From the 7204 installation and upgrade guide:
Do not mix power supplies in the Cisco 7204. In dual power supply router configurations, both power supplies must be of the same type (two AC-input power supplies or two DC-input power supplies).
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The power supply has the router's main power switch and either an AC-input power receptacle or a hardwired DC-input power cable (depending on the type of installed power supply).
Link to the entire guide:
http://www.cisco.com/en/US/products/hw/routers/ps341/products_installation_guide_chapter09186a00801fb71c.html