140CPS11420: Manual Anthologies
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There exists some important design differences between various models of Quantum power supplies that require careful consideration by the system designer in order to achieve maximum system performance. The principal difference lies in the generation within the power supply of important backplane signals related to the health of the power supply and the status of the input power. All Quantum power supplies include on-board early power fail detection logic which is used to signal all other modules on the backplane that input power has failed. This signal is called POK (power OK) and is active high (i.e., when the signal is high, power is OK). There is both an internal (to the power supply) and an external (as seen by the backplane and all other modules) version of the POK signal. The internal POK signal is represented by the Pwr ok LED (light emitting diode) on the front panel of all power supplies. The system POK signal is generated so that there is sufficient time between the negative going edge of system POK (power has failed) and the actual interruption of power to the backplane. This early warning of power failure is necessary for the Quantum executive to perform an orderly system shutdown.
The 140CPS11420 is a Summable Power Supply. A summable power supply may be used as a standalone power supply without reservation in any Quantum system.
For systems configured with a mix of CPS, NOM, expert, and l/O modules, whose total current consumption from the backplane exceeds the current provided by one summable supply, two summable supplies may be used in a single backplane. In such a system, the total current available on the backplane is the sum of the capability of both supplies (i.e., 2 x 8 A = 16 A). The summable supplies are designed so that they split the current supplied to the load almost equally, which also has the added benefit of increasing total system MTBF, and to distribute the thermal load across the backplane. Summable supplies should be installed in the end module positions in the Quantum backplane to maximize the system thermal performance. The Quantum system POK signal in systems powered by two summable power supplies is only true (power is OK) when both internal POK signals (in the 140 CPS •14 •0) are true. Quantum summable power supplies are not hotswappable.
The proper method for starting systems powered by summable power supplies is to insert both supplies in the backplane in an unpowered state, and then apply power to each supply. There is no requirement to turn on each supply simultaneously. The system designer must realize that the operation of the summable supply described above is independent of total backplane load, i.e., even if the total load on the backplane is less than 8 A, if there are two summable supplies installed in the backplane, the system POK is generated as shown in this section. For the special case of a single summable supply used as a standalone, the system POK generation reverts to the standalone case as shown previously in this section.
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