Configuring PID Loops on S7-300 CPU 312: FB58 Memory & Tuning

Overview

The Siemens SIMATIC S7-300 CPU 312 (MLFB 6ES7 312-1AD10-0AB0) is an entry-level compact CPU frequently deployed in small HVAC, building-services, and process-control retrofits. Engineers who try to implement multi-loop PID on this CPU encounter two distinct constraints that are not obvious from the catalog datasheet alone: (1) the 16 kB of integrated work memory limits the number of PID instances that can be resident simultaneously, and (2) the S7-300 family splits its PID blocks between the firmware (SFB 41/42/43, available only on CPU 314 and above) and the STEP 7 Standard Library (FB 41/42/43 and FB 58, which can be loaded onto any CPU with sufficient memory). This reference documents the correct block selection, memory budgeting, and self-tuning procedure for FB 58 (TCONT_CP) running on a CPU 312, using a five-zone air-conditioning application as the worked example.

CPU 312 Hardware Identification

The catalog number 6ES7 312-1AD10-0AB0 identifies a CPU 312 with the following relevant characteristics for PID applications:

Source: Siemens SIMATIC S7-300 CPU 31xC and CPU 31x operating instructions and the ST 70 catalog. Because the CPU 312 has only 16 kB of work memory, the entire PID program (block code + instance DBs + any analog scaling logic) must fit inside that envelope.

PID Block Library: SFB vs FB Selection

The STEP 7 Standard Library contains two parallel families of PID blocks. Choosing the wrong one is the single most common reason a CPU 312 PID program fails to build.

The firmware-resident SFB 41/42/43 are integrated into the operating system of higher-tier CPUs only; the CPU 312 firmware does not expose them. Engineers who try to call SFB 41 from a CPU 312 program receive a "Block does not exist on CPU" or download error. The fix is to delete the SFB calls and replace them with the identically-numbered FB blocks copied from the Standard Library. The FB versions run from load memory and instance DBs the same way as SFBs but consume more work memory because the code is downloaded rather than firmware-resident.

Memory Budget Calculation

Memory sizing for PID on the CPU 312 uses the formula:

Work memory required ≈ (code footprint of FB × N) + (IDB size × N) + OB35 overhead + analog scaling + safety logic

Field-measured values for FB 41 (CONT_C) on the CPU 312:

Applying the rule of thumb to a 16 kB work-memory budget:

N_max ≈ (16 kB − OB35/scaling overhead) / 2 kB per loop

With 4 kB reserved for OB35, alarm OB, and analog scaling, the practical ceiling is:

N_max ≈ (16 − 4) / 2 ≈ 6 loops

Siemens field guidance places the realistic production maximum at 4 to 5 PID loops in parallel on a CPU 312 once additional logic (alarms, bumpless transfer, manual stations, diagnostic blocks) is added. Exceeding the budget generates SF (system fault) errors at download or OB1 priority-class errors at runtime, depending on the memory region overflowed.

How to Verify on a Live Project

1. Open the STEP 7 project, right-click the S7 program and select Object Properties.
2. Open Resource > Memory in the online view to see live work-memory consumption.
3. Build a dummy OB35 containing N copies of FB 41 with a single instance DB each.
4. Download the project and read the "Used work memory" value.
5. Increment N and re-download until the next call exceeds the budget; the last successful N is your hardware ceiling.

FB 58 TCONT_CP Architecture

FB 58 (TCONT_CP) is the temperature-specific PID block in the PID Control section of the STEP 7 Standard Library. It bundles a continuous PID core, a pulse generator for digital heating/cooling outputs, and a self-tuner that performs process identification followed by controller design during a startup phase. The block is the recommended choice for HVAC zone control, plastic-extrusion zones, and similar multi-loop thermal applications.

Auto-Tuning Procedure: TUN_ON and TUN_ST

The self-tuner in FB 58 is invoked by setting the input parameters TUN_ON = TRUE and then triggering a rising edge on TUN_ST. If the PHASE and STATUS outputs remain at zero, the most common root causes are: (a) the setpoint in SP_NT is not far enough above the current process value, (b) the process value is not updated inside the calling OB, or (c) FB 58 was copied into the project without its associated instance DB (the block requires DB 58 in the same program).

Step-by-Step Commissioning Sequence

1. Copy FB 58 and the associated DB 58 from the STEP 7 example project into the user program; do not generate a new instance DB — use the one shipped with the FB.
2. Wire the analog input (e.g. PT100 via SM 331) through FC 105 to FB 58 input PV.
3. Set initial conservative PID values: GAIN = 1.0, TI = T#30s, TD = T#0s.
4. Set TUN_ON = TRUE in the static logic.
5. Set the new SP_NT to 10–15 °C above the measured ambient; do not trigger TUN_ST first, because the tuner checks the step size before it will start.
6. Apply a single rising edge to TUN_ST from the HMI or a one-shot in the user program.
7. Monitor PHASE; values 1–7 indicate active identification stages, value 0 + STATUS ≠ 0 indicates a fault (see diagnostic table below).
8. When PHASE returns to 0 and STATUS = 0, FB 58 has written tuned values for GAIN, TI, and TD into the instance DB. Clear TUN_ON if no further re-tuning is desired.

SP_NT Setpoint Jump Requirement

FB 58 needs a measurable excitation to identify the process. The SP_NT parameter is the effective setpoint seen by the closed loop; the setpoint step is the difference Δw = SP_NT − PV(t=0). Siemens guidance for the TCONT_CP family requires Δw to be large enough to drive the process out of its noise band within one sample interval of the calling OB (typically OB35 at 100 ms). For HVAC air zones, the practical minimum is 10–15 °C above ambient; for water-loop temperature control, 5–10 °C is usually sufficient.

The 10–15 °C value is application engineering practice, not a hard-coded FB 58 limit. The tuner simply requires that the resulting process-value deviation exceed the analog-input noise floor by at least 5× to achieve an identifiable step response. If the setpoint step is below this, FB 58 stays in PHASE = 0 with a non-zero STATUS indicating "insufficient excitation."

HVAC Zone Application: Five Air-Conditioning Zones

The reference application uses a CPU 312 to control five independent air-conditioning zones, each with a supply-air temperature sensor (PT100 or 0–10 V transmitter) and a modulating chilled-water valve (4–20 mA) driven by an SM 332 analog output. Memory and execution budgets for this configuration:

The five-zone configuration fits inside the 16 kB budget with less than 1 kB of headroom. Field practice is to drop the project onto the CPU early in the engineering phase, read the live work-memory usage, and confirm that the 5th loop does not push the project past the available memory. If the result is borderline, three options exist:

1. Reduce the number of HMI-tag-visible parameters to keep IDB 58 smaller (use S7-300 "Only store in load memory" for non-diagnostic tags).
2. Consolidate the OB35 wrapper into a single multi-instance call (saves ~0.5 kB).
3. Upgrade to a CPU 313 (32 kB work memory) or CPU 314 (48–96 kB depending on variant), which removes the budget constraint.

Diagnostic Outputs: PHASE and STATUS

When PHASE = 0 and STATUS = 0 after a rising edge on TUN_ST, the self-tuner has not started. The most common root causes and their verification are:

Alternatives for Larger PID Counts

If the application requires more than five PID loops, the CPU 312 is the wrong hardware. Evaluate the following alternatives, in order of increasing capability:

Loop counts above are derived from the same 2 kB per loop working figure, with 4 kB reserved for non-PID overhead. They are conservative; actual usable counts depend on instance DB configuration, HMI tag count, and any safety/messaging blocks. Always confirm with a live memory test before committing hardware.

Commissioning Checklist for FB 58 on CPU 312

1. Verify CPU 312 MLFB is 6ES7 312-1AD10-0AB0 (or compatible variant) and MMC is inserted.
2. Confirm FB 58 (not SFB 41/42/43) is being used; FB 58 and DB 58 are copied from the STEP 7 example program.
3. Calculate memory budget; target 4–5 PID loops maximum; document remaining headroom.
4. Wire AI → FC 105 → FB 58.PV in OB35 at 100 ms (or 200 ms if process is slow).
5. Set conservative initial PID values and run in manual first to verify scaling direction.
6. Switch to auto, raise SP_NT by 10–15 °C above measured PV, then apply rising edge to TUN_ST.
7. Monitor PHASE and STATUS; verify tuner completes (returns to 0) within expected identification time.
8. Record tuned GAIN, TI, TD in DB 58 and back up the project.
9. Capture the online work-memory reading; archive in the project folder for future expansion planning.

FAQ