Timers, counters, comparisons and data handling

Real control problems involve more than switching outputs on and off according to inputs. A conveyor should stop five seconds after the last box passes, a star-delta starter must change over after the motor accelerates, a packing machine must close a case after 24 bottles, a pump must not run if the tank level is below 20 %, and an alarm should only sound if high pressure persists for three seconds. PLC timers, counters, comparison and math instructions make these behaviours easy to program and change. This lesson covers on-delay, off-delay and retentive timers, up and down counters, comparison and limit instructions, arithmetic and data movement, data types and their pitfalls, and how these instructions combine to build typical machine functions such as sequences, batching and alarm handling.

Timer basics

A PLC timer measures how long a condition has been true. Each timer has a preset (the target time), an accumulated value (elapsed time), and status bits: enable (EN, rung true), timer timing (TT) and done (DN, accumulated value has reached the preset). In tag-based systems, a timer is a structure (for example Timer1.PRE, Timer1.ACC, Timer1.DN); in IEC 61131-3 terms, timers are function blocks with an input (IN), a preset time (PT), an output (Q) and an elapsed time (ET). Time bases and resolutions vary, commonly 1 ms, and presets are given in milliseconds or as time values such as T#5s.

• On-delay timer (TON): starts timing when its rung becomes true; the done bit turns on after the preset elapses, and everything resets as soon as the rung goes false. Used for delays before starting, alarm confirmation delays and debouncing.
• Off-delay timer (TOF): the output turns on immediately when the rung goes true, and stays on for the preset time after the rung goes false. Used for run-on times, such as keeping a fan running after a heater stops, or a conveyor running until it clears.
• Retentive timer (RTO or TONR): accumulates time while its rung is true, holds the accumulated value when the rung goes false, and must be reset explicitly. Used to total run time for maintenance scheduling or cumulative process times.

Counters

Counters count rising edges (false-to-true transitions) of their rung, regardless of how long the rung stays true:

• Count up (CTU): increments the accumulated value on each rising edge; the done bit turns on when the accumulated value reaches the preset.
• Count down (CTD): decrements the accumulated value on each rising edge.
• Reset (RES): clears the accumulated value and status bits.

An up counter and a down counter sharing the same accumulated value can track items entering and leaving an area, such as parts in a buffer or vehicles in a parking garage. Counters may also have overflow and underflow bits. The counting rate is limited by the scan: an input pulse must be on for at least one scan and off for at least one scan to be counted, so fast pulses from encoders or high-speed lines need high-speed counter modules (Lesson 6).

Comparison instructions

Comparison instructions test numerical relationships and act as conditional contacts: equal (EQU), not equal (NEQ), greater than (GRT), greater than or equal (GEQ), less than (LES), less than or equal (LEQ), and limit (LIM), which is true when a value lies between two limits. They compare timer and counter values, analog measurements, setpoints and counts. Examples include starting a fill pump when level is less than 20 %, stopping it when greater than 90 % (with the gap providing hysteresis to prevent rapid cycling), triggering a warning when motor current is between 90 % and 100 % of rating, and activating outputs at specific timer values to build simple timed sequences from one timer.

Comparing analog or floating-point values for exact equality is unreliable, because measured values rarely equal a setpoint exactly; use greater-than or limit tests with a tolerance band instead.

Math and data movement

PLCs provide arithmetic (add, subtract, multiply, divide, modulo, square root, trigonometric functions), compute or expression instructions that evaluate formulas, and data movement (move, masked move, copy, fill, file and array operations). Typical uses include scaling analog values (Lesson 9), calculating flow totals and averages, converting units, computing production rates and efficiencies, and storing recipes and setpoints. In structured text, calculations are written as expressions, such as Flow_Total := Flow_Total + Flow_Rate * Scan_Time / 3600.0, which is often clearer than chains of ladder math instructions.

Data types and their pitfalls

Every value in a PLC has a data type, and mistakes with types cause subtle bugs:

• Overflow: adding to an INT beyond 32,767 wraps around to negative values or faults the processor, depending on the PLC. A production counter in an INT that counts beyond 32,767 parts will fail; use DINT.
• Integer division truncates: 7 ÷ 2 = 3 in integer arithmetic. Scale before dividing or use REAL.
• Floating-point precision: a REAL has about seven significant digits, so adding small increments to a very large total (such as adding 0.001 m³ each scan to a total of 10,000,000 m³) may stop changing the total at all; use larger types, periodic accumulation or separate integer and fractional totals.
• Type conversion: moving a REAL into an INT rounds or truncates, and moving out-of-range values causes errors.

Building sequences

Many machines run in sequences: a press cycle (clamp, press, hold, release, eject) or a tank batch (fill, heat, mix, drain). Simple sequences can be programmed with a step number held in an integer, using comparisons (EQU Step 10) to enable each step's outputs, and transitions that move to the next step when conditions are met (for example a timer done or a sensor on). This "state machine" pattern is easier to understand, troubleshoot and extend than webs of latches. Sequential function charts (SFC) provide a graphical form of the same idea. Every sequence needs defined behaviour for stops, faults and restarts: what happens if the stop button is pressed in step 30, and how does the operator safely return the machine to its starting position?

Timers and counters in troubleshooting

Timers and counters appear constantly when troubleshooting machines. A sequence that seems stuck may be waiting for a timer that will never complete because its rung keeps resetting, for example a TON enabled by a sensor that flickers. A counter that never reaches its preset may be missing pulses because the input is shorter than the scan or because the sensor sees the gaps between products as one long object. When a machine behaves differently from expectations, watch the accumulated values online: they show exactly what the logic is doing and whether inputs are behaving as expected.

Using timers for diagnostics

Timers are also valuable diagnostic tools. A watchdog timer on each step of a sequence detects when a step takes longer than it should, for example a cylinder that should extend within 2 s, and raises a specific alarm such as "Clamp cylinder did not extend", pointing the electrician directly to the problem instead of leaving a stopped machine with no explanation. Similarly, comparing the actual time between events with the expected time reveals slow valves, dragging mechanisms or low air pressure before they cause stoppages. Well-designed programs include these watchdogs for every motion and sequence step.

Run-time totals from retentive timers and cycle counts from counters support maintenance planning, for example scheduling lubrication or replacing a component after a set number of operations. Recording these values on the HMI or in a historian helps maintenance teams move from reactive repair to planned maintenance.

Finally, remember that changing timer presets and counter values can change how a machine behaves in ways that affect safety and quality. Presets should be adjusted only with authorization and documented like other program changes.

Example: a fill-and-drain sequence

Consider a small tank that fills to a high level, heats to a temperature, holds for a set time, then drains. A step-number program uses an integer Step. In step 0 (idle), pressing Start sets Step to 10. In step 10, the fill valve opens until the high-level switch turns on, then Step becomes 20. In step 20, the heater runs until the temperature exceeds the setpoint, then Step becomes 30. In step 30, a TON times the hold period, keeping the heater cycling under temperature control, and when it is done Step becomes 40. In step 40, the drain valve opens until the low-level switch turns on, then Step returns to 0 and a batch counter increments. Each output is driven by one rung that checks which steps need it, for example the fill valve rung examines EQU Step 10. A fault in any step, such as a fill taking longer than expected, moves Step to 99, which turns off all outputs and raises an alarm until an operator resets it.

Written this way, the sequence is easy to read on the HMI (the current step number and its description), easy to troubleshoot (the step shows exactly where the machine is waiting), and easy to extend with additional steps.