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The clients Active Drains contained a legacy GEM80-130 PLC, now obsolete and requiring replacement. Although there was an upgrade path to GEM80-500, there were a number of issues identified as follows:

  • The lead time on the GEM 500s.
  • The battery life within the GEM80-500 is 10 years and requires return to factory for replacement.
  • As Converteam had been acquired by GE, a ‘Nuclear Risk Evaluation’ is required for each GE component to be supplied to the nuclear industry, whether for new systems or for spares. GE has already started to withdraw manufacture of certain GEM80 components raising the question of the long term support for GEM80 series.

Servelec Controls had been involved in many of the clients control systems including the DMS Replacement, Pond Water Treatment Plant, AETP and Active Drains. Servelec Controls proposed a migration to a Siemens S7-300 solution as used elsewhere by the client.

The proposed solution addressed both hardware and software migration. Migration technology was used to mitigate risk, reduce testing and provide a long term upgradeable solution.

Hardware Migration

The picture following shows the original GEM80 rack comprising of four 6U chassis. Each chassis contains the I/O cards with the top rack also containing the main GEM80 PLC. The I/O cards were connected to the field I/O terminals using standard GEM80 front connectors.

The objective was to create a hardware solution that would minimise site installation times both in terms of changeover and testing. The solution involved creating a 6U chassis with the top 3U being populated by the Siemens’ modules and the bottom 3U containing Servelec Controls’ specially designed I/O cards to accept the existing GEM80 plant connectors.

This solution allowed the GEM80 I/O connectors to be disconnected, the 6U racks to be swapped and the GEM80 I/O connectors to be plugged in to the new rack. The advantages of this solution were as follows:

  • Installation time was kept to a minimum
  • All loops between the Servelec cards and the Siemens’ kit could be fully pre-tested
  • Minimal disruption to plant wiring with plug and play swap over
  • Low Risk Changeover strategy with a roll-back position possible

Software Migration

Once decided that the target platform was a Siemens S7 PLC, a further decision was required. This was whether a full rewrite in the Step 7 environment or a migration process using the Siemens Ladder Logic environment was to be used. To minimise risk, it was decided to use the Ladder Logic migration option was chosen.

Prior to the software migration being undertaken, the structure of the PLC Code was analysed to determine the Control sequences, Hardware Interfaces and Computer Interfaces.

The functionality of the GEM80 PLC was also analysed to determine behaviour under fault conditions, such as hardware faults and recovery from power loss.

The existing PLC functionality (along with a number of Station requested improvements) was migrated to the new S7 environment rung by rung to ensure the software did not deviate from the approved working operational code.

A manual documented verification process was performed independently from the implementer to check that every rung had been migrated correctly and no errors introduced.

In addition to the improvements requested by the client, Servelec Controls also provided a number of additional enhancements available within the Siemens’ technology platform such as:

  • Hardware diagnostics
  • “Hot swappable” modules
  • “Self-healing” Profinet I/O network, used to connect the I/O racks




The SCADA system was updated from an obsolete bespoke computer system to a standard workstation running Siemens’ WinCC SCADA application. The operator interface was re-designed and based on a standard approach previously provided by Servelec Controls during the Pond Water Treatment Plant upgrade. The objective was to create a common user interface used on both systems to reduce operator training and increase familiarity.

The new SCADA offered a number of additional features such as:

  • Historical Alarm and Event Logging
  • Improved Graphical Displays
  • Trend facilities

This flexible platform allowed future enhancements to be easily integrated, including:

  • Detailed diagnostics (down to channel level)
  • Straight forward Mimic modifications
  • Open connectivity for additional I/O, HMIs or interfaces into other systems

GEM80 PLC Replacement - Removing the burden of replacing a legacy system

Servelec Controls has in depth experience working with such legacy systems and using migration technology to mitigate risk, reduce testing and provide a long-term upgradeable solution.


Read our capability profile here

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