Holding REAL Values in PCS 7 CFC: Track-and-Hold Methods

1. Overview: The Track-and-Hold Problem in PCS 7 CFC

In SIMATIC PCS 7 V7.0 and the later releases through V9.1, the Continuous Function Chart (CFC) editor is the standard tool for programming the automation station (AS). Unlike ladder logic (LAD), function block diagram (FBD), or SCL, CFC is a free-form placement editor: function blocks, I/O blocks, and operator blocks are positioned on a chart sheet and connected by signal lines, while the runtime execution order is defined by numbered "tasks" (runtime groups) with configurable cycle time and phase offset. The CFC language is therefore extremely well suited to track-and-hold (T/H) functions, but the standard library does not ship a single dedicated HOLD_R or SAMPLE_HOLD_R block. Three engineering patterns cover the vast majority of real-world use cases without writing a custom function block:

1. EN input freezing of a downstream block that consumes the value.
2. SEL_R with output fed back to an input for an explicit sample-and-hold.
3. Runtime group scheduling so the block is only re-evaluated every N seconds.

These techniques are valid for any REAL signal in a CFC chart: process measurements, controller outputs, calculated values, or operator-entered setpoints. They are also valid for other elementary types (BOOL, INT, DINT) by substituting the appropriate select block (SEL_BO, SEL_I, SEL_DI).

2. Prerequisites

Before implementing a track-and-hold function in CFC, verify the following:

• Engineering environment: SIMATIC PCS 7 V7.0 or later with the CFC option installed. CFC V7.0+ is included with every PCS 7 ES installation. Earlier PCS 7 V6.x versions use the same CFC engine but the runtime editor is slightly different.
• AS hardware: AS 400 series CPU (e.g., CPU 414-3, CPU 416-3, CPU 417-4 or H-CPU 417-4H) with sufficient free work memory. Each REAL feedback path consumes one block I/O slot.
• Library access: The SEL_R block is located in the Standard Library under Selection. In PCS 7 V7.0 SP1 and later, the same block is also exposed through the PCS 7 APL (Advanced Process Library).
• CFC license: A valid CFC license is required on the Engineering Station (ES). The license is enforced at compile / download time, not at edit time.
• Runtime groups: At least one user-defined runtime group with the desired cycle time (OB35 by default, 1000 ms) is available to host the T/H logic.
• Online access: An established online connection to the AS for the verification step (recommended for trace recording).

For the elementary-block definitions used in this document, refer to the official Siemens manual CFC Elementary Blocks for S7 (PDF, Siemens Industry Online Support).

3. The Three Methods Compared

4. Method 1 - Freezing a Value with the EN Input

Every CFC block carries an implicit EN (enable) binary input and an ENO (enable output). When EN = FALSE, the block is not processed during the OB tick; its output pins retain the last computed value. This is the simplest track-and-hold pattern and requires no additional block at all.

4.1 Wiring procedure

1. Place the consuming block (e.g., a CTRL_PID, OP_A_LIM, or any APL block) on the CFC sheet.
2. Open its input list (View → Inputs/Outputs).
3. Wire a BOOL tag (or a constant TRUE) to the EN input.
4. Connect the REAL process value to the block's primary input (e.g., PV for CTRL_PID).

4.2 Behaviour

• EN = TRUE - the block updates each OB cycle with the live process value.
• EN = FALSE - the block is skipped; PV retains the last value it had when EN was last TRUE. The value is preserved across the entire OB scan, including in the output image sent to the OS.

5. Method 2 - SEL_R as a Track-and-Hold

The standard SEL_R block implements a binary selection between two REAL inputs. The block is the canonical S7 CFC element for building a digital sample-and-hold. Its I/O definition is:

5.1 Track-and-hold wiring (HOLD-when-K = TRUE)

1. Place a SEL_R block in the chart.
2. Wire the live process value (e.g., a temperature from an AI block) to IN1.
3. Wire the output OUT back to IN0. This creates the holding feedback path.
4. Wire a BOOL HOLD_CMD tag to the K input.
5. Wire OUT to the downstream consumer (HMI, controller, batch record, etc.).

5.2 Resulting truth table

When K transitions to TRUE, the block outputs its previous value (because IN0 = OUT of the previous cycle), and on every subsequent cycle IN0 still equals OUT, so the output remains latched. When K is released (FALSE), the output follows IN1 again with a one-cycle latency inherent to the feedback path.

5.3 Automatic hold for N seconds with a TP timer

To release the hold automatically after a fixed duration, combine the SEL_R with an IEC pulse timer from the Standard Library:

1. Place a TP (pulse, IEC 61131-3) block.
2. Wire the hold trigger (rising edge BOOL) to TP IN.
3. Set TP PT to the desired hold time, e.g. T#5s for 5 seconds.
4. Wire TP Q (the active pulse output) to the SEL_R K input.

Result: when the trigger pulses, the SEL_R holds its value for 5 seconds regardless of further trigger events, then automatically reverts to TRACK mode. This is the standard PCS 7 pattern for "latch on alarm, release after timeout".

6. Method 3 - Runtime Group Cycle Scheduling

Every CFC chart has a "Run sequence" that lists the runtime groups installed on the AS. Each runtime group is associated with one of the S7-400 cyclic interrupt OBs. The PCS 7 V7.0 default mapping is:

The cycle time of OB30 to OB38 is configured in HW Config → CPU properties → Cyclic Interrupts. PCS 7 will not start the AS if the configured cycle time is shorter than the worst-case OB execution time (CPU goes into OB85 "Organization block error" if the OB is missed).

6.1 Configuring a new runtime group

1. Open the CFC chart.
2. From the menu, select Options → Run sequence (or right-click the chart background and choose Run sequence).
3. In the Run sequence dialog, click Insert to add a new runtime group.
4. Assign a descriptive name, e.g. TEMP_HOLD_GROUP.
5. Assign the OB (e.g., OB35 for 1 s).
6. Set the reduction ratio (default 1) and the phase offset (default 0). The phase offset is the delay, in OB-tick units, applied at startup to spread CPU load.
7. Click Install to download the runtime group to the AS. Without "Install" the group exists only in the ES project.

6.2 Why cycle scheduling works as a hold

If a block is placed in OB35 (1 s), its outputs are recomputed once per second. If a downstream block is placed in OB38 (100 ms), it samples the OB35 output ten times per second. Between two OB35 evaluations, the downstream block sees the same frozen REAL value ten times. This is the cheapest possible "hold" because no logic is required - the value simply cannot change more often than the producing OB allows.

7. Step-by-Step Implementation: Latch Peak Temperature on Alarm

Combining methods 1 and 2, the following procedure latches a temperature reading on a high-high alarm and releases it 10 seconds after the alarm clears. This is a field-proven PCS 7 pattern used in fired-heater and reactor interlock applications.

1. Create a CFC chart named TEMP_LATCH in the AS program.
2. From the PCS 7 APL, drop an CTRL_PID (or a MONO block) for the temperature signal as the source.
3. Drop a SEL_R block (Standard Library → Selection).
4. Wire the temperature measurement to IN1.
5. Wire OUT back to IN0.
6. Drop a high-limit alarm block (e.g., APL LIMIT or MONO alarm) to detect the high-high condition. Wire the alarm output to a BOOL HH_FLAG.
7. Drop a TP (pulse) block from the IEC Timers folder. Set PT = T#10s.
8. Wire the rising edge of HH_FLAG to TP IN (use an edge-detect R_TRIG block in front of TP for a clean edge).
9. Wire TP Q to K of the SEL_R.
10. Wire SEL_R OUT to the consumer (HMI faceplate tag, archive tag, or interlock).
11. Open Run sequence and place the chart in OB35 (1 s).
12. Compile and download to the AS.

8. Verification and Commissioning

8.1 Offline checks (S7-PLCSIM)

1. Start S7-PLCSIM and load the compiled S7 program.
2. Force the process value at the AI block to a known constant (e.g., 25.0 degC).
3. Observe the SEL_R output: it must follow IN1.
4. Toggle the TP input (set HH_FLAG high, then low). The SEL_R output should hold 25.0 degC for 10 s and then resume tracking.
5. Repeat with the process value changing during the hold - the output must remain pinned to 25.0 degC for 10 s.

8.2 Online checks (live AS)

1. Open the chart online and use Control & Monitoring (CFC Online) to watch the SEL_R input/outputs in real time.
2. Insert a trace in S7-SCOPE (PCS 7 V7.0+ ships with this) on IN1, OUT, and K. Trigger on K rising edge.
3. Confirm hold duration is within ± one OB cycle of the configured PT.
4. Verify that ENO of the consuming block goes FALSE when EN is forced FALSE.

9. Common Errors and Diagnostics

10. Block Diagram of the SEL_R Track-and-Hold

11. Performance and Sizing Notes

• Cycle time impact: a SEL_R feedback T/H adds two block executions per OB tick (one read, one write). On a CPU 416-3 with 1 s OB35, this is negligible (< 0.05 ms per block).
• Scan latency: a one-cycle latency exists between K transition and OUT change. For an alarm-latch function this is irrelevant; for a closed-loop interlock that must respond in < 50 ms, place the chart in OB38 (100 ms) or OB37 (200 ms).
• Memory: each block I/O consumes 4 bytes for BOOL and 4 bytes for REAL. A T/H chain of 100 tags costs approximately 800 bytes of work memory and 1.6 kB of load memory on the CPU.
• HMI: every latched value that is sent to WinCC must be configured in the tag management. Add the SEL_R output to the AS-OS connection in NetPro if not already in the default transfer list.

12. Frequently Asked Questions