Scheduling S7-300/400 Outputs by Day-of-Week with SFC1 READ_CLK

Overview

Time-window output scheduling on a SIMATIC S7-300 or S7-400 controller is one of the most common application patterns in HVAC, irrigation, lighting, batch release, and machine-shift control. The requirement is to drive a digital output, e.g. Q1.1, to a high logical level between a configured day-of-week and time (Friday 13:00:00) and a second configured day-of-week and time (Monday 15:30:00), and to repeat the cycle every week indefinitely. The implementation requires careful handling of the 8-byte DATE_AND_TIME BCD structure, the time-of-day interrupt OBs, and the edge case where the active window crosses midnight or the week boundary.

Two production-grade methods exist on the S7-300/400 platform:

1. Polling method: call SFC1 READ_CLK from OB1 on every scan, compare the returned DATE_AND_TIME against a configured window, and set/reset the output with S / R.
2. Time-of-Day interrupt method: configure OB10 through OB17 in HW Config to fire at the start and end of the window, and use SFC28 SET_TINT and SFC30 ACT_TINT to arm them. The OBs each contain a single S Q x.y or R Q x.y statement.

The polling method is more flexible, supports arbitrary windows, and survives CPU restart without re-arming. The time-of-day interrupt method is more deterministic and CPU-efficient, but only supports up to eight hard-wired OB slots per CPU (OB10 only on S7-300; OB10–OB17 on S7-400). Most production applications use the polling method in OB1 wrapped inside an FB for reusability across multiple outputs.

Prerequisites

• STEP 7 V5.5 SP4 or later (or TIA Portal V13 SP1 / V15 / V16 with the S7-300/400 add-on installed) for SFC0 / SFC1 / SFC28–SFC31.
• CPU 31x or CPU 41x with integrated real-time clock. All standard CPU 312 through CPU 319 and CPU 412 through CPU 417 include a hardware RTC backed by the front-panel battery or super-capacitor. See the SIMATIC S7-300 CPU 31xC and CPU 31x Operating Instructions and the SIMATIC S7-400 CPU 41x Manual for the RTC backup specifications of your specific CPU.
• CPU battery in service — the RTC retains time only while the backup power source is functional. On CPU 31xC and CPU 31x the backup is a single CR 1025 lithium cell; replace every 3–5 years. A depleted battery on cold restart causes the time to revert to 01.01.1990 00:00:00 and your scheduler to misbehave silently.
• HW Config project loaded to the CPU so the time-of-day OB numbers are recognized in the CPU firmware.
• A configured output address, e.g. Q1.1 on an SM 322 DO16 digital output module, or a built-in CPU output on CPU 31xC.

DATE_AND_TIME (DT) Data Type Specification

STEP 7 represents date and time as a fixed 8-byte BCD-encoded structure. The layout follows the IEC 61131-3 DT definition used by S7-300/400 and WinAC:

The weekday field is critical: a direct BCD comparison against a literal such as 6 for Friday will fail because the high nibble of byte 6 contains part of the milliseconds counter. You must mask the byte with W#16#0F (or B#16#0F for a byte-AND) before comparing the weekday.

SFC0 SET_CLK and SFC1 READ_CLK

Two system functions are needed to read or set the CPU clock. Both are part of the standard library shipped with STEP 7 and require no additional license. Full descriptions are in the SIMATIC S7-300/400 System Software — System and Standard Functions Reference Manual.

Typical call in STL or SCL:

CALL "READ_CLK"   // SFC1
     RET_VAL := #iRetVal
     CDT     := #dtNow;     // DATE_AND_TIME, 8 bytes

After the call, #dtNow contains the current date and time. The individual fields are accessible either by symbolic access (if you assign a DATE_AND_TIME symbol in the symbol table) or by absolute byte addressing against the variable's base address. To set the clock from the user program (e.g. for synchronized time from a master clock), call SFC0 with a DT variable pre-loaded with the desired date and time.

Method 1: Polling with SFC1 in OB1 (Reference Implementation)

The cleanest correct implementation reads the clock once per cycle in OB1, masks the weekday field, and performs the window comparison inside an FB so the same logic can be reused for multiple outputs. The following SCL implementation covers both the same-day topology and the cross-week topology with a single bWrap input flag.

FB100 — Weekly Time Window Scheduler (SCL)

FUNCTION_BLOCK FB 100
TITLE   = 'Weekly Time Window Scheduler'
VERSION : '1.0'
VAR
   iStartWd   : INT;    // 1=Sun .. 7=Sat
   iStartHour : INT;    // 0..23
   iStartMin  : INT;    // 0..59
   iStartSec  : INT;    // 0..59
   iEndWd     : INT;
   iEndHour   : INT;
   iEndMin    : INT;
   iEndSec    : INT;
   bWrap      : BOOL;   // TRUE = window crosses midnight or week boundary
   bActive    : BOOL;   // computed output state
   dtNow      : DATE_AND_TIME;
   iRetVal    : INT;
END_VAR
VAR_TEMP
   iWd : INT;
   iH  : INT;
   iM  : INT;
   iS  : INT;
END_VAR
BEGIN
   // 1. Read CPU clock
   SFC1(CDT := dtNow, RET_VAL := iRetVal);

   // 2. Extract fields. Byte access on DATE_AND_TIME uses BCD;
   //    mask weekday, keep raw BCD for time fields (compared as INT).
   iWd := WORD_TO_INT(dtNow[6] AND W#16#0FFF);   // weekday low nibble
   iH  := BCD_TO_INT(BYTE#16#00 OR dtNow[3]);    // hour
   iM  := BCD_TO_INT(BYTE#16#00 OR dtNow[4]);    // minute
   iS  := BCD_TO_INT(BYTE#16#00 OR dtNow[5]);    // second

   // 3. Compute "in window"
   IF bWrap THEN
      // Topology: start weekday > end weekday (crosses week)
      //           or same weekday but start time > end time (crosses midnight)
      IF (iWd > iStartWd) AND (iWd < iEndWd) THEN
         bActive := TRUE;
      ELSIF (iWd = iStartWd) AND
            ((iH > iStartHour) OR
             (iH = iStartHour AND iM > iStartMin) OR
             (iH = iStartHour AND iM = iStartMin AND iS >= iStartSec)) THEN
         bActive := TRUE;
      ELSIF (iWd = iEndWd) AND
            ((iH < iEndHour) OR
             (iH = iEndHour AND iM < iEndMin) OR
             (iH = iEndHour AND iM = iEndMin AND iS <= iEndSec)) THEN
         bActive := TRUE;
      ELSE
         bActive := FALSE;
      END_IF;
   ELSE
      // Topology: start and end on the same weekday
      IF (iWd = iStartWd)
         AND ((iH > iStartHour)
              OR (iH = iStartHour AND iM > iStartMin)
              OR (iH = iStartHour AND iM = iStartMin AND iS >= iStartSec))
         AND ((iH < iEndHour)
              OR (iH = iEndHour AND iM < iEndMin)
              OR (iH = iEndHour AND iM = iEndMin AND iS <= iEndSec))
      THEN
         bActive := TRUE;
      ELSE
         bActive := FALSE;
      END_IF;
   END_IF;
END_FUNCTION_BLOCK

Instantiate the FB in OB1 as a multi-instance or as a separate instance DB100. For the example, configure DB100 with: iStartWd = 6 (Friday), iStartHour = 13, iStartMin = 0, iStartSec = 0, iEndWd = 2 (Monday), iEndHour = 15, iEndMin = 30, iEndSec = 0, bWrap = TRUE.

Driving Q1.1 from FB100

CALL FB 100 , DB 100 ;
     iStartWd   := 6,
     iStartHour := 13,
     iStartMin  := 0,
     iStartSec  := 0,
     iEndWd     := 2,
     iEndHour   := 15,
     iEndMin    := 30,
     iEndSec    := 0,
     bWrap      := TRUE,
     bActive    := bActiveRun;

A   "DB100".bActive;
S   Q 1.1;
AN  "DB100".bActive;
R   Q 1.1;

Method 2: Time-of-Day Interrupts (OB10 – OB17)

For deterministic, low-overhead scheduling, configure a time-of-day interrupt OB. S7-300 CPUs support OB10 only; S7-400 CPUs support OB10 through OB17 (eight slots). Each OB is configured in HW Config with a start time and periodicity. The OB body contains a single line: S Q x.y for the start OB and R Q x.y for the end OB.

HW Config Settings

1. Open HW Config and select the CPU.
2. Open Properties > Time-of-Day Interrupts.
3. For OB10 (or OB11 – OB17 on S7-400), set:
   • Execution: "Once" for one-shot, or "Every minute / hour / day / week / month" for cyclic.
   • Start time: e.g., 13:00:00 on Friday → format DD-MM-YY HH:MM:SS.
   • Phase offset (for periodic): leave at 0 for a fixed weekly start time.
4. Save & download the hardware configuration.

Arming from the User Program

After a CPU restart the time-of-day OBs are disarmed by default. To re-arm them from the user program (e.g., from OB100 warm restart), call SFC30 ACT_TINT:

CALL "ACT_TINT"          // SFC30
     OB_NO   := 10
     PERIOD  := W#16#0001     // once; see manual for cyclic values
     RET_VAL := #iRetVal;

To cancel an already-armed time-of-day interrupt, call SFC29 CAN_TINT. To query the status, call SFC31 QRY_TINT, which returns STATUS as a WORD with bit 0 = enabled, bit 1 = active, bit 2 = expired. To change the configured start time without re-running HW Config, call SFC28 SET_TINT with a new SDT parameter.

Cross-Day and Cross-Week Boundary Logic

The most common source of bugs in weekly schedulers is the implicit assumption that the start and end moments fall on the same calendar day. The reference FB100 above handles three topologies:

For the wrap branch, the algorithm treats the week as a circular index. A time is "in window" if its weekday lies strictly between the start and end weekdays, OR (if it equals the start weekday) the time-of-day is >= start time, OR (if it equals the end weekday) the time-of-day is <= end time. The iWd > iStartWd AND iWd < iEndWd condition relies on the wrap branch only being entered when iEndWd < iStartWd or when the time-of-day crosses midnight on a single weekday — the strict inequality is then geometrically correct on the circular scale.

Step-by-Step Commissioning Procedure

1. Create the FB: insert FB100 with the declarations shown above. Compile with no errors.
2. Instantiate the FB in OB1 as a multi-instance or as DB100 (instance DB).
3. Configure the start and end values in the instance DB. For the example: iStartWd = 6 (Friday), iStartHour = 13, iStartMin = 0, iStartSec = 0; iEndWd = 2 (Monday), iEndHour = 15, iEndMin = 30, iEndSec = 0; bWrap = TRUE.
4. Wire the output: drive Q1.1 from the bActive BOOL with two parallel S / R networks in OB1.
5. Download the blocks to the CPU and place the CPU in RUN.
6. Set the CPU clock to a value just before the start (e.g., Friday 12:59:50) using SFC0 SET_CLK or the online Set Time dialog in STEP 7 (PLC > Set Time of Day).
7. Monitor Q1.1 in the VAT or in the online block view. Confirm that Q1.1 goes high within one OB1 scan after the clock crosses Friday 13:00:00.
8. Advance the clock through the entire active window and confirm Q1.1 drops within one scan after the clock crosses Monday 15:30:00.
9. Cycle the CPU power (cold restart) and confirm the schedule resumes from the new current time without re-arming. Polling-based scheduling has no arming state.

Verification

After commissioning, run the following verification matrix. Adjust the CPU clock to each test point using SFC0 SET_CLK and confirm Q1.1 matches the expected state within one OB1 scan (typically 10–100 ms). All ten states must match before the schedule is considered production-ready.

If test 1 or test 10 fails, the wrap logic is reversed. If test 5 or test 6 fails, the boundary comparison is off-by-one. If the output toggles intermittently at the boundary, the cycle time is longer than the granularity required — consider moving the FB call from OB1 to OB35 (cyclic interrupt, e.g. 100 ms) to bound the worst-case latency.

Troubleshooting Matrix

Alternate Platforms

• S7-1200 / S7-1500 (TIA Portal): use RD_SYS_T to read DTL (64-bit structured time), then compare against a configured window. The DTL layout is similar but uses binary encoding, not BCD, and weekday numbering matches ISO 8601 (1 = Monday, 7 = Sunday). Direct code port from S7-300/400 BCD logic will not work — convert the literals from BCD to decimal before porting.
• LOGO! 8: built-in weekly timer function block in LOGO! Soft Comfort with on-screen configuration. No user code required; supports up to 3 on/off pairs per weekly timer, with optional second weekly timer for second window.
• S7-200 (STEP 7 Micro/WIN): use READ_RTC (SBR6) and write the read result into a VB area. No DATE_AND_TIME type; manual byte comparison against BCD literals and manual weekday masking with AND B, 0x0F.
• WinAC RTX / SoftPLC: same DATE_AND_TIME layout, same SFC1 call, same FB logic. Verified identical behavior on WinAC RTX 2010 and SoftPLC 4.x.

FAQ