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D20/D200 Technical Overview
2.9.1 D20/D200 Redundancy
To provide high reliability, two D20s (with VME chassis) or D200s can be joined via a dedicated
communication channel to form a redundant RTU assembly, as illustrated below:
Each D20/D200 contains identical hardware and software. The connection to field equipment is
made through an RS-232 Switch Panel. The position of the RS-232 Switch Panel, set either
automatically or manually, determines which D20/D200 is active.
On power up, the active D20/D200 initializes to an "Active Polling" state, acquiring data from
all substation devices and any local peripheral I/O reporting to it. Controls are executed as
directed by the master host system.
The standby D20/D200 monitors the state of the active D20/D200 via the inter-CCU
communications circuit. The standby D20/D200 operates in a warm standby mode; it does not
actively poll for data or execute controls. Some data may therefore be lost during a fail-over,
depending on the timing of the fail-over with respect to polling activity (i.e., the system is not hot
standby).
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Figure 44 D200 Redundancy
PRPI-019-3.00-10
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