting new tokens/frames onto said ring;
• wherein said token or frame priority protocol circuit includes;
• means for detecting the arrival of an incoming or internally generated token or frame;
• means for comparing a priority indicator "P" of an arriving token/frame at a given station with a new priority indicator "I" of the last token whose priority was modified by that station;
• means for comparing a reservation value "R" of a token/frame with the value of an old priority indicator "T" of the last token whose priority was modified by that station;
• means for comparing the value of "R" to the value of "P";
• means for replacing "P" of the token/frame by "R" if "R" is greater than "P" and storing the value of "R" as "I" and the value of "P" as "T" and clearing the "R" and monitor count of the outgoing token/frame;
• means for replacing "P" of the token by the value of "T" if "P" equals "I" and discarding the last stored values of "I" and "T" and clearing the monitor count;
• means for replacing "P" of the token by "R" if "P" equals "I" and "R" is greater than "T" and substituting "R" for the last stored "I" value and clearing the reservation and monitor count of the token;
• discarding the values of "I" and "T" if "P" is less than "I";
• and
• means for inactivating fairness when the values of "I" and "T" are zero and "R"=0.