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To use the default configuration, download this driver from the jTDS website, and copy the driver JAR file to server/default/lib.
You may also instead use the Microsoft JDBC driver, but you'll have to modify the datasource files directly to reflect the correct driver name (com.microsoft.sqlserver.jdbc.SQLServerDriver) and JDBC URL (jdbc:sqlserver://localhost:1433;SelectMethod=cursor;DatabaseName=pacsdb). Using the Microsoft distributed driver may have some benefit, based on your usage, because it provides support for mirrored database deployments. Do your research and make your own decision, but the jTDS driver will get you up and running quickly with the least hassle.
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