S7-300 Weekly Timer: Day-of-Week Fan Scheduling in STEP 7

Overview

Scheduling routine start/stop events on a Siemens SIMATIC S7-300 is one of the most common application tasks for HVAC, ventilation, irrigation, and process-cooling systems. The S7-300 family — including the widely deployed CPU 315-2N/DP (6ES7 315-2AH14-0AB0) and similar variants — does not expose a "Clock Alarm" tile in the same way an S7-1200/1500 does in TIA Portal, so the engineer must build the weekly scheduler from the system clock using standard SFCs and a few lines of comparison logic.

This guide shows how to read the real-time clock with SFC 1 (READ_CLK), convert the BCD-encoded date/time bytes into usable integers, and then build a ladder-logic decision tree that drives multiple fan outputs on a per-day, per-time basis. The result is a compact, reusable function block that the maintenance team can reconfigure by editing a data block (DB) — no re-programming required.

Prerequisites

Confirm that the CPU has a real-time clock. Every S7-300 CPU 31x shipped after 2002 has one; the buffer capacitor or backup battery retains it through power loss. The status LED BATF must be OFF, otherwise the clock will reset every time the rack powers down.

S7-300 Date/Time Data Structure

SFC 1 READ_CLK returns the current date and time as eight contiguous bytes in BCD format. The structure is fixed across the entire S7-300/400 family and is the key to all schedule comparisons:

Reading the Real-Time Clock with SFC 1

Call SFC 1 "READ_CLK" in OB1 (or in a low-priority cyclic OB such as OB35 at 1 s) to refresh a shared date/time buffer. The function has no error return path that matters for non-redundant CPUs, but the BOOL RET_VAL can still be evaluated.

LAD / FBD symbol call (STEP 7 V5.x):

      SFC1          DB_RTC
  |---| READ_CLK |---ENO
  |   |           |
  |   EN          |   RET_VAL   MW200
  |               |   CDT       DB10.DBD0   // 8 bytes starting at DBB0
  O---|   |---

After this call, DB10.DBB0 through DB10.DBB7 hold the live date/time. In TIA Portal the same SFC is found under Instructions → Extended Instructions → Date and Time as RD_SYS_T; the call is identical in behaviour.

Extracting Day-of-Week and Hour

To compare a BCD byte against an integer constant (e.g. "Monday = 2"), use FC 38 (BCD_I) on the low byte and then mask the weekday nibble. The cleanest pattern is to place the conversion inside a dedicated FC so the rest of the program sees clean INT variables:

Mask the weekday nibble with W#16#000F to remove the milliseconds bits:

// STL excerpt for STEP 7 V5.x
L     DB10.DBB7          // Load BCD weekday
AW    W#16#0F            // Mask to 0..15
BTI                     // Convert BCD to INT (16-bit)
T     MW 210             // iWeekday

L     DB10.DBB3
BTI
T     MW 212             // iHour

L     DB10.DBB4
BTI
T     MW 214             // iMinute

From here, every subsequent comparison is a plain INT == INT test that fits in a single ladder rung.

Defining the Weekly Schedule Data Block

Put the on/off times in a shared DB (here DB20 "Weekly Schedule") so the maintenance team can edit them online. A two-window-per-day model gives the example requested in the original brief (e.g. Monday 08:00–10:00 and 15:00–19:00):

Storing time as "minutes since midnight" (0–1439) collapses hour and minute into a single integer, so the comparison is one rung per window:

iNowMin = (iHour * 60) + iMinute   // computed in FC

Comparison Logic in Ladder

Build a single rung that decides whether the fans should run right now. Only one rung is needed per fan group; the rung references the current day's two windows in DB20.

Example for Monday (weekday = 2):

  --| |--|---------|/|------------------------------------( )--
   iWeekday = 2     AND                                    Fan1_On
                 ( iNowMin >= DB20.DBW0
                   AND iNowMin <  DB20.DBW2 )
                  OR
                 ( iNowMin >= DB20.DBW4
                   AND iNowMin <  DB20.DBW6 )

Repeat the same structure for Tuesday, Wednesday, etc., with a 7-way OR on the weekday input. In practice, a small FC/FB scales better:

1. Index into DB20 using (iWeekday - 1) * 8 as the base offset.
2. Read the two ON/OFF pairs (DBWbase+0, +2, +4, +6) into local tags.
3. Evaluate the two window comparisons and OR them together.
4. Assign the result to the fan output tag (e.g. Q 4.0).

Reusable Function Block "FB_Scheduler"

To avoid duplicating the comparison tree for every fan group, package it as an FB with an instance DB per fan. The same block can drive 1 fan or 32 fans simply by calling it multiple times.

FB interface (LAD view):

FB_Scheduler
    iWeekday     : INT
    iNowMin      : INT
    wScheduleBase: WORD
    bHandAuto    : BOOL
    bManOverride : BOOL
    bRun         : BOOL   // output, wired to Q area or coil

Inside the FB, use OPN DB20 to open the schedule DB, then L DBW[ wScheduleBase + 0 ] through +6 to pull the four minute-of-day values. Two ANDs, two ANDs, one OR, and a final OR with bHandAuto AND NOT bManOverride produces the run command.

Wiring the Fan Outputs

Each fan is wired through a digital output module (e.g. SM 322 DO16 DC24V/0.5A). For motor loads, always use an interposing relay or contactor coil — never drive a motor starter directly from the SM card. A typical output assignment is:

For a S7-300 315-2N/DP, slot 4 is a convenient location for the DO module, but consult the hardware configuration in HW Config for the actual slot assigned in your rack.

Setting the Clock

If the S7-300's RTC drifts or after a battery replacement, the time must be set. The cleanest method from HMI is to write to SFC 0 "SET_CLK" using a faceplate that mirrors the same 8-byte BCD layout. An alternative is to enable time-of-day interrupts (OB10–OB17) from HW Config so the CPU can call a service routine once per minute. Combined with the LGF Library's LGF_SetRTC block (if a CP is installed), the controller can keep itself synchronised to an NTP server.

Verification and Commissioning

1. Open the project in STEP 7 and download hardware + software to the CPU in RUN-P.
2. Force the clock to a value just before the first ON time (e.g. 07:59 on Monday) and observe the watch table VAT_Schedule in online mode.
3. Confirm iWeekday, iHour, and iMinute match the forced clock and that Fan1_Run transitions high at the programmed minute.
4. Step the clock through 24 hours, checking each on/off transition in the watch table.
5. Power-cycle the rack and verify the schedule resumes after the buffer capacitor has discharged — this confirms the RTC and DB retentive settings are correct. Mark DB20 as retentive in the CPU properties (Retain → Data Blocks).
6. Switch a fan to manual mode using the operator panel and verify the schedule is bypassed without affecting the other fans.

Troubleshooting Matrix

Alternative: Time-of-Day Interrupt OB10

For an S7-300 315-2N/DP you can also configure OB10 in HW Config to call a routine once per minute. The OB10 start time is set with a BCD DT constant; the runtime can then update internal flags that feed the same comparison logic. This reduces CPU scan load and gives 1-second resolution. Combined with the LGF library's LGF_TimerSwitch (S7-1200/1500), the two platforms share an identical schedule XML, simplifying multi-PLC projects.

Sample STL Listing (OB1 excerpt)

// --- Read RTC once per second (call in OB35 if available) ---
CALL SFC 1
     RET_VAL  := MW200
     CDT      := DB10.DBD0

// --- Extract weekday (low nibble of byte 7) ---
L     DB10.DBB7
AW    W#16#0F
BTI
T     MW210              // iWeekday 1..7

// --- Extract hour and minute ---
L     DB10.DBB3
BTI
T     MW212              // iHour

L     DB10.DBB4
BTI
T     MW214              // iMinute

// --- Compute minutes since midnight ---
L     MW212
L     60
*I
L     MW214
+I
T     MW216              // iNowMin 0..1439

// --- Drive three fans via FB_Scheduler ---
CALL FB 100, DB21
     iWeekday      := MW210
     iNowMin       := MW216
     wScheduleBase := 0            // fan 1 base = Monday offset 0
     bHandAuto     := DB30.DBX0.0
     bManOverride  := DB30.DBX0.1
     bRun          := Q 4.0

Field-Commissioning Checklist

• CPU diagnostic buffer clear of "Time-of-day interrupt not started" entries.
• Battery voltage > 2.7 V at the BAT holder.
• DB20 online view shows expected ON/OFF minute values for each day.
• Watch table confirms transition at scheduled minute within ±1 s.
• Manual mode override works independently of the schedule.
• Power-down test: schedule resumes correctly after at least 30 minutes offline.
• Operator HMI has a faceplate to force-edit the schedule DB online.

FAQ