PLC Previous Value Comparison in Ladder Logic: Scan-to-Scan

1. Overview: The Scan-to-Scan Comparison Problem

Detecting a decreasing trend on a process variable is a recurring requirement in PLC applications: drop in line pressure, falling tank level, derating of a motor current, loss of vacuum in a pick-and-place head, or creep of an analog setpoint. The classic engineering pattern is to keep the value of the previous execution in non-volatile memory, subtract the current value from it, and arm an alarm when the difference crosses a threshold.

The reference requirement used throughout this article is:

• Read a 4–20 mA pressure transmitter scaled to engineering units (bar).
• Each cycle, compare the previous sample with the current sample.
• If the previous sample − current sample ≥ 4 bar, latch an alarm bit.
• If the current sample is also < 4 bar, raise a hard alarm.

The same logic structure works for any single-value comparison: it is independent of whether the variable is real, integer, or scaled analog. The principles transfer directly to SIMATIC S7-1200 Programmable Controller and SIMATIC S7-1500 Programmable Controller with the TIA Portal engineering framework (V17, V18, V19, or V20).

2. The Core Pattern: Previous, Current, Delta

The control-flow structure is the same in every IEC 61131-3 dialect and reduces to three operations executed in order every cycle:

1. SUB — compute the delta: delta = previous - current
2. Compare — if delta >= threshold AND current < low_limit, set the alarm.
3. MOVE — copy current into previous for the next cycle.

The reason for storing previous last is fundamental: executing the MOVE before the SUB would corrupt the difference. In ladder, FBD, and SCL the ordering must be respected or the result is always zero on the first cycle and undefined thereafter.

3. Implementing the Pattern in TIA Portal (S7-1200 / S7-1500)

Create a new FB in the project tree (right-click Program blocks > Add new block > Function Block), assign an Instance DB automatically, and switch to the FBD or LAD editor. The following ladder segment implements the core pattern in FBD-style contacts because the network is small and reviewable.

3.1 Tag Declaration in the FB

VAR
    current_value  : REAL;       // scaled pressure in bar
    previous_value : REAL := 0.0; // retained across scans
    delta          : REAL;
    threshold_drop : REAL := 4.0; // bar
    low_limit      : REAL := 4.0; // bar
    alarm_pressure : BOOL;        // latched output
    cycle_enable   : BOOL;        // run the comparison
END_VAR
VAR RETAIN
    previous_value : REAL;        // survive warm restart
END_VAR

Mark previous_value as Retain so that a brief power dip does not zero the history. S7-1500 supports fine-grained retain; S7-1200 supports retain on a per-DB basis. Refer to the S7-1500 system manual, section on retain behavior.

3.2 Ladder Network

Network 1 - Compute delta and test the two conditions
  cycle_enable                                       (    )
        |-----[ SUB ]-----( delta := previous_value - current_value )-----|
        |                                                                |
        |-----[ GE   ]-----( decreasing_flag := delta >= threshold_drop )-----|
        |                                                                |
        |-----[ LT   ]-----( low_flag := current_value < low_limit )-----|
        |                                                                |
        |-----[ AND  ]-----( trigger := decreasing_flag AND low_flag )-----|
        |                                                                |
        |-----[ S    alarm_pressure ]-----|

Network 2 - Save current as previous for next scan
  cycle_enable                              (    )
        |--------[ MOVE  current_value → previous_value ]--------|

The S (Set) coil is used so the alarm sticks until an operator acks it from the HMI. Use R (Reset) on a separate network driven by AlarmAck.

4. Choosing the Execution Context: OB1, OB30–OB38, or Timer

A common question is whether to run the comparison in the main OB (OB1 on Siemens), in a cyclic interrupt (OB30–OB38), or inside a timer-driven block. The choice depends on scan time, signal noise, and whether the comparison is a fast control loop or a slow trend detector.

Cyclic interrupt OBs run at a fixed phase regardless of OB1 scan time. The phase is configured under Properties > Cycle time of the OB. Refer to the S7-1200 system manual and the S7-1500 system manual for the supported phase values per CPU type.

For a 5-second comparison (the example requirement) configure OB35 with a 1 s phase and use a counter that triggers the comparison every 5th call:

VAR
    tick_counter : INT;
    compare_now  : BOOL;
END_VAR

IF tick_counter >= 5 THEN
    compare_now := TRUE;
    tick_counter := 0;
END_IF;
tick_counter := tick_counter + 1;

5. Building a Reusable Function Block (FB) in SCL

Once the pattern is implemented, encapsulate it into a typed FB so the same block can be reused for pressure, temperature, level, and flow. The following SCL code is a complete, copy-paste ready FB for TIA Portal V17+.

FUNCTION_BLOCK "FB_TrendDropAlarm"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
   VAR_INPUT
      i_currentValue    : REAL;     // current scaled value
      i_enable          : BOOL;     // run comparison
      i_dropThreshold   : REAL := 4.0; // magnitude of decrease that triggers
      i_lowLimit        : REAL := 4.0; // absolute low threshold
      i_acknowledge     : BOOL;     // rising edge clears latched alarm
   END_VAR

   VAR_OUTPUT
      o_delta           : REAL;     // previous - current (positive = dropping)
      o_alarmActive     : BOOL;     // latched alarm
      o_justTriggered   : BOOL;     // one-shot on the cycle the alarm set
   END_VAR

   VAR RETAIN
      s_previousValue   : REAL;     // retained across power cycle
   END_VAR

   VAR
      s_edgeAck         : BOOL;     // ack edge memory
   END_VAR

BEGIN
   // Compute the delta first, before the previous value is overwritten
   o_delta := s_previousValue - i_currentValue;

   // Trend + absolute condition
   o_alarmActive := o_alarmActive
                    OR (i_enable
                        AND (o_delta >= i_dropThreshold)
                        AND (i_currentValue < i_lowLimit));

   // One-shot trigger for HMI / event logging
   o_justTriggered := (i_enable
                       AND (o_delta >= i_dropThreshold)
                       AND (i_currentValue < i_lowLimit));

   // Operator acknowledgement clears the latch
   IF i_acknowledge AND NOT s_edgeAck THEN
      o_alarmActive := FALSE;
   END_IF;
   s_edgeAck := i_acknowledge;

   // Save the current as previous for the next scan
   s_previousValue := i_currentValue;
END_FUNCTION_BLOCK

Drag the FB into a cyclic OB and connect:

"iDB_TrendDropAlarm"(i_currentValue  := "Scale_PT_101".bar,
                     i_enable        := TRUE,
                     i_dropThreshold := 4.0,
                     i_lowLimit      := 4.0,
                     i_acknowledge   := "HMI".Ack_PT101_Low);\code>

The same FB can be called multiple times with different instance DBs for each monitored tag. TIA Portal will automatically generate one instance DB per call site.

6. Edge Cases and Field-Proven Caveats

7. Platform-Specific Implementations

7.1 Allen-Bradley CompactLogix / ControlLogix (RSLogix 5000 / Studio 5000)

Use an Add-On Instruction (AOI) with the same three steps: SUB, GRT (Greater Than), MOV. The retention is achieved by marking the AOI tag as “Retained” in the tag properties. The cyclic execution is provided by a Periodic Task with a configurable period (10–2000 ms). Refer to the Logix5000 Controllers Design Considerations reference manual.

[ PeriodicTask 100ms ]
  XIC(Enable) SUB(SourceA := previous, SourceB := current, Dest := delta)
  GRT(SourceA := delta, SourceB := 4.0) XIO(AlarmActive) ONS(TriggerOneShot)
     OTE(AlarmActive)
  MOV(Source := current, Dest := previous)

7.2 Beckhoff TwinCAT 3 (PLC)

Use a FUNCTION_BLOCK in Structured Text called from a MAIN task. For cyclic execution independent of the PLC task, use the PlcTaskDef with CycleTime := T#1s. The same SUB / GT / MOVE structure applies; use __RETENTION for the previous_value attribute so it survives an online change.

7.3 CODESYS V3 (Wago, Schneider M241/M251, others)

The pattern is identical. Use PRG blocks called from a cyclic task. For persistence, mark previous_value as RETAIN or PERSISTENT. CODESYS also provides the BLINK and FT_BLINK library blocks for the cyclic interrupt alternative if a dedicated task is not desired.

8. HMI / SCADA Visualization and Trend Setup

For Siemens WinCC Unified / TIA Portal HMI panels, expose o_delta and o_alarmActive as HMI tags. Configure a trend view with the following parameters:

• Trend source: previous_value and current_value from the instance DB.
• Sampling: 1 s (must be a multiple of the OB35 phase).
• Update cycle: 1 s.
• Alarm logging: configure a discrete alarm on the rising edge of o_justTriggered.

For WinCC Professional / SCADA, map the same tags to a tag logging group with a 500 ms acquisition cycle and 10 s archive cycle. Refer to the WinCC Unified System Manual.

9. Commissioning Checklist and Verification

1. Watch table test. Create a watch table with current_value, previous_value, delta, and alarm_pressure. Force current_value from 10.0 → 5.0 in 0.5 bar steps; verify delta updates each cycle and the alarm sets at the expected point.
2. Power-cycle test. Trigger the alarm, STOP the CPU, power off for 30 s, power on, RUN. The retained previous_value must still match the last sample. The alarm should not self-clear.
3. First-scan test. From a clean project download (or after memory reset), the FB should not raise an alarm on the very first sample. Verify the first-cycle / startup OB initializes previous_value correctly.
4. HMI acknowledge. Trigger the alarm from the watch table, then press the Ack button on the HMI; verify alarm_pressure resets and the alarm disappears from the alarm log.
5. Negative-going vs positive-going direction. Confirm whether the requirement is previous − current (drop) or current − previous (rise). Swap IN1 / IN2 of the SUB block to change direction.
6. Cyclic interrupt verification. In TIA Portal, enable Online & Diagnostics > Cycle time for the OB. Confirm the configured phase is being met and that OB1 is not also calling the FB.

10. Frequently Asked Questions