SCADA talks to RTUs over a wired/wireless link using an agreed protocol (e.g., Modbus, DNP3, IEC 60870-5-104). Tags in SCADA map to RTU registers/points. Data is acquired cyclically (polling) and/or event-driven, then archived, visualized, alarmed, and used for control.
1) Typical Topology
Layers:
Field sensors & actuators — transmit process values (AI), states (DI), and receive outputs (AO/DO).
RTU — aggregates I/O, timestamps events, queues data; speaks one or more protocols.
Comms network — RS-485, Ethernet/TCP/IP, 4G/5G, microwave, fiber, or satellite.
SCADA server — polling engine, alarm server, historian/DB; HMI clients visualize data.
2) Communication Channels (Physical Layer)
Wired: RS-232/RS-485 for short-range serial; Ethernet for LAN/WAN.
Wireless: Licensed/unlicensed radio, 4G/5G, microwave; satellite for remote assets.
Optical fiber: Long distance, EMI-immune, high bandwidth.
Design tips: ensure surge/lighting protection, ground properly, and size link bandwidth vs. scan rate.
3) Protocols (Application Layer)
Modbus RTU / TCP: simple polling, ubiquitous in water, oil & gas, and manufacturing.
DNP3: time-stamped events, report-by-exception; favored in power/water utilities.
IEC 60870-5-101/104: European/Asia utilities standard; good for large telemetry fleets.
OPC UA/DA: middleware for SCADA-to-server/app integration.
Rule of thumb: choose the protocol your RTU and SCADA both natively support; prefer time-stamped, buffered protocols when links are lossy.
4) Tag Mapping (Point List)
Create a Tag List that maps SCADA tags to RTU addresses and data types.
| Tag Name | Description | RTU Address | Type | Scale/Units | Alarm |
|---|---|---|---|---|---|
| FT101_Flow | Instantaneous flow | 40001 | Holding Reg (16-bit) | x0.1 m³/h | HiHi 1200 |
| PT102_Press | Line pressure | 40002 | Holding Reg | x0.01 kPa | Low 180 |
| MV201_Open | Valve status | 00001 | Coil | — | — |
| TT301_Alm | Temp alarm | 10005 | Discrete Input | — | Latched |
Best practice: keep a master spreadsheet under version control; include engineering ranges, scaling, and alarm deadbands.
5) Polling vs. Event-Driven Acquisition
Cyclic polling: SCADA reads a block of registers every N seconds (e.g., 1–10 s). Simple and predictable.
Report-by-exception (events): RTU pushes time-stamped changes (SOE) to SCADA (e.g., DNP3). Reduces bandwidth and improves sequence accuracy.
Hybrid: poll analogs periodically; get digitals via events.
6) Time, Quality, and Buffering
Time sync: Use NTP or protocol-based time sync so SOE timestamps are trustworthy.
Quality flags: Good/Bad/Uncertain indicators propagate to trends and alarms.
Store-and-forward: Ensure RTUs buffer data during outages and forward later to avoid gaps.
7) Security & Reliability Essentials
Network segmentation: Field network → (optional) edge gateway → DMZ → SCADA.
Encryption & auth: VPN/IPsec, TLS where available; strong credentials per site.
Least privilege: Separate view/control roles; multi-factor for remote access.
Hardening: Disable unused services, lock protocol write functions if not needed.
Resilience: Redundant comm links for critical stations; watchdogs and local fallback logic.
8) Commissioning Checklist
Addressing: RTU ID/IP, port, unit IDs set; no collisions.
Protocol match: Function codes, object groups, class scans configured.
Scaling: Engineering units verified against a handheld calibrator.
Alarms: Set points and deadbands agreed; nuisance alarm test.
Trends & reports: Historian tags named and retained per policy.
Failover: Simulate comm loss; confirm data buffering and alarm behavior.
Cyber: Change default passwords, verify VPNs, back up configs.
9) Troubleshooting Fast Paths
No data: Check link LEDs, serial parameters (baud/parity/stop bits), firewalls/NAT.
Garbled values: Endianness, word/byte order, wrong register base (0/1 offset).
Polling timeouts: Reduce block size, increase timeout/retries, check bandwidth.
Missing events: Confirm event classes enabled and unsolicited messaging allowed.
10) When to Use Gateways
Introduce a protocol gateway if the RTU speaks one protocol (e.g., Modbus) and SCADA requires another (e.g., IEC 104/OPC UA). Keep the point count small per transaction and monitor gateway CPU/memory.
Conclusion
A robust SCADA–RTU setup is all about clean point mapping, the right protocol, disciplined timing, and secure, resilient networking. Start with a clear tag list, pick polling/event strategies per signal type, and harden the path end-to-end.