SCADA CHANNEL TYPES BASIC INFORMATION AND TUTORIALS

Three types of channels are used:

1) Duplex point-to-point channels

2) Duplex party-line channels

3) Half-duplex channels

Duplex Point-to-Point Channels

For this type of channel, the DCE Request to Send signals at both terminals are activated continuously. The DCE transmitters and receivers are both continuously active while the channel is operational, thereby eliminating DCE response time delays.

Duplex Party-Line Channels

The master terminal DCE Request to Send signal is operated as described in. The corresponding signal at any remote terminal is activated only when that terminal responds to a preceding master terminal message and is deactivated immediately following completion of transmission of the response message. A DCE response time delay is encountered at the remote terminal.

Half-Duplex Channels

The DCE Request to Send signals are activated only when the associated terminal initiates transmission of a message and are deactivated immediately on completion of each message transmission. DCE response-time delays are encountered at both master and remote terminals.

Additional DCE Facilities

Remote terminal DCE equipment operating in party-line communication channels should be equipped with means to monitor the length of their output transmissions and to deactivate their DCE transmitter if an excessive length occurs, presumably due to an equipment failure.

This feature, commonly known as “anti-streaming protection,” minimizes the probability that a failure in one remote terminal will disable communication with other remote terminals that share the channel.

The DCE may also be equipped with means to monitor the quality of the received line signal and to report to the associated terminal when degraded signal conditions occur.

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