Your SCADA RTUs need to communicate with all your on-site equipment and survive under the harsh conditions of an industrial environment. Here's a checklist of things you should expect from a quality RTU:
Rugged construction and ability to withstand extremes of temperature and humidity. You know how punishing on equipment your sites can be. Keep in mind that your SCADA system needs to be the most reliable element in your facility.
Secure, redundant power supply. You need your SCADA system up and working 24/7, no excuses. Your RTU (Remote Telemetry Unit) should support battery power and, ideally, two power inputs.
Redundant communication ports. Network connectivity is as important to SCADA operations as a power supply. A secondary serial port or internal modem will keep your RTU (Remote Telemetry Unit) online even if the LAN fails. Plus, RTUs with multiple communication ports easily support a LAN migration strategy.
Nonvolatile memory (NVRAM) for storing software and/or firmware. NVRAM retains data even when power is lost. New firmware can be easily downloaded to NVRAM storage, often over LAN - so you can keep your RTUs' capabilities up to date without excessive site visits.
Intelligent control. As I noted above, sophisticated SCADA remotes can control local systems by themselves according to programmed responses to sensor inputs. This isn't necessary for every application, but it does come in handy for some users.
Real-time clock for accurate date/time stamping of reports.
Watchdog timer to ensure that the RTU (Remote Telemetry Unit) restarts after a power failure.