5.2.5 If the operation of a key is able to cause dangerous operating conditions, measures should be taken to prevent the instruction in question from being executed by a single action such as：
- use of a special key lock,
- use of two or more keys.
5.2.6 Conflicting control interventions should be prevented by means of interlocks or warnings. The active control status should be recognizable.
5.2.7 The operation of input devices should be logical and correspond to the direction of action of the controlled equipment.
5.3 Output devices
5.3.1 The size, color and density of text and graphic information displayed on a visual display unit should be such that it may be easily read from the normal operator position under all operational lighting conditions. The brightness and contrast should be capable of being adjusted to the prevailing ambient conditions.
5.3.2 Information should be displayed in a logical priority.
5.3.3 If alarm messages are displayed on color monitors, the distinctions in the alarm status should be ensured even in the event of failure of a primary color.
5.4 Graphical user interface
5.4.1 Information should be presented clearly and intelligibly according to its functional significance and association. Screen contents should be logically structured and their representation should be restricted to the data which is directly relevant for the user.
5.4.2 When using general purpose graphical user interfaces, only the functions necessary for the respective process should be available.
5.4.3 Alarms should be visually and audibly presented with priority over other information in every operating mode of the system; they should be clearly distinguishable from other information.
5.4.4 All display and control functions in control stations operated by the same operators should adopt a consistent user interface. Particular attention should be paid to：
- symbols;
- colors;
- controls;
- information priorities;
- layout.
