The default retrofit question. You have a plant of Modbus devices, a new machine arrives speaking OPC UA, and someone has to decide what the site standard is.
| Modbus TCP | OPC UA | |
|---|---|---|
| Specification | Modicon Modbus, TCP encapsulation (Modbus-TCP) | IEC 62541 |
| First published | 1979 (serial), 1999 (TCP) | 2008 |
| Transport | TCP, port 502 | TCP binary on 4840, or HTTPS |
| Data model | Flat 16-bit registers and single-bit coils. No types, no names, no metadata — a register is a number at an address. | A full information model: typed nodes, references, objects, methods and historical access. The richest model in industrial automation. |
| Discovery | None. You need the vendor's register map, usually a PDF. | Built in. Browse the address space, read the type definitions. |
| Security | None in the base specification. Whatever the network gives you. | Built in and mandatory to configure: X.509 certificates, signing, encryption, user authentication. |
| Polling or subscription | Polling only. The client asks; the device answers. | Both. Polled reads and subscriptions with monitored items and dead-banding. |
| Implementation complexity | Lowest of any protocol here. A working client is ~200 lines. | High. The stack is large, certificate handling is a real operational task, and server implementations vary. |
| Typical industries | Everything. Drives, meters, inverters, sensors, PLCs, BESS. | Modern machine tools, robotics, packaging, anything specified after ~2015. |
| Forge support | Adapter shipped. 14 OEM families mapped. | Adapter shipped. 23 OEM families mapped. |
Strength. Universal, trivial to implement, works on hardware from 1985.
What it costs you. Carries no semantics whatsoever. Scaling, byte order and register meaning all live outside the protocol.
Modbus TCP adapter →Strength. Self-describing. You can walk a server you have never seen and learn what it holds.
What it costs you. Self-describing is not the same as standardised — two vendors model the same machine differently, so the names still need mapping.
OPC UA adapter →They are not competitors so much as different eras. Modbus gives you a number and no context; OPC UA gives you context and charges you a stack for it. Most real plants end up running both, which is precisely the situation that makes a normalization layer worth having rather than a nice-to-have.
Forge is not an argument for a protocol. It is the layer that makes the choice stop mattering downstream: 16,908 curated mappings across 14 protocol adapters, resolving to 366 canonical fields with enforced units and physics bounds.
Two devices, two protocols, two vendors, two entirely different tag names — and one canonical field out. Both of these are real mappings from the corpus, not illustrations.
Same field: feed_rate. Same unit contract, same physics
bounds, same shape. Your query does not need to know which protocol the reading
arrived on.