Siemens Enhanced Driver S7 1500 loses all communication when an INT tag value drops below 64

Ignition
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Environment

  • Ignition 8.3.6
  • Siemens Enhanced driver — Symbolic addressing
  • PLC: Siemens S7-1500 — firmware 4.0
  • Affected tag: an INT written by the PLC, inside a UDT instance, read symbolically from an optimized DB (it's a "card number" used in a PLC↔Ignition handshake)

What happens

Whenever that INT tag holds a value below 64, the entire device connection stops communicating. Every tag on the device goes Bad and the connection reports as defined-but-not-established. There is no Gateway log entry, the tags just go Bad with the error "Communication with the datasource is defined but not established".
The behavior is:

  • Value-dependent — only values < 64 trigger it. >= 64 is always fine.
  • Total — it's the whole device that drops, not just that one tag.
  • Self-clearing — I don't restart anything. The moment the PLC writes a value >= 64 again, communication comes back on its own.
  • It's clearly below the script/DB layer: tag event scripts never fire during the fault (nothing in the gateway logs), because no value updates come through at all.

Steps to reproduce

  1. Symbolic-addressing connection to an S7-1500, INT tag in an optimized DB.
  2. Have the PLC write a value < 64 into that tag.
  3. The whole device goes Bad / loses communication.
  4. Write a value >= 64 → communication is restored automatically.

Expected vs actual

  • Expected: the value held by a tag should never influence device communication.
  • Actual: values < 64 reliably take down the entire device connection until the value goes back up.

I'm only using symbolic addressing, so I can't compare against absolute/legacy addressing — I'd be curious whether anyone sees this only with symbolic. Optimized writes is ON.

Workaround (currently stable)

On the PLC I add 100 to the number before writing it to the tag, and I subtract 100 in Ignition after reading. The on-wire value is therefore always >= 100, and the problem hasn't recurred since.

Questions

  • Has anyone hit this with Symbolic addressing on 8.3.x?
  • Is there something about INT values < 64 in symbolic reads/writes I'm missing, or is this a known driver issue?
  • What logs / device diagnostics should I capture to turn this into a proper ticket? Happy to provide gateway logs and a packet capture.
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Open a support ticket with this information and let IA ask for whatever else they need. Include a link back to this forum topic.