When a family is told that their loved one is going to be sent to the State Penitentiary they frequently think that means he is being sent to Huntsville. Actually there are a several different State jails that he could be sent to. The Texas Department of Criminal Justice has set up an inmate locator website to help you find the person you are looking for.
Aside from a name search the site has two different numbers you can use. The TDCJ number is the number that has been assigned to an inmate by the Texas Department of Criminal Justice. They use that number the same way Bexar County uses a sid number. When TDCJ asks for a sid number on their locator site they are referring to what everyone else calls a DPS number.