SCADA MASTER TERMINAL CHANNEL CONTROL FUNCTIONS BASIC INFORMATION


In addition to supporting the basic communication functions, the master terminal is required to provide the following channel management functions. In this section, parameters labelled “user alterable” are required to be readily adjustable to permit user tuning of the communication facilities.

The system supplier is expected to provide default values for these parameters to support initial field operation.

Message Priorities
The capability to manage the transmissions on each channel based on relative priorities assigned by non protocol processes to waiting messages should be supported. The capability to assign different priorities to the various message functional types (e.g., status versus analog data acquisition) should be supported.

A priority order based on remote terminal address should be supported for party line remotes. The assignment of priorities should be user alterable.

Message Queuing
The capability to manage a queue of messages waiting for access to each channel should be supported. The size of each queue shall be user alterable. The capability to establish alarm thresholds indicating imminent queue overflow and excessive queuing time for each queue should be supported. The values of the alarm thresholds should be user alterable.

Message Retransmission
When any master terminal message requiring a response is transmitted, a “No-Response” timer should be started and subsequently reset on receipt of the response. If the user-alterable time-out value is exceeded, the previous master terminal message may be retransmitted.

The capability for user-alterable assignment of the permitted maximum number of retries for each message functional type should be supported. Some message types may not require any retries.

Communication Performance
The capability to alarm channels as inoperative or marginal should be supported. The criteria for declaring a channel inoperative or marginal should be user-alterable and be based on measured channel quality parameters and/or error statistics.

As a minimum, an error count should be accumulated for a user-defined message count to alarm a channel as inoperative or marginal when the error count exceeds a useralterable limit. Channel quality parameters may include signal-to-noise ratio, carrier level, etc., which are monitored by communication equipment.

Rejection of messages by remote or master terminals may be due to conditions such as a low quality channel, marginal modems, or terminal equipment failure.

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