What sensors does a tank fill PLC program typically use?

Most basic tank fill programs use two discrete level switches: one at the low-level setpoint to start filling and one at the high-level setpoint to stop filling. More advanced systems add a 4-20 mA continuous level transmitter so the PLC can monitor exact fill percentage and catch slow-leak faults.

Should the inlet valve or the pump start first in a fill sequence?

Open the inlet valve first, confirm it is open (via a valve feedback contact or a short TON delay), then start the fill pump. Starting the pump against a closed valve causes deadhead damage and pressure spikes. The sequence should be: valve open, confirm, pump start.

How do I stop the tank from overfilling if the high-level switch fails?

Use a separate high-high level switch wired to a hardwired interlock that cuts power to the inlet valve, independent of PLC logic. In the ladder program, also latch a High Level Fault bit on the rising edge of the high-level input and require an operator acknowledge before the sequence can restart.

What is the difference between a latched fault and a regular interlock in ladder logic?

A regular interlock (XIO contact) blocks the output as long as the fault condition is present but resets automatically when the fault clears. A latched fault uses OTL to hold the fault bit even after the condition clears, forcing an operator to acknowledge it before the machine can run again.

How do I add a fill timeout to a tank fill program?

Start a TON timer when the fill sequence starts. Set the preset to the worst-case fill time (e.g. 120 seconds for your tank volume and flow rate). If the timer reaches DN before the high-level switch trips, trigger a fault latch. This catches a stuck-open valve, a failed sensor, or a broken pump.

Can I use a PLC counter to track the number of fill cycles?

Yes. Add a CTU counter driven by a one-shot (OSR) on the rising edge of the Fill_Complete bit. The counter accumulates fill cycles, which you can display on an HMI for maintenance planning or to trigger a filter-change reminder at a defined count.

What I/O types do level switches connect to on a PLC?

Discrete level switches connect to digital inputs. Most are 2-wire or 3-wire devices. A PNP output switch connects to a sinking (NPN) input module; an NPN switch connects to a sourcing (PNP) module. Always confirm sourcing vs sinking before wiring.

How do I simulate a tank fill program without a real tank?

In Studio 5000 you can force the level switch input bits manually while offline or online in emulated mode. Toggle Level_Low TRUE to trigger fill, then force Level_High TRUE to stop it. You can also use RSLogix Emulate 5000 or Studio 5000 Logix Designer's emulation server to run the full program without hardware.

Do I need an analog input for a basic tank fill exercise?

Not for a two-setpoint on/off exercise. Two discrete level switches are enough. An analog 4-20 mA level transmitter adds continuous level feedback, proportional control, and leak detection, which makes a good follow-on exercise once the discrete version works cleanly.

How do I wire a 3-wire PNP level switch to a Studio 5000 digital input?

Connect the brown wire to 24 VDC, the blue wire to 0 V (common), and the black signal wire to the PLC input terminal. The input module must be sinking (NPN-compatible). Make sure the input module common is connected to the 0 V rail.

Tank Fill Control PLC Exercise: Ladder Logic

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Flat vector diagram of a tank fill control system showing level sensors, inlet valve, transfer pump, and connections to a PLC I/O rack for a ladder logic exercise

A tank fill sequence is one of the first real-world problems you should build in ladder logic. It has everything: sensor inputs, timed sequences, output interlocks, and a fault that can cause a physical mess if you get the logic wrong. This exercise walks you through a complete two-setpoint tank fill program in Studio 5000 (Allen-Bradley ControlLogix or CompactLogix), from the I/O table all the way to the fault latch and operator reset. You can adapt every rung to any platform.

What Is a Tank Fill Control PLC Exercise?

A tank fill control PLC exercise is a structured ladder logic program that automatically fills a tank to a high-level setpoint, stops filling, and waits until the level drops to a low-level setpoint before filling again. The PLC monitors discrete level switches, controls an inlet solenoid valve and a fill pump, and latches a fault if the tank overflows or if the fill cycle takes too long. It is the foundation for real liquid-handling applications like chemical dosing, buffer tanks, and process water systems.

The Scenario and I/O Table

Picture a 500-litre stainless buffer tank. Water enters through a pneumatic inlet valve; a transfer pump on the outlet feeds a downstream process. Two float switches monitor level: one at 20% (low) and one at 90% (high). A third switch at 98% is the high-high trip, wired hardwired AND in software. The HMI has a Start/Stop pushbutton and a Fault Acknowledge button.

Tag Name	I/O Type	Address	Description
Level_Low	DI	Local:1:I.Data.0	Float switch, closes at 20% level
Level_High	DI	Local:1:I.Data.1	Float switch, closes at 90% level
Level_HighHigh	DI	Local:1:I.Data.2	Float switch, closes at 98% (trip)
HMI_Start	DI	Local:1:I.Data.3	Operator start command from HMI
HMI_Stop	DI	Local:1:I.Data.4	Operator stop command from HMI
HMI_FaultAck	DI	Local:1:I.Data.5	Fault acknowledge from HMI
Inlet_Valve	DO	Local:2:O.Data.0	Pneumatic inlet valve, energise to open
Fill_Pump	DO	Local:2:O.Data.1	Fill pump contactor

Tank fill exercise I/O table, Studio 5000 CompactLogix

Float switches are normally open (NO) in most catalogue listings, meaning the contact closes when the float rises to that level. Confirm this with your datasheet before writing a single rung. If you wire an NC switch into an XIC contact expecting NO behaviour, your tank fills to the top and stays there. I have seen it happen on a commissioning trip with a Gems Sensors LS-1800 that was ordered as NC by mistake.

If you need a refresher on how PNP and NPN level switches connect to your input module, the post on 3-wire sensor wiring for PNP and NPN devices covers it clearly. And if you are unsure whether your input module is sinking or sourcing, sinking vs sourcing PLC I/O explains the difference with real wiring diagrams.

Sequence Design Before You Write a Rung

Write down the sequence in plain English first. It sounds obvious, but skipping this step is why people end up with latching logic that never resets. Here is the sequence for this exercise:

Operator presses HMI Start. System is enabled.
If Level_Low is TRUE (tank at or below 20%), open Inlet_Valve.
After a 2-second valve-open delay, start Fill_Pump.
When Level_High goes TRUE (tank at 90%), stop Fill_Pump first, then close Inlet_Valve.
Wait. When level drops and Level_Low goes TRUE again, repeat from step 2.
If Level_HighHigh goes TRUE at any time, latch a High_High_Fault, stop everything, and wait for HMI_FaultAck.
If the fill cycle runs longer than 120 seconds without reaching Level_High, latch a Fill_Timeout_Fault.
Operator presses HMI Stop to disable the system cleanly.

Notice the pump stops before the valve closes in step 4. This prevents water hammer from closing the valve against a running pump. Small detail, but it matters on real pipework. You see the same sequencing logic in the PLC pump station exercise and in the bottle filling machine project, both of which extend this pattern with multiple pumps and product counters.

Tank Fill Ladder Logic: Rung by Rung

The ladder below is written for Studio 5000 notation. XIC is a normally open contact, XIO is a normally closed contact, OTL latches a bit ON, OTU unlatches it, and OSR is a one-shot rising edge. TON is a retentive-on-delay timer. If you are on Siemens TIA Portal, the logic maps directly: use SR/RS function blocks for latches and the IEC TON block. The XIC vs XIO explainer is worth bookmarking if contact types trip you up.

Tank Fill Control: System Enable, Fill Sequence, Fault Latch, and Timeout (Studio 5000)Ladder logic

Toggle inputs

Rung 0

Rung 1

Rung 2

Rung 3

Rung 4

Rung 5

Rung 6

Rung 7

Rung 8

Rung 9

Rung 10

Rung 11

energizedTip: click a contact in the diagram to flip its bit.

Rung 1 latches System_Enabled on HMI Start, blocked by either fault. Rung 2 unlatches on HMI Stop. Rung 3 latches Fill_Active when the tank is below the high-level setpoint; rung 4 unlatches it when Level_High trips. Rung 5 drives the Inlet_Valve output. Rung 6 starts the 2-second valve-open delay timer. Rung 7 energises Fill_Pump only after the valve delay expires and the tank is not full. Rung 8 runs the 120-second fill timeout timer. Rung 9 latches Fill_Timeout_Fault on a one-shot if the timer expires before Level_High closes. Rung 10 latches High_High_Fault on the rising edge of the 98% switch. Rungs 11-12 unlatch each fault only when the operator acknowledges AND the triggering condition has cleared.

Why the Fault Latch Uses OSR (One-Shot Rising)

Driving OTL directly from Level_HighHigh without a one-shot seems simpler, but it creates a problem: if the float switch bounces or the level hovers right at the setpoint, the OTL instruction fires repeatedly and the unlatch logic fights it. The OSR catches exactly one rising edge per event. The fault latches once, stays latched, and only clears on a deliberate acknowledge. This pattern is described in more detail in the post on normally open vs normally closed contacts in ladder logic.

The High_High_Fault bit must also feed a hardwired interlock, not just ladder logic. If your PLC CPU faults or the program stops executing, the software latch is useless. Wire Level_HighHigh through a safety relay or a hardwired contactor to cut power to the Inlet_Valve independently. The emergency stop circuit wiring post explains the categories of hardwired protection.

The Fill Timeout: Picking a Realistic Preset

120 seconds is a placeholder. You need to calculate it for your tank. Take the tank volume between Level_Low and Level_High (say 400 litres), divide by your minimum inlet flow rate (say 5 litres per second), and add 30% margin. In this example: 400/5 = 80 seconds, plus 30% = 104 seconds, round to 110. If your inlet flow rate changes with supply pressure, use the worst-case low-flow rate. The TON, TOF and TONR timer explainer is useful if you want to understand how the timer accumulator behaves across scan cycles.

Adding a Cycle Counter for Maintenance Tracking

Once the basic sequence is working, add a CTU counter to track fill cycles. Drive it from an OSR on the rising edge of Level_High (the fill-complete event). Display the accumulated value on your HMI and reset it after a maintenance interval. The PLC counter instructions post shows exactly how CTU, CTD and CTUD work, which saves time when you wire in the reset logic.

Extending the Exercise: Analog Level Feedback

The two-switch version is solid for learning, but real tanks often have a 4-20 mA ultrasonic or hydrostatic level transmitter alongside the discrete switches. The transmitter gives you a continuous percentage so you can display a level bar on the HMI and catch a slow leak (level dropping when Fill_Active is FALSE). Wiring and scaling that signal is covered step by step in PLC analog input wiring for 4-20 mA and the matching 4-20 mA scaling formula guide. Once you have the raw count, use the analog scaling calculator to check your EU conversion numbers before you type them into the program.

Flat vector illustration of two tank fill control ladder rungs showing level switch contacts, OTL fill active latch, and TON fill timeout fault logic for a PLC exercise — Key ladder rungs: Fill_Active latch driven by level switches (left) and the 120-second fill timeout fault latch (right).

Testing and Troubleshooting the Exercise

Before connecting real field devices, simulate the program by forcing input bits in online mode. Run through every branch:

Force HMI_Start TRUE: System_Enabled should latch ON.
Force Level_Low TRUE with Level_High FALSE: Fill_Active should latch, Inlet_Valve output should energise, and Fill_Pump should come on after 2 seconds.
Force Level_High TRUE: Fill_Pump should drop out immediately, then Inlet_Valve should de-energise as Fill_Active unlatches.
Force Level_HighHigh TRUE: High_High_Fault should latch, Inlet_Valve and Fill_Pump should both drop out. Confirm they stay out even if you force Level_Low TRUE.
Acknowledge the fault with HMI_FaultAck TRUE while Level_HighHigh is FALSE: High_High_Fault should unlatch.
Let Fill_Timeout accumulate to 120 seconds without Level_High going TRUE: Fill_Timeout_Fault should latch.

If you do not have hardware, the post on how to simulate ladder logic without a PLC covers software emulation options. For testing with real I/O, how to test a PLC input and PLC I/O fault diagnosis with a multimeter are the right next reads.

If you want to go further with this exercise on a Siemens platform, the S7-1200 first program in TIA Portal guide shows you how to create the program block and download it, and S7-1200 timers in TIA Portal maps the TON block directly to the IEC syntax you will use instead of the Studio 5000 style shown here.

What to Read Next

The tank fill sequence is a great foundation. From here, try the PLC pump station exercise to add duty/standby pump alternation on top of your fill logic, or work through the car wash PLC exercise to practice multi-step sequencing with timer-driven state transitions. If you want to push the analog side, add a 4-20 mA level transmitter and scale it with the steps in 4-20 mA scaling in a PLC using ST and ladder.