Diddy Smiles, Chats with Fellow Inmates at Fort Dix

Diddy's just one of the boys behind bars ... laughing and chatting with other inmates in the yard in new photos from Fort Dix.

We've obtained pics of the star hanging out with a few dudes in the prison yard Saturday -- and, ya gotta check 'em out ... 'cause Diddy's got the biggest smile on his face.

Diddy's bundled up in a thick dark blue coat, gray pants and gray beanie ... shootin' the you-know-what with the other guys.

At one point, Diddy seems to be shaking hand with someone ... perhaps making a new friend. Regardless, all these guys seem interested just to chat with the guy -- no tension in the air whatsoever.

This is the second set of pics we've seen that show Diddy ... after we published a photo of the guy in prison yesterday where he appeared a bit more solemn in a bright orange beanie.

Diddy was assigned to Fort Dix earlier this week ... a major win for the mogul who asked to go there. It's a low-security prison in New Jersey where Diddy will serve his time after he was convicted on two counts of violating the Mann Act.

We hadn't seen a pic of Diddy in more than a year -- cameras weren't allowed in the courtroom during his federal trial -- and these photos gave us the first look at the rapper's graying beard.

These times in the yard are a nice break for the mogul who's already got a job in the joint ... a little R&R after grinding on laundry duty.