LOGO! 8 0BA1 Digital Input Wiring: 230V Grouping and DM16 24R

Overview

Siemens LOGO! 8 (catalog generation suffix 0BA1) ships with several logic modules and digital expansion modules that look similar from the front but differ in their mains/power-supply range and the voltage they accept on the digital inputs. Two wiring rules are interpreted inconsistently in the field:

1. The 230 V AC input group rule (two galvanically isolated groups of four inputs) is often read as applying only to the display-less variant, when in fact it applies to every 230 V module of the 0BA1 generation.
2. The DM16 24R is sometimes wired to a 24 V AC supply because the smaller DM8 24R is documented as 24 V DC/AC. The DM16 24R is 24 V DC only; the supply column in the manual is correct.

This reference pins both points down against the catalog matrix, shows the internal topology, lists the P/N action rule, and ends with a verification procedure and a diagnostic matrix for the most common field errors. Cross-reference the Siemens LOGO! 8 product page and the Siemens Industry Online Support for the latest manual revision.

LOGO! 8 (0BA1) Module Identification Matrix

Each module's catalog order code encodes the supply and I/O family. The 0BA1 generation variants covered here are:

230 VAC Input Grouping Rules

Every 230 V AC input variant of the LOGO! 8 (0BA1) — both the 230RCE and 230RCo base modules, and the DM8 230R / DM16 230R expansion modules — has its digital inputs internally organised as two galvanically isolated groups of four inputs on the base module and the DM16 230R, and as two groups of two inputs on the DM8 230R expansion.

The rules printed in the LOGO! 8 system manual are correct, and they apply to all 230 V AC variants, not just the 230RCo. The 230RCE and 230RCo share the same input PCB; only the display, the Ethernet port, and the firmware build flags differ. The 230 V expansion modules reuse the same input circuit and the same group topology.

Concretely, the rules are:

1. The four inputs of one group (I1–I4 or I5–I8 on a base module; C1–C4 or C5–C8 on a DM16 230R) must be connected to the same phase. The four inputs share a common return inside the module.
2. The two groups may be wired to different phases. For example, group A on L1 and group B on L2 of a three-phase 400 V system is permitted, provided the neutral reference of each group is the system neutral.
3. Phase-to-phase voltage (e.g. 400 V) must never be applied between the line terminal of an input and the module's N terminal, because that exceeds the 240 V AC absolute maximum rating of the input optocoupler.
4. The neutral terminal of the module is shared by both groups internally on the 230 V base module; it is brought out as a single terminal. Each group must be wired so that the line conductor of the field switch is the switched conductor, and the neutral returns to the module's N terminal.

The two-group layout exists because the 230 V input is a half-wave-rectified optocoupler fed from the AC line. Each optocoupler pair shares the same return, and the four-input grouping is the natural layout of the input PCB. The 230RCE and 230RCo share the input PCB; therefore the 230RCE base module — with display and Ethernet — is subject to the same wiring rule as the 230RCo.

Input Group Wiring Topology

Two design rules follow from the diagram:

• Do not mix phases within a single group. If I1 and I2 are wired to L1 but I3 is wired to L2, the optocouplers see a line-to-line potential across the internal return and either latch permanently or fail at first power-on.
• If a field switch is wired as "switched neutral" instead of "switched line", the input is biased to the same potential as the module's N terminal and the input never reaches the "1" threshold (≥ 79 V AC for the 230 V variant). The input reads 0 in the LOGO! display even with the contact closed.

DM16 24R Power Supply Specification

Where the printed manual column for the DM16 shows "24 V DC" in the power supply row, that is not a typographical error. The DM16 24R (6ED1055-1NB10-0BA1) and the DM16 24 (6ED1055-1BA00-0BA1) are 24 V DC-only devices. Only the DM8 24R (6ED1055-1HB00-0BA1) accepts a true 24 V AC supply on its power terminals, because the DM8 24R includes a bridge rectifier at the supply input while the DM16 modules do not.

Feeding 24 V AC into a DM16 24R will power the module briefly through the input protection Zener diodes, but the internal 5 V and 3.3 V rails sag on every zero-crossing of the 24 V AC source. Typical field symptoms:

• ERR LED on the DM16 24R blinks at approximately 1 Hz; the module does not enumerate on the internal expansion bus.
• The base module loses the DM16 24R from the I/O list shown in LOGO! > Diagnostics > I/O Status approximately 1–3 s after power-on, then re-attempts enumeration, producing a "DM missing / re-appear" cycle.
• If a 24 V AC source with significant source impedance is used, the DM16 24R may appear to "work" for a few seconds, then drop out. This is the most common misdiagnosis in the field.

P/N Action Compatibility Matrix

The LOGO! 8 system manual statement that "digital inputs can be operated either with P or with N action" applies to the 24 V variants only:

• LOGO! 8 24RCE / 24CE / 12/24RCE / 12/24CE base modules
• DM8 24R and DM8 12/24R
• DM16 24R and DM16 24

For the 230 V variants, the 230 V input circuit references the L (line) terminal, not the module's own +24 V rail. There is no 24 V rail accessible on the input terminal strip, so the LOGOSoft selector for P/N action does not appear for the 230 V family. Only P action is implemented in hardware, and the input latches when the line terminal is energised relative to the module's N terminal.

Selection in LOGO! Soft Comfort (version 8.x for 0BA1 hardware):

1. Open the project and select File > Properties > I/O Settings (or on older builds: Edit > Properties).
2. For each input that supports it, set the type to P (default) or N.
3. On the device itself, navigate to I/O > Inputs > I1..I8 > Type and confirm. The 230 V menu path does not show the N option.

On the 12/24RCE / 12/24CE, the four "fast" inputs I1–I4 (24 V mode) and the four analog-capable inputs I5–I8 (0–10 V mode) are the same physical terminals. The P/N action is set per input, and analog mode for I5–I8 is enabled from the same dialog.

Wiring Verification Procedure

Verify the wiring against the rules above before powering the field for the first time.

1. Identify each module by its catalog order code. The label is on the right side of the housing. Record the order code on the as-built drawing.
2. For every 230 V module, separate the eight inputs into group A (I1–I4) and group B (I5–I8). With a multimeter on the diode/continuity range, verify that the four L terminals of each group share a common return to the module's N terminal. The group returns are not bridged between groups on the PCB.
3. With the supply de-energised and locked out, measure insulation between the L terminal of group A and the L terminal of group B. The reading should be > 1 MΩ at 500 V. A low reading indicates a wiring error (e.g. two phases tied together by mistake) or a foreign object on the terminal strip.
4. For the DM16 24R, measure the DC component of the supply rail at the module terminals with an oscilloscope. The ripple should be < 5 % of the nominal 24 V (i.e. < 1.2 V peak-to-peak). If the ripple exceeds 1.5 V peak-to-peak at 100/120 Hz, the source is rectified but unsmoothed — replace with a regulated 24 V DC supply.
5. Force a single input of each group with the rated voltage (e.g. 230 V AC for the 230RCE; 24 V DC for the DM16 24R) and confirm the corresponding bit toggles in the LOGO! display or the LOGOSoft monitor (online > I/O status). A bit that does not toggle indicates a wrong-phase or wrong-polarity connection inside the group.
6. For N-action verification, switch the input type in LOGOSoft to N, force the input with 0 V, then with 24 V DC, and confirm the inverted logic: 0 V → bit = 1, 24 V → bit = 0. Repeat for every input that is to be used in N action.
7. With the supply on, measure the current drawn on L1, L2 (if used) and N with a clamp meter. The imbalance should not exceed 30 % of the larger reading; a larger imbalance indicates that one group is feeding a larger load or that the neutral reference is not shared.

Diagnostic Matrix for Common Field Errors

Commissioning Checklist

• ☐ Catalog MLFB recorded on the as-built drawing for every module
• ☐ Supply type (DC, AC, DC/AC) labelled on every terminal strip
• ☐ Group A and Group B return terminals identified and tagged on the schematic
• ☐ For 230 V modules, neutral reference for each group verified with a continuity check
• ☐ Insulation > 1 MΩ between L of group A and L of group B at 500 V
• ☐ P/N action confirmed per input in the LOGOSoft project and on the device menu
• ☐ DM16 24R fed from a regulated 24 V DC source (ripple < 5 %, < 1.2 V peak-to-peak)
• ☐ DM8 24R may be fed from 24 V AC or 24 V DC, but not from rectified-but-unsmoothed AC
• ☐ Expansion bus terminated on the rightmost module (terminator slider closed)
• ☐ First-time power-on with a clamp meter on L1, L2 (if used) and N to confirm no wiring imbalance
• ☐ Online I/O status display exercised: every input toggles 0 → 1 → 0 with a field-forced test pulse
• ☐ LOGO! Soft Comfort project saved with the same firmware version as the target (0BA1)
• ☐ LOGO! display backlight timeout and password set if required
• ☐ Ethernet connection (if used) configured with a fixed IP outside the DHCP range and pinged from a laptop

Specifications Reference

Three electrical caveats follow from the table:

1. The 230 V "1" threshold of ≥ 79 V AC is what allows the 230 V input to be cross-wired in 110 V systems (e.g. North American 120 V AC control) without reconfiguration — the threshold is well below 120 V AC, so the input reads "1" reliably.
2. The 230 V "0" ceiling of 40 V AC is what makes 24 V AC drive on a 230 V input impossible — 24 V AC peak (≈ 34 V peak for a 24 V AC RMS source) is below the "0" ceiling, so the input never latches to "1".
3. The 24 V group-to-group isolation is functional (i.e. the groups share the module's 0 V rail) — this is why the 24 V variants can use N action (the module's own 24 V rail becomes the source), while the 230 V variants cannot.

FAQ