Fail-safe input module for ET200S, 30 mm, SIL 3
The Siemens 6ES7138-4FA03-0AB0 is a fail-safe digital input module for the ET200S distributed I/O system, delivering 8 inputs configurable as 8 single-channel or 4 two-channel with PROFIsafe communication. Rated SIL 2 (single-channel) and SIL 3 (two-channel) per IEC 61508, and Category 3 (single-channel) / Category 4 (two-channel) per EN 954 — the two-channel wiring path is what gets you to the highest safety integrity for emergency-stop and light-curtain circuits. The module draws 300 mA from the backplane bus and dissipates 4 W typ — within the ET200S power budget for a 30 mm slice in a mixed I/O rack.
Input characteristics and wiring constraints
Signal '1' threshold is +15 to +30 V with a typical sink current of 3.7 mA; signal '0' is -30 to +5 V. The module does not accept 2-wire proximity sensors — it expects a separate 24 V DC supply for the field loop. Input filtering is parameterizable: the '0' to '1' transition can be set between 0.3 ms and 17 ms, same for '1' to '0'. For a fast safety reaction (e.g., light curtain), set the minimum debounce; for noisy environments, the 17 ms filter avoids nuisance trips. Maximum cable length is 200 m for both shielded and unshielded runs — the limiting factor is the voltage drop on the 24 V supply, not the signal integrity. Each output channel is electronically short-circuit protected with a response threshold between 0.7 A and 1.8 A — it will trip before the backplane fuse blows.
Physical integration and diagnostics
Diagnostics are reported via the group error SF (red) LED and the backplane bus — the module signals a group fault to the safety CPU, which then identifies the failing channel through PROFIsafe telegrams. Channel-to-channel isolation is present between channels and the backplane bus, but not between channels and load voltage L+ — so a short from L+ to a channel input will not propagate to adjacent channels.