An HSMS Reply's Session ID Is Not Always the One You Sent
Select.rsp and Linktest.rsp carry the Session ID the tool chose, not the one you sent. Data replies echo it. Three real HSMS captures.
Industrial Knowledge Inventory
SCADA
Things
Practical notes for host and MES developers who need to understand SECS/GEM and HSMS before they have equipment to test against.
191 entries indexed · EN / KO · updated continuously
Latest entry Online
/SECS/GEM/8 min read
An S2F0 Doesn't Tell You Why: Unimplemented Function or Closed Communication Gate
Two captures from one HSMS session show S2F23 aborted twice for different reasons, and the reply bytes differ only in SystemBytes.
Continue reading →
Latest Entries
All entries →The Tool Acked the S5F1 Your Host Should Never Have Sent
/SECS/GEM/8 min read
Alarm integration passes every test and no alarm ever arrives. A real capture: S5F1 sent by the host draws ACKC5 0, and S10F3 draws an abort.
GEM Is Not GEM300: Which SECS Streams E87, E40 and E90 Actually Use
/SECS/GEM/9 min read
One socket, five primaries: S1F13 answered, S3F17, S14F1 and S16F11 all came back SxF0. What SECS/GEM support does and does not include.
HSMS Linktest Timeout While Data Still Flows: Close the Socket Anyway
/SECS/GEM/11 min read
A swallowed Linktest.req burns T6 while an S1F1 on the same socket answers with a full S1F2 in 0.3 ms, and the host should close the connection anyway.
An S1F4 Carries No SVIDs — Get the Names From S1F11 First
/SECS/GEM/11 min read
An S1F4 body carries no SVIDs at all. Real S1F11/S1F12 bytes showing where a host learns SVID numbers and names, and how S2F29 differs.
Your HSMS Session Is SELECTED and S1F3 Still Comes Back as S1F0
/SECS/GEM/11 min read
SELECTED is E37, COMMUNICATING is E30. Real bytes showing an S1F3 sent before S1F13/S1F14 aborted as S1F0, and the same S1F3 answered after.
Reading Equipment Constants From the Host: S2F29 for the Namelist, S2F13 for the Values
/SECS/GEM/12 min read
You asked S2F13 for one ECID and S2F14 came back with two values. Real S2F29 and S2F13 bytes, and the length check host code must run.
Field Sections
Start here§01→§02→§03→
SCADA Basics
Core architecture, responsibilities, and vocabulary for HMI/SCADA projects.
Industrial Protocols
Practical notes on Modbus, OPC UA, MQTT, SECS/GEM, and network troubleshooting.
Project Checklists
Templates for naming, alarms, historian planning, commissioning, and field troubleshooting.
Debug without equipment
Reproduce a host-side SECS/GEM problem in your browser.
Explore SECS/GEM message exchanges in the shared browser demo, then compare them with a captured Select failure. It gives you a practical starting point before you have equipment to test against.
What this is
A public technical notebook: definitions, design notes, protocol reminders, checklists, and lessons learned from real industrial automation work. Clarity over marketing, useful references over generic content.