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Cisco 2900 and 3900 Series Hardware Installation
OL-18712-01
Chapter 3 Installing and Connecting the Router
Connecting Power
Approved Scenarios and Scenarios Not Approved for Dual DC Power Supply Configuration on Cisco
2911, 2921, and 2951 Routers
You can connect a single DC power source to either the A input or the B input. If there are dual power
sources, connect one source to the A input and one source to the B input. Both sources must be the same
polarity (with respect to ground) and voltage (within 0.25 volts). Do not connect –DC grounded and +DC
grounded dual sources to Cisco 2911, Cisco 2921, and Cisco 2951 routers.
Caution
Dual sources with opposite-polarity grounding damage equipment.
In Figure 3-23, either the positive source terminal or the negative source terminal is tied to ground.
Figure 3-23 Connecting to One Source Only—Source A or Source B
In Figure 3-24, source A and source B share common negative terminal connections.
Figure 3-24 Connecting Source A and Source B with Common Negative Terminals
In Figure 3-25, source A and source B share common positive terminal connections. This is allowed only
if Va equals Vb (within 0.25 V).
Caution
When connecting source A and source B with common positive terminals, if source A and source B
voltages are unequal by more than 0.25 V, the higher-voltage source can discharge into the lower-voltage
source through the A- and B- input terminals. Excessive discharging currents through these terminals
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