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Module 13Integration & Real Systems11 min.md

Gateways & Mixed Networks

▮ Overview

What you'll learn

TCP↔RTU routing, unit IDs, latency, and how to keep mixed deployments diagnosable.

Sections
11
Labs
1
Quiz
9 Qs
What you'll be able to do
  • Route a TCP request to a serial server using the Unit ID.
  • Predict gateway-induced latency and exception behavior.
  • Diagnose 'works in dev, fails through the gateway' problems.
Why you'll need this
  • "Your SCADA polls Unit ID 5 over TCP; the gateway holds three RTU buses. How do you confirm the routing is right?"
Three things people get wrong
  1. 1.
    Assuming the gateway forwards exceptions verbatim
    Fix Some gateways synthesize their own exceptions on timeout. The exception code may not match the device's behavior.
  2. 2.
    Polling faster than the slowest serial bus
    Fix The serial side has a max throughput. Over-polling causes queueing and gateway-side timeouts.
  3. 3.
    Reusing Unit ID 1 for two devices
    Fix Each device behind the gateway needs a unique Unit ID — this is the gateway's only addressing handle.
From the field

The gateway with a 50 ms quiet window

A perfectly working integration started timing out after a firmware bump. The new gateway enforced a 50 ms gap between RTU polls; the SCADA was issuing back-to-back reads. Spacing the polls fixed it; the lesson was to always profile the actual round-trip after a gateway change.

Cited sources

Primary sources come from protocol and standards publishers. Secondary sources provide supporting tool, vendor, or reference context.

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