The MD485 RS-485 Multidrop Interface is a general communication interface or serial converter. Additionally, the MD485 can be used without a datalogger and offers collision avoidance for PakBus communications.
The SDM-SIO1 Serial Input/Output Module is designed for acquiring data from smart serial sensors, rather than for general communications like data collection or program downloads. The SDM-SIO1 can operate in half- and full-duplex modes, has a smaller form factor, works with RS-422, and can be daisy-chained in quantities of up to 15 on a single three-wire SDM port.