This European Standard is applicable to safety-related electronic systems using for digital communication purposes a transmission system which was not necessarily designed for safety-related applications and which is– under the control of the designer and fixed during the lifetime, or– partly unknown or not fixed, however unauthorised access can be excluded, or– not under the control of the designer, and also unauthorised access has to be considered.Both safety-related equipment and non safety-related equipment can be connected to the transmission system.This standard gives the basic requirements needed to achieve safety-related communication between safety