The Saturn control pad is a very simple beast. Two 74153 chips are combined into a custom Sega chip. Two Select lines are used to poll for four groups of four buttons. If you want to make your own Saturn controller you need only shove two 74153 chips into it, and wire them according to this diagram.
Note: If a nonstandard controller is attached the pinout below may not be accurate. This pinout is only accurate for Saturn pads, sticks and twin-sticks. The Saturn port has many functions and uses. MegaDrive controllers (including the 3- and 6-button pads, mouse and TeamTap) as well as Saturn pads, keyboards and guns can be used on each port. While the SMPC (System Manager + Peripheral Control) chip automatically handles all the communication for these devices, it DOES tell the software what kind of device is attached, and these devices are not guaranteed to work in-game even though the hardware supports them. The pinout for connecting a MegaDrive controller to a Saturn is not known, but Sega's documentation says they are supported with this port. If you have one of these controllers please let me know!
Information for this page was taken from two sources: Sega's SMPC User's Manual, and Kashi's page.