Great question—let’s break this down based on standard industrial automation architectures, since there’s no one-size-fits-all answer but most setups follow consistent patterns.

1. Is an OPC Historian deployed between SCADA and an OPC UA Server? #

In almost all standard configurations: No.

OPC Historians are built specifically for storing historical time-series data, not acting as middleware or a proxy between SCADA and OPC UA servers. Typically, the Historian connects directly to either the OPC UA Server or the SCADA system as a separate client to pull and archive historical data. It doesn’t sit in the real-time communication path between SCADA and OPC UA—inserting it there would add unnecessary latency to real-time operations, which defeats the core purposes of both SCADA (real-time monitoring/control) and OPC UA (standardized, efficient data exchange).

A common setup might look like:

• OPC UA Server ↔ SCADA (real-time control/monitoring)
• OPC UA Server ↔ OPC Historian (historical data archiving)
  Or:
• PLC ↔ SCADA ↔ OPC UA Server
• SCADA ↔ OPC Historian

2. Is there an OPC Historian between PLC/SCADA and an OPC UA Server when they establish a connection? #

Again, in standard deployments: No.

When a PLC or SCADA connects to an OPC UA Server, this is a real-time, bidirectional link for operational data (like tag values, device statuses, control commands). OPC Historians don’t participate in this direct connection flow—they operate independently, usually polling or subscribing to historical data streams after the real-time data has been exchanged between the PLC/SCADA and OPC UA Server.

The only exception would be a highly customized, non-standard architecture where someone repurposes a Historian as a data proxy, but this is extremely rare and not recommended. It would introduce avoidable latency, complicate troubleshooting, and go against the intended design of both OPC UA and Historian systems.

内容的提问来源于stack exchange，提问作者jik177