no connection to OBD2

I have a 1991 camaro rs with a305 engine, my check engine light came on and flashed a few times before it shut down the engine. I connected my OBD2 scanner to the connection and the scanner said no connection. I checked the plugs to the scanner and the connection under the dash turn the key in the on positon and still its saying no connection. Can anyone tell me what I need to check on the car to get a connection to the scanner? I need help asap please.