Why will my code reader not work in my 2000 chevy S10's OBD port, but it works in my other GM vehicles ?

I just purchased a 2000 Chevy S10 and getting it ready for state inspection. We only have emissions testing through the OBD port. I plugged my code reader in and nothing happens. I have two other vehicles that are right around the same year and are both Chevy trucks also. The reader works fine on them. Any help would be greatly appreciated. Thanks