Scheduling S7-1200 Weekly Outputs with TIA Portal V15 LGF

1. Problem Definition: Weekly Output Scheduling on S7-1200

A home or building automation system running on a Siemens SIMATIC S7-1200 CPU 1217C must activate physical outputs on a weekly cadence. Some loads (lighting, irrigation pumps, signage, HVAC setback relays) need daily on/off windows at fixed clock times; other loads only run once per week (cleaning cycle, generator exercise, test routines). A typical requirement looks like:

• Output Q0.0 (lobby lights) — daily 11:00 → 12:00, every day of the week.
• Output Q0.1 (irrigation pump) — Mondays 09:00 → 10:00 only.
• Output Q0.2 (backwash valve) — Wednesdays 03:00 → 03:15 only.
• Output Q0.3 (signage) — daily 17:00 → 23:30.

The logic must be parameterised at runtime from a WinCC HMI so the end user can change the active days, start time, and stop time without reloading the PLC program.

Two specific problems must be solved in TIA Portal V15:

1. Build a reusable weekly scheduler block that evaluates the real-time clock (RTC) of the CPU 1217C and asserts an output for the configured day/time window.
2. Expose the schedule parameters to a WinCC runtime so an operator can edit the active days, on-time, and off-time from the panel and persist the changes through a power cycle.

The cleanest engineering path is the Siemens Library of General Functions (LGF), shipped as a free global library for TIA Portal and explicitly maintained for S7-1200/S7-1500. The LGF contains a pre-built weekly timer switch block that handles the day-of-week mask, time-of-day window, and edge detection — exactly the function required.

2. Prerequisites

3. Solution Architecture

The scheduler sits between the HMI tag interface and the PLC outputs. The HMI writes the schedule entries into a global data block (DB). On each PLC scan, the LGF weekly timer block reads the configured day mask and on/off time, compares them to the CPU's RTC, and sets a per-slot boolean. The boolean drives the physical output directly via an assignment or via a separate logic stage (interlocks, manual override, etc.).

4. Installing the Library of General Functions (LGF) in TIA Portal V15

1. Download the LGF V15.1 global library from the Siemens support page 109750448. The archive is named 109750448_LGF_V15_1.zip (or the V15.0 equivalent).
2. Extract the archive locally. The extracted folder contains LGF_V15_1.al15_1 (or the matching version file).
3. In TIA Portal V15, open the target S7-1200 project.
4. From the menu, choose Options → Global libraries → Open.
5. Browse to the extracted .al15_1 file and open it.
6. If you downloaded the V14 SP1 library by mistake, V15 will prompt to upgrade; click Upgrade. The V14 SP1 master copy folder is converted in place.
7. After upgrade, expand the library tree: LGF_V15_1 → Types → Timer. The weekly scheduler block (LGF_TimerSwitch) lives here.
8. Drag the timer block from the library types into your project's Program blocks folder. The LGF adds its own data types and auxiliary FBs automatically; these are placed in the System blocks → LGF subtree.

5. Using LGF_TimerSwitch for Weekly Time Slots

The LGF weekly timer block (LGF_TimerSwitch) provides a fully parameterised weekly time switch. The instance is called once per scheduled output. The minimum interface looks like:

Edge behaviour: onTime == offTime is treated as "always OFF" by most LGF revisions. If offTime < onTime (crosses midnight), the block holds the output from onTime through 23:59:59 and from 00:00:00 through offTime. Verify the exact behaviour in the LGF version you are using — it is documented in the LGF help page 109750448.

6. Building the Schedule Data Block

Place the per-slot configuration in a global DB so the HMI can read and write it. A typed structure keeps OB1 readable and lets the HMI use a single symbol for the whole array.

// DB "SchedData" — mark Retain = true on the DB properties
TYPE UDT_SchedSlot
  STRUCT
    enable   : BOOL;        // HMI checkbox
    reserved : BOOL;        // pad to byte boundary
    dayMask  : BYTE;        // bit 0=Mon … bit 6=Sun
    onTime   : TOD;         // TOD#11:00:00
    offTime  : TOD;         // TOD#12:00:00
  END_STRUCT;
END_TYPE

DATA_BLOCK SchedData
  STRUCT
    slot : ARRAY[0..31] OF UDT_SchedSlot;  // 32 schedule entries
  END_STRUCT
BEGIN
END_DATA_BLOCK

In the DB properties, set the Retain attribute. On the CPU 1217C, the retain area is limited to 10 kB for the data block region; a 32-slot UDT of 8 bytes each is 256 bytes, well within budget.

7. Mapping Schedule Slots to PLC Outputs

Drop one LGF_TimerSwitch instance per schedule slot into OB1. With 32 slots, use a small loop in SCL or manual call instances. An SCL approach:

// Inside OB1 or FB "Scheduler" — SCL
#rtc := RD_SYS_T(RET_VAL := #err);                  // current DTL
FOR #i := 0 TO 31 DO
  #timer[#i](enable   := "SchedData".slot[#i].enable,
             dayMask  := "SchedData".slot[#i].dayMask,
             onTime   := "SchedData".slot[#i].onTime,
             offTime  := "SchedData".slot[#i].offTime,
             currentDT:= #rtc,
             active   => #active[#i],
             error    => #errArr[#i],
             status   => #statArr[#i]);
END_FOR;

// Optional override
"Outputs".lobby  := #active[0] OR "Overrides".lobbyManual;
"Outputs".irrig  := #active[1] AND NOT "Faults".pumpFault;

Always add an Override word for each critical output so maintenance can force the output on from the HMI without disturbing the schedule DB.

8. Exposing the Schedule to WinCC HMI

1. In the TIA Portal project tree, open HMI Tags and create a tag array that mirrors the UDT_SchedSlot structure: HMI_SchedSlots[0..31] with the same fields (enable, dayMask, onTime, offTime).
2. Set the PLC tag's access method to Symbolic with S7-1200. Symbolic access is preferred on S7-1200 firmware ≥ V4.2 — the data block must be marked as optimisable. The LGF blocks are compatible with optimised blocks in V15.1.
3. Configure the HMI connection in Devices & Networks with the standard S7-1200 protocol. The HMI default poll rate of 1 s is fine for minute-resolution schedules.
4. For Comfort Panels, also enable the Area pointer — Date/Time under connection properties. The panel will then synchronise its clock from the PLC, avoiding drift between operator edits and the active schedule.

9. HMI Screen Design for Schedule Editing

For each output that has a schedule, build a screen with:

• Seven checkboxes (Mon … Sun) bound to the bits of the HMI tag's dayMask BYTE. A script on the HMI converts "checkbox X clicked" → "set/clear bit X of dayMask".
• Two I/O fields, format HH:mm, bound to onTime and offTime.
• One "Enabled" checkbox bound to enable.
• One "Status" output field bound to the active BOOL of the LGF instance for that slot.

Use the WinCC "Date/Time Picker" or a custom keypad for time entry. Validation: clamp onTime and offTime to 0 … 86 399 999 ms in TOD scale, then reformat to HH:mm:ss for the operator.

For a more compact UI, the seven day bits can be displayed as seven round buttons in a row, lighting green when active. This pattern matches typical building-automation dashboards.

10. Runtime Verification and Diagnostics

1. Clock check: Add a watch table in TIA Portal online mode showing SchedData and the LGF active[] array. Set the PLC time one minute before an expected on-time using Online & Diagnostics → Set time. Watch the active bit assert exactly on the minute.
2. Day mask test: With dayMask = 16#7F (all seven days), the block should fire every day. Reduce to a single day, force the CPU date to that weekday, and verify only that day's output asserts.
3. Cross-midnight test: Set onTime = 23:00, offTime = 02:00. Verify the output is TRUE from 23:00 to 23:59 and from 00:00 to 02:00, and FALSE from 02:00 to 23:00.
4. HMI round-trip: From the panel, change a day checkbox. Watch the corresponding bit of SchedData.slot[i].dayMask update over the S7-1200/Comfort panel connection. Confirm retention by power-cycling the PLC and checking the schedule is still active.
5. Override test: Force Overrides.lobbyManual = 1 from the HMI. Confirm the output follows the override regardless of the schedule.

11. Troubleshooting Matrix

12. Performance, Memory and Scaling

Each LGF_TimerSwitch instance on the S7-1217C consumes roughly 200 bytes of work memory and around 50 µs of OB1 execution time. 32 instances therefore add about 6.4 kB of work memory and 1.6 ms of scan time — negligible on a CPU 1217C (150 kB work memory, 0.04 ms/kInst typical). If you need hundreds of schedules, switch to a different scheduler architecture: store all slots in a single DB array, loop in OB1, and call a single FB instance that processes the array internally. This drops memory to a few hundred bytes total and gives a deterministic scan-time impact proportional to the number of slots.

For redundant scheduling with logging, route each active transition through a logging FB that writes timestamped events to the SD card. The S7-1200 supports the standard "DataLog" instructions for this purpose (refer to the S7-1200 programmable controller system manual at 109751325).

For 24-hour day-of-year and astronomical schedules (sunrise/sunset), the basic LGF timer switch is not sufficient. Combine the LGF weekly timer with a DTL/TimeOfDay conversion FB and a sunrise/sunset table that the PLC reads at startup. The LGF distribution contains helper FBs for DTL math, but the actual sunrise calculation must be supplied by the user or by a custom library (for example, the open-source "AstronomicalClock" example project shipped with the S7-1200 application examples).

FAQ