LOGO! 3-State Switch OFF/AUTO/MANUAL with Single Input

Overview

A three-position selector (OFF / AUTO / MANUAL) is one of the most common operator interfaces on a heating, ventilation, irrigation, or pump control panel. On a Siemens LOGO! 8 (6ED1052-1xx08-0BA0 series) the standard hardware approach is a 3-position cam switch wired to two digital inputs. That gives unambiguous position decoding and survives a wire break because the controller reads the state as INVALID and forces the load off.

This reference documents the alternative demanded when only a single input is physically available: a momentary push-button that cycles OFF → AUTO → MANUAL → OFF on each press. A counter block, an on-delay debounce, and three exclusive state flags implement the state machine. A second implementation shows the two-input safe selector. Both are written in FBD inside LOGO!Soft Comfort V8.3 / V8.4 and download to LOGO! 8.3 firmware 1.82.x or later. The remote visibility portion uses the LOGO! Web Editor / LOGO! App and a single-line Message Text so the operator sees one stable line: Mode: AUTO.

Prerequisites

Refer to the LOGO! 8 system manual (Siemens entry ID 109751654) for electrical specifications and to the LOGO!Soft Comfort V8.3 online help (entry 109751655) for block parameter details.

State Machine: One Input, Three States

The single push-button is wired to digital input I1. Each falling edge of I1 increments an Up/Down counter. The counter modulus is 3, so the values cycle 0 → 1 → 2 → 0. Three comparators decode the counter value into mutually exclusive flags S_OFF, S_AUTO, S_MAN. Because the flags are decoded from one integer, exactly one is true at any time, which is the property required to drive a single text line without flicker.

Decoding arithmetic uses the integer counter CV directly. Comparator B001 fires when CV = 0 (OFF), B002 when CV = 1 (AUTO), B003 when CV = 2 (MANUAL). The Q output of the matching comparator latches for the entire time the counter holds that value.

Counter / Debounce Wiring in FBD

The full network fits in a single diagram page of LOGO!Soft Comfort. The blocks required are: one Up/Down counter (B004 in the local block address space), one On-delay (B005), three Analog comparator / Threshold trigger blocks (B001, B002, B003), and one OR gate to build the heater output.

Network 1 — Debounce and count
   I1 ──[ B005 On-delay 1.0 s ]──(edge)──┐
                                          ├──> B004 Cnt (Up, threshold = 0)
   I2 (clear, optional) ─────────────────┘──> B004 R

Network 2 — Decode state
   B004.CV ──┬─> B001 thr= 0 ──> S_OFF
             ├─> B002 thr= 1 ──> S_AUTO
             └─> B003 thr= 2 ──> S_MAN

Network 3 — Build Q1 (heater)
   S_AUTO AND I2 ──┐
                   ├──> Q1 (relay output)
   S_MAN ──────────┘

The On-delay B005 enforces a 1.0 s minimum spacing between valid button presses. The same timer serves as a debouncer for contact bounce (typical 5–20 ms) and as a guard against accidental double-taps. The internal block address B005 is shown here because it is the convention used in LOGO!Soft Comfort when the user does not rename blocks; the address is not significant to the logic and may be reassigned by the compiler.

Converting Counter Value to Output

The user's original sticking point — "I hit a wall when I tried to convert a counter value into an output" — is solved by recognising that the Up/Down counter block already exposes its current value on CV. The output conversion is therefore not an arithmetic step but a decoding step: do not try to write the integer 0/1/2 directly to a digital output. Instead, route the value through comparators so that one comparator per state becomes a boolean flag. Boolean flags drive boolean outputs without type mismatch.

Single-Line Text Display

The complaint that the three states "move around" comes from a layout mistake: three separate Message Text blocks, each enabled by its own flag, are stacked at the same Y-position in the display editor and shift position when the active one changes. The fix is to keep three Message Text blocks but anchor them at three different Y-positions, or — preferred — use one Message Text block whose enable condition is a priority encoder that activates exactly one message at a time.

Network 4 — Priority encoder for one stable line
   S_OFF  ──> M1 enable  (text = "Mode: OFF  ")
   S_AUTO ──> M2 enable  (text = "Mode: AUTO ")
   S_MAN  ──> M3 enable  (text = "Mode: MANUAL")
   M1, M2, M3 share the same Y-line on the LOGO! TDE

Because the three flags are decoded from a single counter, the priority encoder is implicit: only one enable is ever high. The TDE / on-board BM shows the active message on a single line; the line does not shift. For visualisation, place the line at Y = 2 of the TDE (counted from the top) and add a bar graph on Y = 4 showing 0 %, 50 %, 100 % corresponding to OFF / AUTO / MANUAL. The bar graph uses a single Message Text with bar block driven by the integer counter CV multiplied by 50 to give the 0 / 50 / 100 % range.

Wire-Break Detection and Safety

A break in the I1 conductor is electrically indistinguishable from the button being open. With a 24 V DC input the simplest detector is a 10 kΩ pull-down resistor in parallel with the contact, and a 1.5 kΩ pull-up to 24 V through a 4.7 kΩ current-limiting resistor; the resulting analog voltage at I1 is monitored by an AI in the LOGO! 8 base module (AI1 on the 12/24 RCE variant). Configure AI1 in the LOGO!Soft Comfort variable table as 0–10 V. A wire break drops the analog reading below 0.3 V, well below the 1.0 V threshold comparator B010, which then forces S_OFF and asserts the system fault flag F_WIRE. The fault flag is latched and must be cleared by a deliberate key press on the TDE or via the LOGO! App.

For installations where the analog input is already in use, an alternative is to add a second digital input I3 wired in parallel through a 100 kΩ resistor. A closed contact pulls both I1 and I3 high; a wire break pulls neither. A NOR of I1 and I3 yields F_WIRE. The disadvantage is that the operator can no longer claim a "single input" topology — at that point, switch to the two-input safe selector described next.

Alternative: Two-Input Three-Position Selector

The two-input topology is the recommended baseline. Wire a 3-position cam switch with the common pole to 24 V and the OFF / AUTO / MANUAL throws to I1 and I2. Three input combinations are valid, the fourth is reserved as the fault state.

The decoded state is built from a 2-to-4 line decoder built from a single LOGO! AND + OR network, or more concisely by routing (I1, I2) through a custom UDF that returns the state as an integer 0/1/2/3. With this topology a wire break manifests as 0/0 and the operator sees a permanent MANUAL state with the fault flag asserted, which the dispatcher should treat as a fail-safe condition. The Q1 output is forced off whenever F_INVAL is high, regardless of the decoded state, satisfying SIL-1 style behaviour for non-critical loads.

Mobile App and Remote Visibility

The LOGO! 8 base module ships with an integrated Ethernet port and a built-in Web Editor. The LOGO! App for iOS / Android is configured by scanning the QR code shown on the TDE or by entering the controller's IP address. The mode variable S (an integer 0/1/2) is exposed in the project's variable table and can be assigned a custom name Mode with a custom string list. The App renders the value as a drop-down; selecting AUTO remotely writes the integer 1 to the variable, and the state machine increments the counter on the rising edge of Mode_remote_changed. The remote write and the push-button share the same counter, so the operator on site and the operator on the phone see identical state.

For full remote control rather than just remote view, expose the push-button function as a Network Input NI1. The App dashboard widget Pulse button writes a one-cycle pulse to NI1, which is then OR'd with the physical I1 inside the LOGO!. The counter increments identically for a local press or a remote press.

Commissioning and Verification

1. Power the LOGO! with the I1 input open. The TDE should display Mode: OFF and Q1 should measure 0 V at the load terminals.
2. Press and release the push-button once. After 1.0 s (B005) the display must change to Mode: AUTO. Apply 24 V to I2 (thermostat). Q1 should track I2 with the same polarity. Remove I2 — Q1 must drop to 0.
3. Press the button a second time. Display changes to Mode: MANUAL. Q1 must read 230 V (or 24 V depending on the BM variant) at the load terminals, independent of I2.
4. Press a third time. Display must return to Mode: OFF and Q1 must drop. Confirm there is no flicker between modes by holding the button for 5 s; the debounce timer must keep the state stable.
5. Disconnect the I1 wire. Display stays on the last valid state and F_WIRE should assert within one scan (≤ 100 ms on a 0.0 cycle time). Q1 must drop immediately if the system is configured fail-safe, or hold the last state if the operator has acknowledged the fault.
6. From the LOGO! App, send a remote pulse to NI1. The display should advance by one state and the App dashboard should reflect the new Mode value within 500 ms.
7. Run the panel for 24 h on a chronometer to confirm no spurious state changes. A correctly debounced design shows zero state changes without operator action.

Troubleshooting Matrix

Specifications and Limits

For a complete list of block counts, remanent flags, and program-size limits, see the LOGO! 8 system manual (Siemens support entry 109751654). For firmware release notes and known issues affecting counter and Message Text blocks, consult the LOGO! 8 firmware update history (entry 109972551).

FAQ