Motorola V186 User Manual - Page 101

MDLC Data Transfer Methods

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Communications MDLC Data Transfer Methods Three messaging methods are commonly used by the Motorola RTU: Contention (transmission upon change-of-state; also called burst), Polling (interrogation), and Report-by-Exception. The Contention method has the RTU report upon a change-of-state (COS) of conditions/values without waiting for a poll from the SCADA Manager. The RTU recognizes a COS and reports relevant data to the SCADA Manager or to another site as soon as the shared communication medium becomes available. The RTU will repeat the data message until confirmation of reception is received. The RTU listens to the shared communication medium before sending a message and then uses a slotted channel acquisition method to avoid synchronized message collisions. This is the messaging method most often used by Motorola RTUs because it uses the shared communication medium properly. The Polling (interrogation) method is a periodic activity used to confirm the proper operation of the normally silent RTUs and/or to update the SCADA Manager database at specified intervals or when manually instructed by the operator. The Report-byException method has the RTU report only the conditions/values that have changed since the last poll. The SCADA Manager retains all data conditions and values in a local database for instant use. 97

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Communications
MDLC Data Transfer Methods
Three messaging methods are commonly used by the Motorola RTU: Contention
(transmission upon change-of-state; also called burst), Polling (interrogation), and
Report-by-Exception. The Contention method has the RTU report upon a change-of-state
(COS) of conditions/values without waiting for a poll from the SCADA Manager. The
RTU recognizes a COS and reports relevant data to the SCADA Manager or to another
site as soon as the shared communication medium becomes available. The RTU will
repeat the data message until confirmation of reception is received. The RTU listens to
the shared communication medium before sending a message and then uses a slotted
channel acquisition method to avoid synchronized message collisions. This is the
messaging method most often used by Motorola RTUs because it uses the shared
communication medium properly.
The Polling (interrogation) method is a periodic activity used to confirm the proper
operation of the normally silent RTUs and/or to update the SCADA Manager database at
specified intervals or when manually instructed by the operator. The Report-by-
Exception method has the RTU report only the conditions/values that have changed since
the last poll. The SCADA Manager retains all data conditions and values in a local
database for instant use.
97