Product Overview

The 6ES5431-8MC11​ is a SIMATIC S5-431 series digital input module from Siemens, configured as an 8-channel, 115 V AC/DC, floating (opto-isolated) input module in the compact “Version M” mechanical form. It belongs to the S5 compact / distributed-I/O branch rather than the S5-115U central-rack line we covered with the 420/430 entries — this distinction matters. Where the 6ES5420-4UA11​ and 6ES5430-4UA14​ sit in a S5-115U/135U central rack as 32×24 V DC modules, the 6ES5431-8MC11​ is the narrower “M” form factor designed for the S5-90U, S5-95U, S5-100U central racks and for ET 100U / ET 200U distributed I/O stations hung off an IM (interface module) link. The “431” family is Siemens’ AC-input DI branch inside S5: 431-8MC11 = 8 × 115 V AC/DC; 431-8MD11 = 8 × 230 V AC (the next step up). Both are floating via optocouplers, grouped 8-per-group (i.e. all 8 share one isolation barrier to the S5 backplane, which itself is galvanically separated from the 115 V AC/DC field side).The 6ES5431-8MC11​ accepts 115 V AC (47–63 Hz) or 115 V DC on the field side, with Siemens-specified thresholds of “0” = 0–40 V and “1” = 85–135 V, making it well-suited to picking up signals that are natively at mains-related potential — MCC auxiliary contacts (115 V AC coil-derived), 115 V AC pilot-lamp circuits, 120 V AC pushbutton strings, or 110–120 V DC drive-status relays in DC-traction or substation contexts. Input current is typ. 12 mA @ 115 V AC (typ. 2.5 mA @ 115 V DC), inherent delay is ~10 ms 0→1 / ~20 ms 1→0, and the module supports unshielded cable runs up to 100 m. Like other 431 units, the 6ES5431-8MC11​ can connect Siemens BERO two-wire proximity switches on the AC side (with a listed residual current allowance of 4 mA), a useful detail for machinery OEMs who hung BERO sensors off a 115 V AC sensing bus.From an obsolescence standpoint, the 6ES5431-8MC11​ was officially cancelled by Siemens on 10 Jan 2020 without replacement — the same fate as most of the S5 catalogue — but the S5-90U/95U/100U installed base it serves is still active in smaller machinery, packaged skids, water-treatment compact stations, and Eastern European/Asian legacy OEM exports where a S5-100U was specified as the cost-efficient PLC in the 1990s–2000s. NOS and refurbished 6ES5431-8MC11​ units trade because a failed 115 V AC DI on a S5-100U rack is a stop-event: MCC “motor run” aux contacts at 115 V AC won’t be seen by the CPU, the interlock chain breaks, and the plant isn’t migrating that skid to S7 over the weekend. Supplying the 6ES5431-8MC11​ means supplying the exact 8×115 V AC/DC floating density in compact M — not a 430 (24 V DC, wrong voltage threshold) and not a 431-8MD11 (230 V AC, would under-read a 115 V signal as “0”). The model suffix is the spec: 6ES5431-8MC11​ = 8 inputs, M = compact mechanical, C = 115 V AC/DC.

Technical Specifications