Siemens LOGO! Biweekly Timer Alternating Weekly Schedule

Overview: The Biweekly Timer Problem on LOGO!

A biweekly (alternating-week) timer is one of the most common scheduling requests on the Siemens LOGO! 8 platform (6ED1052-xxx08-0BA1 / -0BA2) programmed in LOGO!Soft Comfort V8.x and V9.x. The classic requirement is:

• Week A (odd): output ON from Thursday 19:00 through Monday 07:00 of the following week.
• Week B (even): output ON from Friday 07:00 through Monday 07:00 of the following week.
• The cycle then repeats indefinitely.
• Monday 07:00 behavior must remain ON every Monday, regardless of which week the controller is in.
• A 2-week rhythm where Tuesday (and other mid-week days) is ON for one week and OFF the next must be respected.

This pattern cannot be expressed with a single weekly timer because the LOGO! Weekly timer (function block class B6, B007) only supports one recurring on/off window per week. It requires at least one Weekly Timer (or Yearly Timer) feeding an AND/OR gating structure that is then qualified by a Up/Down counter clocked by a once-per-cycle edge.

Prerequisites

Confirm the BM clock is running and on the correct weekday before commissioning. Navigate LOGO! menu → Setup → Date/Time and verify Mo Tu We Th Fr Sa Su index and 24-hour time. If the BM clock is wrong the weekly timers will mis-trigger by the same offset.

LOGO! Time Block Inventory

LOGO!Soft Comfort provides four time-related blocks that are relevant to biweekly scheduling:

The Weekly Timer block (B007) accepts up to three ON-cam and three OFF-cam entries. Each entry has its own Day-of-week mask (bit-pattern Mon..Sun). For our base pattern, define:

Weekly Timer Channel A (Week-A schedule)

Weekly Timer Channel B (Week-B schedule)

These two channels give you two independent weekly patterns. The next step is to alternate them every seven days using a counter.

Biweekly Alternation: Counter-Based Week Selector

The cleanest way to toggle a 2-week rhythm on a LOGO! 8 is to use an Up/Down counter (function block B003) clocked by a single rising edge per ISO week. A single weekly timer configured with a 1-minute ON pulse at Sunday 23:59 provides the clock source. The counter threshold On = 1, Off = 0 produces a clean 0/1/0/1 output that perfectly aligns with Week-A / Week-B selection.

Clock generator (pulses once per week)

1. Insert a Weekly Timer (B007) named WT_CLK.
2. Cam 1: On 23:59, Off 23:59, Day mask Su only.
3. Insert a Pulse Generator (B002, optional) with PT = 1 s to widen the edge to a reliable counter input.

Week selector counter

1. Insert an Up/Down counter (B003) named CNT_WEEK with:
   • On threshold = 1
   • Off threshold = 0
   • Count direction = count up
   • Reset input = M8 (cold-start flag) OR a manual reset via digital input I1
2. Wire the rising edge of WT_CLK to Cnt.
3. Output Q of CNT_WEEK is 0 in Week-A and 1 in Week-B (assuming initial state 0 after power-up).

Selecting Week-A vs Week-B Output

Use a 2-to-1 multiplexer pattern with two AND gates and one OR gate:

WT_A  ----+---[ AND ]---+
         |             |
~CNT_WEEK -----+        +---[ OR ]--- Q1 (final output)
         |             |
WT_B  ----+---[ AND ]---+
         |
CNT_WEEK -----+

Where ~CNT_WEEK is the inverted output (use NOT block B004). The final OR drives the LOGO! output Q1. This guarantees that:

• When the counter is 0, only WT_A is allowed to pass.
• When the counter is 1, only WT_B is allowed to pass.
• Either channel can independently switch OFF in the middle of its own week without affecting the other week.

Forcing Monday ON Every Week

The base requirement that Monday 07:00 must turn the output OFF (end of cycle) is handled implicitly by both weekly timers: WT_A cam 2 ends at Monday 07:00, and WT-B cam 3 ends at Monday 07:00. To instead force Monday to be always ON regardless of which week the controller is in, OR a third weekly timer onto the final OR gate:

This block is gated by a direct feed (not the counter), so Monday always wins. If you need Monday to be ON only after the weekend cycle, then leave the Monday cam inside WT_A and WT-B as drawn above; do not add a third block.

Alternate Parity Derivation (No UPS, No Battery)

If the application must self-correct the week parity on every cold start (e.g., unreliable power), replace the once-per-week counter clock with a day-of-year counter whose modulus is 14:

1. Daily pulse generator: Weekly Timer with cam at 00:00:01 every day → pulse relay to debounce.
2. Up/Down counter B003, threshold On=14, Off=0, count up.
3. When the counter reaches 14, pulse a reset pulse (B002) with PT=1 cycle to clear it back to 0 and emit a single edge.
4. Wire that edge into a second B003 configured as a toggle (threshold On=1, Off=0). Its Q is the week parity.

This pattern is self-healing after a power loss of any duration because the parity is regenerated from elapsed days, not from a non-volatile event log. The trade-off is that you lose the ability to manually skip a week, since the parity is always derived from real time.

Step-by-Step Build in LOGO!Soft Comfort

1. Open LOGO!Soft Comfort and select your BM from Tools → Select Hardware → LOGO! 8.
2. Drag three Weekly Timer blocks (B007) onto the diagram. Name them WT_A, WT_B, WT_CLK.
3. Configure the cams per the tables above. Use 24-hour format. Day-mask bits: Mo Tu We Th Fr Sa Su, click the days to enable.
4. Insert one NOT block (B004), two AND blocks (B001), and one OR block (B001 OR variant) for the multiplexer.
5. Insert one Up/Down counter (B003). Set On=1, Off=0, Remanence=ON (right-click the block, set Retentive so the count survives power loss with battery present).
6. Wire WT_CLK output → CNT_WEEK.Cnt.
7. Wire WT_CLK output → CNT_WEEK.R through a normally-closed input or skip; do NOT reset weekly, only on cold start.
8. Wire WT_A → AND-A input 1, ~CNT_WEEK.Q → AND-A input 2.
9. Wire WT_B → AND-B input 1, CNT_WEEK.Q → AND-B input 2.
10. Wire AND-A.Q and AND-B.Q to OR block; OR.Q → Q1.
11. Mark Q1 as Retentive if your application requires the last output state to persist through power-down.
12. Save the project as biweekly_timer.lsc. Click PC → LOGO! to download. Use the Simulation tool first to step through a full 14-day cycle in accelerated mode.

Verification

Always run a 14-day accelerated simulation before live deployment. Set time acceleration to 1:500 (one real second = 500 simulated seconds), start on a known Monday 00:00, and let it run for ~5 minutes real time. Q1 should produce exactly two distinct active windows matching Week-A and Week-B.

Troubleshooting Matrix

Alternative: LOGO! 8 Web Editor / TDE HMI

For larger installations where you want to display the active week (A/B) and remaining time on the LOGO! TDE text display or via the integrated web server, expose two virtual flags:

• M1 = Week-A active flag (set when CNT_WEEK.Q = 0)
• M2 = Week-B active flag (set when CNT_WEEK.Q = 1)

Drag these onto the LOGO! display under Tools → Message Text for live monitoring. The web server (http://<logo-ip>) shows the same flags under Variables.

Edge Cases and Field-Proven Caveats

• DST transitions. LOGO! 8 with firmware ≥ FS:04 handles DST automatically. Older 0BA1 units do NOT; verify DST behavior with the seasonal override logic or use a Yearly Timer to bracket the DST weekend.
• Leap second. LOGO! 8 inserts a leap second at 23:59:60 UTC. If your application crosses UTC midnight, plan a one-minute allowance in the cams.
• Multi-BM networks. If you have multiple LOGO! BMs coordinating one schedule, use one BM as the master (with the counter) and broadcast Q1 over Ethernet using a Network input (function block B090 / NI) referencing the master's output.
• First-power parity. If the system is commissioned in the middle of an active Week-A window, the counter starts at 0 and is already correct. If commissioned mid Week-B, force a reset of the counter via a digital input wired to I1 so the next Sunday pulse aligns parity correctly.
• Counter wrap protection. The Up/Down counter with On=1, Off=0 will not wrap on its own; once it hits 1 it stays at 1 until reset, even if you keep clocking. That is exactly the behavior you want.

FAQ