The MR-52-S3 SIO is a versatile 2 door, 4 reader subpanel. The MR-52-S3 enables simple system expansion with Mercury Controllers via RS-485 wiring. The MR-52-S3 offers an attractive expansion solution due to its versatility, reliability, and cost efficiency. The auxiliary inputs and outputs provide expanded flexibility with simplified point control and monitoring. The MR-52-S3 is the direct replacement to the Series 2 MR-52 and uses the identical footprint and interface as the Series 2 MR-52. This allows panels to seamlessly be upgraded in existing deployments. Panels upgraded from Series 2 to Series 3 will not require any software reprogramming. The commitment to interoperability simplifies the steps to upgrade software and prevents issues in the field. The Series 3 subpanels maintain compatibility with older EP Series controllers and SCP models. The MR-52-S3 supports a variety of reader technologies including Wiegand, F/2F, and OSDP with Secure Channel. Using OSDP Multi-drop technology allows 2 readers to me attached to a single reader port. The layout of the board provides flexibility with 4 auxiliary inputs and 4 auxiliary outputs allowing for a variety of security devices to easily connected. The auxiliary components are programmable for simplified automation and programmable supervised inputs provide an additional layer of security.