What is a master terminal unit?
A master terminal unit (MTU), also called the SCADA master station or SCADA server, is the central computer in a SCADA (supervisory control and data acquisition) system. SCADA systems monitor and control spread-out industrial processes and infrastructure, such as power grids, water and wastewater systems, pipelines and manufacturing plants. The MTU communicates with the devices in the field, processes and stores the data they send, and presents it to operators.
Not to be confused with MTU as in maximum transmission unit, the largest packet a network link carries (1500 bytes on standard Ethernet).
SCADA components
| Component | Role |
|---|---|
| MTU (master terminal unit) | Central server: polls field devices, logs data, raises alarms, sends commands |
| HMI (human-machine interface) | The operators' screens: process diagrams, live values, alarms and controls |
| RTU (remote terminal unit) | A device at a remote site that reads sensors, operates equipment and reports to the MTU |
| PLC (programmable logic controller) | A rugged controller that runs local control logic and is often polled like an RTU |
| Field devices | Sensors (pressure, flow, temperature) and actuators (valves, pumps, breakers) |
| Communications links | Connect the MTU to the field: radio, cellular, leased lines, fiber or Ethernet |
How the MTU works
- The MTU polls each RTU or PLC on a schedule using an industrial protocol such as Modbus or DNP3. Some protocols also let field devices report changes on their own.
- It stores the readings, often in a historian database, and checks them against alarm limits.
- Operators watch the process on HMI screens and issue commands, such as opening a valve or changing a setpoint.
- The MTU sends each command to the right field device, which carries it out.
Fast, moment-to-moment control usually runs locally on the PLCs and RTUs; the MTU supervises. Because the whole system depends on it, the MTU is often deployed as a redundant pair of servers.
Security concerns
SCADA systems often stay in service for many years on legacy operating systems that are hard to patch, and many industrial protocols were designed for closed networks, with no authentication or encryption. A compromised MTU could let an attacker falsify readings or send commands to physical equipment. Common protections include separating the SCADA network from the business network with firewalls, tightly limiting remote access, and monitoring for unusual traffic.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.3 lists legacy and embedded systems, naming SCADA, alongside the server roles. MTU isn't on the Core 1 acronym list, but know the SCADA pieces: the MTU is the central server, RTUs and PLCs work in the field, and operators use the HMI. Also know why these often-legacy systems are kept isolated from other networks.