BECKHOFF EL1904 - Manual (Page 13)

System description
EL1904 11
TwinSAFE digital input (EL1904) and output terminal (EL2904)
 All current safety sensors can be connected
 Operation with a TwinSAFE logic terminal
 EL1904 with 4 fail-safe inputs for sensors (24 V
DC
) with floating contacts
 EL2904 with four safe channels for actuators (24 V
DC
, 0.5 A per channel)
 Conforming to IEC 61508:2010 SIL 3 and DIN EN ISO 13849-1:2006 (Cat 4, PL e) requirements.
2.2.3 EL1904, EL2904 – Bus Terminals with 4 fail-safe inputs or outputs
The EL1904 and EL2904 Bus Terminals enable connection of common safety sensors and actuators.
They are operated with the EL6900 TwinSAFE logic terminal. The TwinSAFE logic terminal is the link unit
between the TwinSAFE input and output terminals. It enables the configuration of a simple, flexible and
cost-effective decentralized safety control system.
Therefore, there are no safety requirements for the higher-level controller! The typical safety functions
required for the automation of machines, such as emergency stop, protective door, two-hand etc., are
already permanently programmed in the EL6900. The user configures the EL6900 terminal according to
the safety requirements of his application.
2.2.4 EL6900 - TwinSAFE logic terminal
The TwinSAFE logic terminal is the link unit between the TwinSAFE input and output terminals. The
EL6900 meets the requirements of IEC 61508:2010 SIL 3, EN 954 Cat. 4 and DIN EN ISO 13849-1:2006
(Cat 4, PL e).
2.2.5 The fail-safe principle (Fail Stop)
The basic rule for a safety system such as TwinSAFE is that failure of a part, a system component or the
overall system must never lead to a dangerous condition. The safe state is always the switched off and
wattless state.