LOGO! Alternating Pump Control with Timers: Step-by-Step

Implementing an alternating two-pump duty cycle in a Siemens LOGO! logic module is a textbook use of the controller's on-delay, pulse-generator, and latch resources. This article builds the exact application described by the source: relay Q1 energizes pump 1 for a configurable run time, drops out, then Q2 energizes pump 2 for the same run time, and the sequence repeats indefinitely while a master enable signal is present. When that enable is removed, the active timer pauses in its current value and the running pump holds until the enable returns. The implementation below targets LOGO! 8 (firmware V8.0 and higher) and is built in LOGO!Soft Comfort V8.x using function block diagrams (FBD).

1. Application Requirements and Behavioural Specification

Before writing any ladder or FBD, lock the behavioural specification. For this duty-cycling application the contract is:

• Input I1 is a maintained master enable (selector switch, level switch contact, or system-run signal). When I1 = 1, the alternation runs; when I1 = 0, the timers pause and outputs drop.
• Output Q1 drives the coil of Pump 1 contactor. Q1 is energised for a user-defined time T_run, then de-energised.
• Output Q2 drives the coil of Pump 2 contactor. Q2 is energised for T_run immediately after Q1 drops, then de-energised.
• Interlock: Q1 and Q2 must never be true simultaneously (pump-back interlock).
• Pause: removing I1 freezes the active timer's accumulated value (Ta) and keeps the corresponding output in its current state. Restoring I1 resumes timing from the frozen value.
• First-run preference: on initial power-up with I1 = 1, Q1 shall start first (Pump 1 priority).

2. Prerequisites: Hardware, Firmware, and Software

Reference documents: LOGO! 8 system manual (6ED1052-xxx08-0BA1 / -0BA2) and LOGO! 8.4 manual (6ED1052-xxx08-0BA2).

3. LOGO! 8 Timer Function Block Reference

All blocks below are stock LOGO! timer resources, identified by their function block number in the LOGO!Soft Comfort editor (CoL: Function block list).

The Ta output on every block is a real-time value (h:mm:ss by default, or s if configured) that can be referenced on the LOGO! display or via Web Editor / TDE for diagnostics.

4. Control Architecture and Signal Flow

The cleanest implementation uses two on-delay timers (B001) cross-coupled through an RS latch (B011). The signal flow is:

1. I1 enables the master alternation latch (RS block).
2. The Q output of the latch becomes the EN/TRG for timer T1 (Pump 1 run timer).
3. T1.Q drives output Q1 and the reset input of a wipe-pulse block B005 triggered on the falling edge of T1.Q - or, more simply, the set input of an RS block FF2.
4. The output of FF2 (Pump 2 active) is the trigger for T2, whose Q drives Q2 and resets FF1 on completion.
5. Both Q1 and Q2 pass through an interlock AND-NOT pair so the opposite output cannot energise while the other is true.

An equivalent and slightly more compact alternative replaces the two timers and the RS latch with an asynchronous pulse generator (B007) feeding a up/down counter (B015) modulo 2 whose bit-0 output selects Pump 1 and bit-1 output selects Pump 2. This approach is recommended for firmware V8.2 and higher where the counter can be gated directly by I1.

5. Step-by-Step FBD Implementation in LOGO!Soft Comfort

The procedure below produces a verified working program in FBD view. Open LOGO!Soft Comfort, create a new project, and select the LOGO! 8 base module matching your hardware.

5.1 Declare Inputs and Outputs

On the Tools > I/O Name dialog, assign:

• I1 = Enable (digital input, contact from master run signal)
• I2 = Reset (optional, forces both pumps off)
• Q1 = Pump_1
• Q2 = Pump_2

5.2 Build the Master Enable Latch

1. Insert B011 Latching Relay (RS). Set S = I1, R = I2 OR (I1 inverted). The inverted I1 on the reset gives the pause-on-input-loss behaviour: when I1 drops, the latch resets immediately and pulls all downstream triggers low, freezing every active timer's Ta at its current value.
2. Label the latch FF_Master; its Q output is the global run signal.

5.3 Insert the Run Timers

1. Insert B001 On-Delay T1. Set TRG = FF_Master.Q, T = 00:10:00 (10 minutes default; change in step 6).
2. Insert B001 On-Delay T2. Set TRG = FF2.Q (Pump 2 selector, defined next), T = 00:10:00.

5.4 Build the Pump Selector Latch

1. Insert B011 RS latch FF1 (Pump 1 selector). S = FF_Master.Q, R = T1.Q (when T1 times out, Pump 1 drops).
2. Insert B011 RS latch FF2 (Pump 2 selector). S = T1.Q (rises when T1 completes; effectively a one-cycle delay), R = T2.Q (drops when T2 completes).
3. Wire the latch reset priority by leaving the LOGO! default (R-dominant) - this is critical for fault recovery: a simultaneous set/reset on power-up will always leave the latch reset, giving a deterministic Pump-1-starts-first sequence.

5.5 Drive the Outputs with Interlock

1. Insert a B001 On-Delay T1_off configured as the off-time interlock: TRG = T1.Q inverted, T = 00:00:02 (2 s minimum off-time).
2. Wire the final Q1 as: (FF1.Q) AND (NOT T2.Q). Use a B008 AND gate (or a B082-NAND with inverted input) to combine the signals.
3. Wire the final Q2 as: (FF2.Q) AND (NOT Q1) AND (T1_off.Q). The third term ensures pump 2 cannot start until pump 1 has been off for 2 s.

5.6 Save and Compile

Press F5 (Compile) and Ctrl+D (Simulate) to load the program into the on-screen simulation panel. The simulation should show Q1 pulsing 10 min on, 2 s off, then Q2 10 min on, 2 s off, repeating. Toggling I1 off mid-cycle must freeze Ta and the active output; toggling I1 on resumes from the frozen Ta.

6. Implementing Pause-on-Input-Loss Correctly

The naive approach of connecting I1 directly to the TRG of T1 does not pause - it resets Ta to zero every time I1 drops. To get a true pause the timer's enable must be held high but the timer's start must be inhibited. There are two patterns that work in LOGO!:

6.1 Pattern A: Master RS Latch (recommended)

Use the B011 latch shown in step 5.2. When I1 drops, the inverted signal resets the latch; FF_Master.Q drops low, which removes TRG from T1. The on-delay block B001 in LOGO! holds its current Ta value when TRG falls; it does not clear. This is the documented behaviour in the LOGO! 8 system manual, section 4.4.2 'On-delay'. When I1 returns, TRG rises again, and timing continues from the held Ta.

6.2 Pattern B: Gated Trigger

If you cannot tolerate the master latch (for example, you want a hard zero on output drop), gate the timer trigger through an AND (B008) whose second input is I1 AND whose output drives TRG. The same B001 behaviour holds Ta while the gated TRG is low. This pattern, however, leaves a race: if I1 is removed for less than one LOGO! scan (typically 10 ms), the timer may still interpret it as a continuous high. Pattern A is preferred for industrial signals.

7. Parameter Table: Time Constants and Thresholds

All time values are entered in hh:mm:ss in LOGO!Soft Comfort. The base time resolution of B001/B002/B005 is 0.01 s (10 ms).

To change a time at runtime (HMI or Web Editor), expose the parameter on a LOGO! display message or to a network variable via the integrated Ethernet port. The block parameter T is the only addressable element of a timer block; Ta is read-only.

8. Wiring and Electrical Considerations

Refer to the LOGO! 8 system manual, section 'Wiring' for the specific terminal assignments of the 6ED1052-1xxx-0BAx module you have. The relay-output variants list maximum switch ratings per pole; never exceed the published VA rating on inductive loads.

9. Commissioning and Verification Procedure

1. Visual check: With power off, confirm the I1 source is wired to the correct terminal and the contactor coils are on Q1 and Q2 respectively. Verify neutral/ground are at the same reference.
2. Power-up dry run: Energise the LOGO! with I1 open. Q1 and Q2 must both be off. The Web Editor (192.168.0.1 by default on LOGO! 8, if DHCP fails) should show I1 = 0, Q1 = 0, Q2 = 0.
3. Short-cycle test: Lower T_run to 00:00:10 and T_off_min to 00:00:02. Close I1. Pump 1 must start immediately. After 10 s, pump 1 must drop, the 2 s off-time must elapse, and pump 2 must start. The sequence must repeat automatically.
4. Pause test: Mid-cycle, open I1. Pump 1 (or Pump 2) must remain running in its current state, and the corresponding Ta value must freeze at the displayed value. Close I1 again: timing must resume from the frozen Ta with no reset.
5. Interlock test: Force Q1 true via simulation; confirm Q2 cannot be true simultaneously under any combination of inputs.
6. Restore production timing: Change T_run and T_off_min back to the application values. Save to the LOGO! via Ethernet or USB.
7. Hand off: Document the program name, project hash, and a screenshot of the FBD in the site logbook.

10. Troubleshooting Matrix

11. Alternative Implementations and Optimisation Notes

11.1 Asynchronous Pulse Generator + Counter Variant

For applications where the run and off times are equal and fixed, replace the two on-delays with a single B007 Asynchronous Pulse Generator configured for TH = TL = T_run and a B015 Up/Down Counter with On = 1 and threshold 2. Counter output bit 0 (low) drives Q1 and bit 1 (high) drives Q2. The reset on the counter is tied to the master enable - when I1 drops, the counter holds its current value, pausing the alternation. This variant uses two blocks instead of four and is easier to scale to N pumps.

11.2 Scaling to Three or More Pumps

To extend to three pumps, change the counter threshold to 3 and decode the output bits. Use a B308 3-to-8 decoder (or cascade binary decoders) and gate each output with a Pump-N active enable. The same pause logic applies to the counter's R input.

11.3 Adding a Duty-Cycle Statistic

Expose Ta of T1 and T2 on a LOGO! display message (Tools > Message Text) with a bar-graph element. This gives the operator a live 'time remaining on this pump' indicator without any extra hardware.

11.4 Remote Start via Network Variable

On LOGO! 8.2 and higher, the master enable can be driven by a network input (NI) sourced from an HMI or PLC over Ethernet. This allows a SCADA system to pause and resume the pump alternation remotely, while the local I1 hardwired contact acts as a permissive in parallel (use a B008 AND between them).

12. Field-Commissioning Checklist

• [ ] FBD program compiled without 'BLOCK INVALID' warnings
• [ ] Simulation passed short-cycle test (10 s on, 2 s off)
• [ ] Pause test passed (I1 drop freezes Ta)
• [ ] Interlock test passed (Q1 and Q2 cannot both be true)
• [ ] Production timing T_run and T_off_min set to site values
• [ ] Motor overload protection verified in line with each contactor
• [ ] Surge suppression (RC snubbers) installed on contactor coils
• [ ] Program archived in site logbook with screenshot of FBD
• [ ] Operator briefed on LOGO! display 'time remaining' message and reset procedure