Two Environments, One Device
An online dissolved gas analysis (DGA) monitor must interface with two very different environments: traditional substations, where point-to-point telemetry dominates, and digital substations, where the IEC 61850 standard governs everything. Getting the protocol stack right is what turns measurement data into actionable telemetry for SCADA, asset performance management, and control-room displays. This article maps the mainstream protocols — MODBUS RTU/TCP, IEC 61850 (MMS/GOOSE), IEC 60870-5-104, DNP3.0, RS485/RS232, and MQTT — to the scenarios where each belongs.
Protocol Selection by Scenario
| Application scenario | Recommended protocol | Notes |
|---|---|---|
| Intra-station integration, traditional stations | MODBUS RTU/TCP | Most widely deployed; simple to implement; flexible point-table mapping |
| Dispatch / central-control uplink | IEC 60870-5-104 or DNP3.0 | 104 is the grid telecontrol protocol over TCP; DNP3 is widely used in North America |
| Digital substation | IEC 61850 MMS/GOOSE | Device connects as an IED at station-control level; GOOSE supports fast interlocking |
| Fieldbus | RS485 / RS232 | RS485 supports a multi-node bus (1 Mbps, up to 32 nodes) |
| Cloud platform / IIoT | MQTT | Lightweight publish/subscribe, suited to cloud and mobile endpoints |
MODBUS RTU/TCP: The Workhorse
For intra-station integration in traditional substations, MODBUS RTU/TCP remains the default. It is the most widely deployed industrial protocol, simple to implement, and its point-table mapping makes it easy to expose each gas concentration, moisture value, and device status as a telemetry point. Its strengths are ubiquity and flexibility; its limits are the absence of standardized semantics — the meaning of each register lives in the point table, so master-data discipline matters.
IEC 61850 MMS/GOOSE: The Digital Substation IED Role
IEC 61850 is the core standard for digital substation communication. MMS (Manufacturing Message Specification) is oriented to client/server interaction at the station-control level; GOOSE is oriented to fast tripping and interlocking. When an online DGA monitor connects at the station-control level, it acts as an IED (intelligent electronic device), and its measurement data can be read uniformly by station monitoring and fault-recording systems. This is the integration path for digital substations, and it removes the ambiguity of vendor-specific point tables.
Telecontrol Uplink: IEC 60870-5-104 and DNP3.0
When data must leave the station for dispatch or central control, the choice is usually between IEC 60870-5-104 and DNP3.0. IEC 104 is the grid telecontrol protocol carried over TCP and is the norm in Europe and much of Asia; DNP3 is widely deployed in North America. Both support the telemetry, telesignaling, and time-tagged event reporting that operators expect from protective and monitoring equipment.
RS485 and MQTT: Fieldbus and Cloud
At the physical layer, RS485 supports a multi-node bus — up to 32 nodes at 1 Mbps — which suits short, industrial cabling runs inside a substation. On the other end of the spectrum, MQTT is a lightweight publish/subscribe protocol designed for cloud and IIoT endpoints, making it the natural choice for feeding group-level or cloud platforms and mobile push alerts, especially on unmanned sites.
Master-Data Consistency and Time Sync
The decisive factor in integration quality is usually not the protocol itself but the consistency of master data. Equipment ledgers, channel point tables, range and unit conversion, and time synchronization (NTP/SNTP) must be managed uniformly; otherwise you get “protocols connected but data mismatched,” which causes substantial rework later. A monitor that stamps every value with a consistent timestamp and transmits through a well-governed point table makes the rest of the integration straightforward.
The 4-20 mA Analog Backup and the Protocol Stack
Older integrations sometimes rely on 4-20 mA analog uplinks to carry a gas reading as a single current loop. The limitation is structural: one loop carries one channel, with no diagnostics, no time tag, and no fault-type context. For a multi-gas monitor that is a poor fit. In practice, 4-20 mA has been progressively replaced by digital protocols even in retrofit projects, which is why a modern DGA monitor treats analog output as a backup for a single critical channel rather than the primary integration path.
In practice, most projects do not pick one protocol — they specify a protocol stack. A typical stack pairs MODBUS RTU/TCP for intra-station integration, IEC 104 or DNP3 for the dispatch uplink, and MQTT for cloud or mobile endpoints. Specifying the stack up front, together with the point table and time-sync method, is what makes the integration contract testable and avoids rework at commissioning.
PAS DGA for online DGA monitoring
Commercial L-PAS devices commonly support MODBUS RTU/TCP, IEC 61850 MMS/GOOSE, IEC 60870-5-104, DNP3.0, and RS485 — covering both digital and traditional substation scenarios. The DGA-900 carries this protocol stack, making it suitable for everything from a legacy MODBUS RTU retrofit to a fully digital IEC 61850 substation. For context on where these protocols fit the monitoring architecture, see our DGA fundamentals and the standards comparison IEC 60599 vs IEEE C57.104.
To confirm protocol support and integration scope for your project, contact PAS DGA.