The Practice
When adding new signals to an existing SCADA system, there is often a choice: create new points or reuse old spare/decommissioned points. In many established facilities, the preference is to reuse spare points that already exist in the database.
Why Reuse
- License limits — some SCADA platforms are licensed by point count. Reusing existing points avoids exceeding the license or requiring an upgrade.
- Tested communication paths — a spare point already has its Modbus register, poll group, and communication path configured and proven. A new point requires setting all of this up and testing it.
- Historian continuity — if the SCADA historian has pre-configured slots for spare points, reusing them avoids restructuring the historian database.
How to Do It Right
- Verify the spare point is truly unused. Check the PLC program, the HMI displays, alarm lists, reports, and historian tags. A point labeled “spare” may still be referenced somewhere.
- Update the tag description and engineering units. Do not leave the old description in place — this will cause confusion during future troubleshooting.
- Update alarm limits to match the new signal. Inheriting alarm setpoints from the previous usage will generate false alarms or miss real ones.
- Update HMI displays to reflect the new tag name, description, and units.
- Document the change — record what the spare was previously used for and what it is now mapped to.
When to Create New Points
If no suitable spare exists in the correct register range, poll group, or data type — create a new point. Forcing a REAL value into an INT spare (or vice versa) to avoid creating a new point creates more problems than it solves.