Register Magento resource

config.xml

<global>
    <models>
        <custom_module>
            <class>Custom_Module_Model</class>
            <resourceModel>module_mysql4</resourceModel>
        </custom_module>
        <module_mysql4>
            <class>Custom_Module_Model_Mysql4</class>
            <entities>
                <custom_module>
                    <table>custom_table</table>
                </custom_module>
            </entities>
        </module_mysql4>
    </models>
    <resources>
        <custommodule_setup>
            <setup>
                <module>Custom_Module</module>
            </setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </custommodule_setup>
        <custommodule_write>
            <connection>
                <use>core_write</use>
            </connection>
        </custommodule_write>
        <custommodule_read>
            <connection>
                <use>core_read</use>
            </connection>
        </custommodule_read>
    </resources>
</global>