DP83266 datasheet(24/152 Pages) NSC

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50 Functional Description (Ring Engine) (Continued)

The Ending Delimiter followed by the Frame Status Indica-

tors should begin and end on byte boundaries Control Indi-

cators are repeated until the first non R S or T symbol is

received

The processing of properly aligned E A and C indicators by

the Ring Engine is detailed in Table 5-5 Given the shown

received Control Indicator values and the settings of the

internal flags the noted control indicator values will be

transmitted

551 Odd Symbols Handling

When the first T symbol of a frame is received as the sec-

ond symbol of a symbol pair (the T symbol is received off-

boundary) the Ring Engine signals this condition by sending

out the symbol sequence TSII This indicates the end of

frame for a frame which had an error Note that this is a low

probability error event

Reception of symbols other than R S or T during the

Frame Status processing is also a low probability event

56 SMT FRAME PROCESSING

All SMT frames are handled as all other frames with the

exception of the SMT Next Station Addressing (SMT NSA)

frame NSA frames are used to announce this station’s ad-

dress to the next addressed station The current SMT proto-

col requires stations to periodically (at least once every 30

seconds) transmit an NSA frame Since the Broadcast ad-

dress is used and every station is required to recognize the

broadcast address the downstream neighbor will set the A

Indicator A station can determine its upstream neighbor by

finding NSA frames received with the A Indicator received

as R By collecting this information from all stations a map

of the logical ring can be built

TABLE 5-5 Control Indicator Processing

Received Indicators

Flags

Transmitted Indicators

Copy

RRR

XXX

RSR

XXX

RSR

XXX

Flag is set when the local FCS check fails or when the E Indicator is received as anything other than R

Flag is the value of the internal A

Flag or the external A

Flag as indicated by the EA input signal (when OptionEmind is set)

EC represents the value of the External C indicator Input Signal one byte time before then ending delimiter of the frame

The Copy Flag is a one cycle delayed version of the VCOPY input

Flag indicates that an NSA frame is being received This signal is sampled at the same time that the received A indicator is being investigated

X Represents a Don’t Care Condition

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