Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

32 Guidelines for Tricon ControllersMaintenance OverridesThree methods can be used to check safety-critical devices connected tocontrollers:• Special switches are connected to inputs to a controller that deactivate theactuators and sensors undergoing maintenance. The maintenance conditionis handled in the logic of the control program.• Sensors and actuators are electrically disconnected from a controller andmanually checked using special measures.• Serial communication to a controller activates the maintenance overridecondition. This method is useful when space is limited and the maintenanceconsole should be integrated with the operator display.Using Serial CommunicationFor maintenance overrides, two options for serial connection are available:• DCS connection using Modbus RTU protocol (or another approved serialprotocol)• TriStation PC connection, which requires additional, industry-standardsafety measures in a controller to prevent downloading a program changeduring maintenance intervals. For more information on TriStation, seesection “Alarm Usage” on page 68.Tricon Safety Considerations Guide