Remote Terminal Unit (RTU) Guidance
Remote terminal units are an important part of any remote monitoring system or SCADA system, but they can seem fairly complicated if you're unfamiliar. What you need is a quick guide that will teach you the key elements of remote terminal units.
This guide will teach you the basics of remote terminal units (RTUs) in about 6 minutes. You'll learn about the standard features that just about every RTU has, and you'll also discover handy features that only the better units include.
Basic terminology for understanding remote terminal units:
- Discrete alarm inputs / Contact closure inputs / Dry contact inputs
These are the most basic way for equipment to notify your RTU electrical equipment of a problem. To use discrete inputs, you'll connect a pair of wires from the dry contact on your equipment to one of your RTU's inputs (RTU inputs can be either common-ground or isolated/individually ). One input pin on your RTU is constantly outputting 5vDC, while the other is looking for this voltage. When your equipment has a problem (such as a LAN failure), it will "latch" the appropriate relay. This simply means that it will connect the circuit between its two pins. This allows the 5vDC from your remote terminal unit to reach the other pin. This will trigger an alarm on the unit. You'll have pre-programmed a description of the problem ("switch LAN failure") that will appear on your monitoring interface. As you can see, this is a very basic (and therefore less subject to malfunction) method of communication. It's impossible to misinterpret a protocol, since no protocol is used at all.
- Analog voltage/current inputs
Of course, not everything can be measured with a binary (yes/no) value. Some things (like temperature, humidity, battery voltage, tank levels) require a more precise numerical/percentage value. To monitor continuous values, you need an input that can measure a fairly exact voltage or current flow through its pins. Two industry standards have developed for analog inputs: voltage and current. The voltage standard is 0-5v DC. The current flow standard is 4-20mA. Most external sensors that you can purchase will output one or the other of these two standards. Unlike a dry contact, where the voltage is supplied by the remote terminal unit and merely routed through the monitored device's latched relay, the sensor is continuously outputting a voltage or current that reflects the monitored value (ex. Temperature). Sensor manufacturers will give you at least 2 reference values to understand sensor output. With a good RTU, you can input these values. Then, your remote terminal unit will translate relatively meaningless voltage/current into instantly understandable sensor readings.
- Control relay inputs and outputs
Of course, the role of a remote terminal unit extends beyond simply monitoring remote equipment. Sometimes, you need to use the information gathered through remote monitoring to execute a response with control relay outputs. Control relays are the counterparts of discrete alarm inputs. Instead of detecting binary (on/off) conditions, they issue binary (on/off) commands. You can wire control relay outputs to all sorts of equipment. You might use relays to activate generators, unlock doors, activate obstruction lights, raise/lower an antenna, or just about anything else that involves pushing a button or flipping a switch. Using control relays simply (and beautifully) means that you don't have to drive out to a remote site (sometimes for hours each way) just to press a reset button, flip a switch, or turn something on, effectively using the RTU switches as regular physical switches.
- Serial terminal server
If you're like most companies, your network has a lot of different equipment from different technological generations. Some is modern and LAN-based, while other communicates only via serial (RS232/RS485) connection. Even modern equipment will commonly have a more traditional connection like serial available for local diagnosis. When you have equipment that can (or must) use serial connectivity, a remote terminal unit with a serial terminal server provides a way to access that equipment remotely over LAN. Your properly equipped remote terminal unit will act as an intermediary between your LAN and your RS232/RS485 equipment. You'll be able to access the equipment from your desk.
- Dual power inputs
RTUs that only have one power input can fail when the power goes out (exactly when you need them to be working). Dual redundant power inputs mean that you can wire your RTU into 2 power sources. As long as either one is available, your RTU will remain powered.