DP83266 datasheet(19/152 Pages) NSC

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50 Functional Description (Continued)

is not received within TTRT (the token is late) the event is

recorded by setting the Late

Flag If the token is not re-

ceived within twice TTRT (TRT expires and Late

Flag is

set) there is a potential problem in the ring and the recovery

process is invoked

Furthermore the Token Holding Timer (THT) is used to limit

the amount of ring bandwidth used by a station for asyn-

chronous traffic once the token is captured Asynchronous

traffic is prioritized based on the Late

Flag which denotes

a threshold at TTRT and an additional Asynchronous Priori-

ty Threshold (THSH) The Asynchronous Threshold com-

parison (Apri 1) is pipelined so a threshold crossing may not

be detected immediately however the possible error is a

fraction of the precision of the threshold values

The Token Timing Logic consists of two Timers TRT and

THT in addition to the TMAX and TNEG values loaded into

these counters (see

Figure 5-1 )

The Timers are implemented as count-up counters that can

increment every 80 ns The Timers are reset by loading

TNEG or TMAX into the counters where TNEG and TMAX

are unsigned two’s complement numbers This allows a

Carry flag to denote timer expiration

On an early token arrival (Late

Flag is not set) TRT is

loaded with TNEG and counts up On a late token arrival

(Late

Flag is set) Late

Flag is cleared and TRT contin-

ues to count When TRT expires and Late

Flag is not set

Late

Flag is set and TRT is loaded with TNEG

THT follows the value of TRT until a token is captured

When a token is captured TRT may be reloaded with TNEG

while THT continues to count from its previous value (THT

does not wrap around) THT increments when enabled THT

is disabled during synchronous transmission and a special

class of asynchronous transmission THT is used to deter-

mine if the token is usable for asynchronous requests For

these purposes the token is considered late one byte be-

fore it is actually late (to promote interoperability with less

careful implementations)

If TRT expires while Late

Flag is set TRT is loaded with

TMAX and the recovery process (Claim) is invoked (unless

the Inhibit Recovery Required option is set) The Recovery

Required condition becomes true one byte time after TRT

expires (to promote interoperability with less careful imple-

mentations) When TRT expires and the ring is not opera-

tional TRT is loaded with TMAX TRT is also loaded with

TMAX on a MAC Reset

512 Token Recovery

While the ring is operational every station in the ring uses

the Negotiated Target Token Rotation Time TNEG The

MAC implements the protocol for negotiation of this target

token rotation time (TTRT) through the Claim process The

shortest requested token rotation time is used by all of the

stations in the ring as the TNEG

If TRT expires with Late

Flag set a token has not been

received within twice TTRT (Target Token Rotation Time) If

TVX (Valid Transmission Timer) expires the station has not

received a valid token within TVX Max Both these events

require token recovery and cause the Ring Engine to enter

the Claim process

In the Claim process a MAC continuously transmits Claim

frames containing TREQ Should the MAC receive a Claim

frame with a shorter TREQ (larger valueHigher

Claim) it

leaves the Claim State A station that receives its own Claim

frame gains the right to send the first token and make the

ring operational again If the Claim Process does not com-

plete successfully TRT will expire and the Beacon Process

is invoked

The Beacon Process is used for fault isolation A station

may

invoke

the

Beacon

Process

through

Controlrequest(Beacon) When a station enters the

Beacon Process it continuously sends out Beacon Frames

The Beacon Process is complete when a station receives its

own Beacon Frame That station then enters the Claim pro-

cess to re-initialize the ring

52 SERVICING TRANSMISSION REQUESTS

A Request to transmit one or more frames is serviced by the

Ring Engine After a Request is submitted to the Ring En-

gine the Ring Engine awaits an appropriate Service Oppor-

tunity in which to service the Request Frames associated

with the Request are transmitted during the Service Oppor-

tunity The definition of a Service Opportunity is different

depending on the operational state of the ring

TLF11705 – 7

FIGURE 5-1 Token Timing Logic

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