Configuring SFB 47 on S7-314C-2 DP for Encoder Pulse Counting

Overview: S7-300 Compact CPUs and Integrated Counting Functions

Siemens SIMATIC S7-300 compact CPUs (CPU 31xC and the legacy CPU 314 IFM) integrate technology functions inside the central processing unit. Unlike modular PLCs that require Function Modules (e.g. FM 350-1, FM 450, ET 200S 1COUNT) to be plugged into the backplane, compact CPUs include high-speed digital I/O, analog channels, and counter logic inside the CPU module. The technology functions are accessed through dedicated system function blocks (SFBs) that map onto the CPU's internal technology objects.

The most common application of those integrated counters — and the central question that opens this article — is reading pulse-train feedback from a 24 V or 5 V incremental encoder fitted to an AC motor. The compact CPU can deliver an encoder pulse count, derive motor speed, drive a comparator-based trip output, and emit pulse-width modulated control signals for downstream drives, all without an external counter module.

Two system function blocks are relevant when the encoder feedback has to land in the S7 program:

• SFB 47 (COUNT) — used on the current CPU 31xC family (314C-2 DP, 314C-2 PtP, 314C-2 PN/DP), the IM 151-7 CPU, and the ET 200S 1COUNT terminal.
• SFB 29 (COUNT) — used on the legacy CPU 314 IFM only.

The remainder of this reference explains the distinction, walks through the HW Config and wiring of the 314C-2 DP, shows concrete Ladder / FBD / Structured Text calls for SFB 47, and provides a migration path for projects that still call SFB 29 on the discontinued CPU. Source for the block interface descriptions: the Siemens CPU 31xC Technological Functions manual and the Siemens CPU 314 IFM operating manual. The symbolic and numerical naming convention for SFB 47 (block number 47, name "COUNT") and the wider S7-300/S7-400 instruction set is documented in the Siemens TIA documentation portal.

SFB 47 vs SFB 29: Functional Comparison

The decisive difference is not the interface but the platform viability. The CPU 314 IFM was flagged as "cancelled" in 2003 and removed from production in 2012; no firmware updates and no spare parts have been issued since. Every new encoder-pulse-counting project, every spare-part replacement, and every engineering service request should therefore default to the 314C-2 DP with SFB 47. SFB 29 is included here only to ease migration of legacy code.

CPU 314C-2 DP vs CPU 314 IFM: Hardware Comparison

The 314C-2 DP wins on every spec category that matters for AC-motor encoder feedback:

• Six times the counter frequency (60 kHz vs 10 kHz) — drives a 1 024-ppr encoder up to ~3 500 RPM without filter-induced count loss.
• Four counter channels instead of one — multiplies the per-CPU encoder channel count.
• On-board analog I/O — eliminates an extra SM334/SM335 when the motor drive also needs a process variable feedback.
• Current firmware support — STEP 7 V5.5 SP2 or higher hosts the HSP for the 314C-2 DP and exposes the technology dialog under HW Config.

Counter Modes and Channel Configuration

Each of the four technology channels on the 314C-2 DP can be set to one of five operating modes. The mode is fixed at configuration time; changing it requires re-downloading HW Config.

For an AC-motor incremental encoder you typically select mode 1 or mode 3. The decision is whether you want a never-resetting total position or a per-revolution cyclic position. Inside the channel properties in HW Config the following sub-parameters determine behavior:

For the eight HSC-capable digital inputs on the 314C-2 DP the channel-to-pin mapping is fixed:

Hardware Configuration in STEP 7 HW Config

STEP 7 V5.5 SP2 with the latest HSP, or STEP 7 V5.6 / TIA Portal V16 or higher, hosts the configuration dialog for the 314C-2 DP. Procedure to enable SFB 47 on a channel:

1. Open SIMATIC Manager and the project; double-click HW Config.
2. Insert a S7-300 rack with the power supply (PS 305 / PS 307), then drag the CPU 314C-2 DP (6ES7314-6CH04-0AB0) into slot 2.
3. Double-click the CPU → Properties → set the PROFIBUS / MPI address and the DP mode if used.
4. Switch to the Count tab. Each channel (Count 0..Count 3) is listed; click one to open the Channel Properties dialog.
5. In the Operating Mode tab pick one of: continuous counting, single counting, periodic counting, frequency measurement, PWM output.
6. In the Inputs tab set signal evaluation, sensor type, and input filter.
7. In the Outputs tab configure comparator behavior (set, reset, toggle, hysteresis range).
8. In the Gate tab set the HW gate polarity and the SW gate default (open/closed).
9. Press OK. Save and compile HW Config (Station → Save and Compile).
10. Set the CPU mode switch to STOP, download the HW Config (PLC → Download), then return the switch to RUN.

After the download the channels are initialized with the configured mode and the instance DBs from any pre-existing SFB 47 calls in the S7 program become valid. Calling SFB 47 in OB1 starts the operation.

Encoder Wiring to CPU 314C-2 DP

Wiring an incremental encoder to the 314C-2 DP depends on the supply voltage of the encoder. Pick the 24 V path first; move to the 5 V TTL/RS422 path only when the encoder frequency exceeds 60 kHz.

Wiring: 24 V PNP Encoder

Use shielded twisted-pair cable, clamp the shield on a grounding bar near the panel entry, and avoid running the encoder cable in parallel to VFD power cables. The 40-pin front connector pin assignment for channel 0 is shown below.

Channels 1, 2 and 3 follow the same pattern, shifted by bit offsets of +0.3, +0.6 and +1.1 respectively on the same front connector. Confirm the actual terminal block in your hardware kit because the pin numbering on the printed silkscreen may differ from the manual's reverse-slot numbering.

Wiring: 5 V TTL/RS422 Encoder

For high-frequency encoders (above 60 kHz), connect the encoder through the differential terminals and select "5 V TTL" in the channel properties. The differential terminals accept EIA-422 line levels (≥0.2 V differential).

Calling SFB 47 in Ladder, FBD and Structured Text

SFB 47 must be called once per used channel per scan cycle of the OB that drives the application. STEP 7 auto-generates an instance DB at the first call; renaming it (for example DB_COUNT_CH0) makes the project easier to read. The block reads the technology area through the process-image input word at LADDR = W#16#100 on the 314C-2 DP.

Ladder (LAD) call

Network 1 — Read channel 0
"DB_COUNT_CH0"(
   LADDR  := W#16#100,
   CHANNEL:= 0,
   SW_GATE:= "Tag_SWgate",
   CTRL_DO:= FALSE,
   SET_DO := "Tag_SetDO",
   JOB_REQ:= "Tag_JobReq",
   JOB_ID := 1,
   JOB_VAL:= W#16#0,
   STATUS_H=> "Tag_StatusH",
   STATUS_L=> "Tag_StatusL",
   STS_GATE=> "Tag_StatusGate",
   STS_DO  => "Tag_StatusDO",
   STS_C_UP=> "Tag_CountUp",
   STS_C_DN=> "Tag_CountDn",
   COUNTVAL=> "Tag_CountValue");

Function Block Diagram (FBD) call

In FBD the block appears with the inputs on the left and the outputs on the right. Wire LADDR to W#16#100, CHANNEL to constant 0, SW_GATE to a BOOL tag, and bind the seven outputs to seven BOOL/DWORD tags.

Structured Text (ST) call

// One call per channel per OB1 cycle
"DB_COUNT_CH0"(
   LADDR  := W#16#100,
   CHANNEL:= 0,
   SW_GATE:= "Tag_SWgate",
   CTRL_DO:= FALSE,
   SET_DO := "Tag_SetDO",
   JOB_REQ:= "Tag_JobReq",
   JOB_ID := 1,
   JOB_VAL:= DW#16#0,
   STATUS_H=> "Tag_StatusH",
   STATUS_L=> "Tag_StatusL",
   STS_GATE=> "Tag_StatusGate",
   STS_DO  => "Tag_StatusDO",
   STS_C_UP=> "Tag_CountUp",
   STS_C_DN=> "Tag_CountDn",
   COUNTVAL=> "Tag_CountValue");

Counter value to motor speed conversion

Once COUNTVAL is stable in OB35 (a 100 ms interrupt OB) the application can derive motor speed. With a counter running in mode 4 (frequency measurement) the block returns Hz directly. With mode 1 or 3 the application divides by a fixed window:

n_RPM := (("Tag_CountValue" - "Tag_CountValuePrev")
           / REAL#4.0
           / "Encoder_PPR"
           / "Window_Seconds") * 60.0;

Where:

• Tag_CountValue — current COUNTVAL inside SFB 47
• Tag_CountValuePrev — snapshot taken at the start of the window
• Encoder_PPR — pulses per revolution of the encoder (for 4× evaluation: divide by 4)
• Window_Seconds — measurement window in seconds, e.g. 0.5 s for 500 ms

For a 1 024-ppr quadrature encoder (effective 4 096 counts per revolution) the formula becomes:

n_RPM := (delta_counts / 4096.0 / window_seconds) * 60.0;

Validate against a tachometer or an inverter display; the calculated RPM should agree within ±2 counts of measurement noise.

SFB 47 Input and Output Parameter Reference

Inputs

Outputs

JOB_ID values

A job is processed on the rising edge of JOB_REQ. The result reflects in STATUS_L within one OB1 cycle. Always reset JOB_REQ in the application code once the job is acknowledged (read STATUS_L bit 6 = 0x0040, "job complete").

Diagnostic and Status Behavior

The STATUS_L word is bit-coded. Reading the bit pattern correctly is essential for diagnostics on encoder cable faults, compare-threshold overruns, and overflow detection during start-up.

Two recommended patterns in the application code:

1. Sample STATUS_L into a non-volatile word every OB1 cycle, compare against the previous value, and raise a controlled motor stop when bit 4 toggles (cable break) or bit 0 toggles (overflow).
2. After every job edge, confirm bit 6 returns to 1 within two SFB 47 calls. A stuck-zero means the technology unit did not see JOB_REQ, typically the result of an LADDR typo, a wrong CHANNEL, or the CPU being in STOP.

Migrating from CPU 314 IFM (SFB 29) to CPU 314C-2 DP (SFB 47)

Migration is recommended when any of the following is true:

• CPU 314 IFM fails and no spare is available.
• The new project requires more than 10 kHz counter frequency.
• The new project requires more than one counter channel.
• The new project requires a PWM output from the same CPU.
• STEP 7 V5.5 SP2 or higher is the engineering standard for the site.

Procedural migration steps:

1. Open the existing STEP 7 project. In the symbol table capture every DB member used by SFB 29 (typically COUNTVAL, STS_GATE, STS_DO, STATUS, plus any internal flags).
2. Add a new S7-300 station, mount a CPU 314C-2 DP (6ES7314-6CH04-0AB0), copy the rack and SM modules. Re-write any SM part numbers that may have been discontinued.
3. Re-wire the encoder to the new channel mapping (see wiring section).
4. Replace every call of SFB 29 with a call of SFB 47.
5. Adjust the parameter wiring as follows:

6. Migrate the load values, comparator thresholds, and hysteresis from the HMI pages to the new instance DB constants.
7. Re-test with the original motor. Watch COUNTVAL, the comparator DO state, and the SW_GATE transitions.
8. For projects that need to retain the original archive, lock the legacy project read-only and rebuild the active code on the 314C-2 DP profile.

Commissioning and Verification Procedure

1. Power up the CPU. The SF (system fault) and BF (bus fault) LEDs must be off.
2. With the mode switch in STOP, configure HW Config and download to the CPU.
3. Download the STEP 7 user program containing the SFB 47 call in OB1 and a minimal OB100 (startup) OB.
4. Switch the mode selector to RUN. The CPU enters RUN with no fatal error.
5. Open "Monitor / Modify" on the instance DB and force SW_GATE to FALSE; COUNTVAL remains static.
6. Set SW_GATE to TRUE. Rotate the encoder shaft by hand. COUNTVAL increments in steps equal to the configured edge evaluation. For a 1 024-ppr encoder with 4× evaluation one mechanical revolution yields 4 096 increments.
7. Drive the encoder with a fixed-frequency signal generator (e.g. 5 kHz at 50 % duty) and read COUNTVAL and STATUS_L. Bit 6 ("job complete") should toggle within two OB1 cycles after the next JOB_REQ.
8. Run the AC motor up to operational speed. Confirm COUNTVAL updates every OB1 cycle and STATUS_L bit 0 stays zero. If bit 4 toggles, check the encoder cable shield and ground-loop wiring.
9. Drive a controlled stop. Issue JOB_ID := 7 (reset counter) on a rising JOB_REQ; confirm COUNTVAL returns to zero and STATUS_L bit 6 sets.

Field-proven checks

Spare-Parts and Lifecycle Planning

Because the CPU 314 IFM (6ES7314-5AE10-0AB0) has been discontinued, no spare units are produced. Sites that still need an SFB 29-class encoder counter should plan a one-time migration to the 314C-2 DP, even if the legacy code is to be kept in a frozen archive. The manuals at CPU 31xC Technological Functions and CPU 314 IFM operating manual remain available on the Siemens support portal, but no firmware updates will be issued for the 314 IFM. Symbolic and numerical references for SFB 47 (block number 47, name "COUNT") and the wider S7-300/S7-400 instruction set are documented in the Siemens TIA documentation portal.

Frequently Asked Questions