Siemens LOGO! Async Pulse Generator Remaining Time Display

A field-engineering guide to configuring the Siemens LOGO! 8 family (6ED1052 series) so that the remaining time before the next asynchronous pulse output is shown live on the integrated LOGO! display or LOGO! TDE. The pattern solves a recurring need: a periodic process (e.g., a motor triggered every 5 min by an asynchronous pulse generator) where the operator must know how many seconds remain until the next pulse, and where a hard-wired terminal switch (I1) is used to break the cycle.

1. Problem Overview

The reference application contains four cooperating blocks on the LOGO! 8 base module:

1. A weekly timer (block B001) closes its contact between 07:00 and 18:40 on selected days. The HIGH edge arms the latch.
2. A RS-latch (block B002) sets on the timer's rising edge and resets when the operator closes terminal switch I1.
3. An asynchronous pulse generator (block B003) is enabled by the latch output. It produces a 2 s HIGH pulse every 5 min (300 s) when the latch is set.
4. An output block (block B004, Q1) drives the relay contactor for the motor when the pulse is HIGH, while a message text (block B005) on the LOGO! display shows the live remaining time before the next pulse.

The functional question is: which LOGO! parameter exposes the live remaining time of an enabled asynchronous pulse generator, and how do we route that parameter into a message text field? The asynchronous pulse generator (APG) is internally a cycle-time-independent timer pair; it does not expose a "remaining time" terminal in the same way as the On-Delay (TON) block does. The remaining time must be derived from the elapsed-time parameter of the upstream timer or computed with an Arithmetic block from a parallel counter, then pushed to a Variable Memory (VM) word that the message text can reference.

2. Prerequisites

3. Asynchronous Pulse Generator Block Fundamentals

The asynchronous pulse generator in LOGO! Soft Comfort is found under Special Functions > Timers > Asynchronous Pulse Generator. It accepts three configuration parameters and exposes three terminals:

The asynchronous pulse generator is the only LOGO! timing block that is not updated by the scan cycle; its state advances on the hardware timer interrupt inside the CPU. This means Cnt is incremented in real time even when the program scan is busy, which is what makes it suitable for producing precise 2 s pulses on a 5 min schedule.

Crucially, the APG does not present a remaining-time output. It only presents the Q terminal (boolean) and the running Cnt counter. To obtain a "time-to-next-pulse" value we therefore need one of two approaches:

1. Approach A (recommended): Place a parallel On-Delay (TON) block whose time base equals the APG period. The TON block's TH output (remaining time) is naturally available for VM mapping.
2. Approach B: Read the APG's Cnt value via VM, multiply by the base time resolution, and subtract from TH using an Arithmetic block.

Approach A is shown below; it is shorter and reuses well-documented VM mapping for the TON block.

4. Variable Memory Addressing Strategy

The LOGO! 8 CPU exposes 850 Variable Memory words (VM0..VM849) accessible from every function block and from the message text. The default assignment is:

5. Remaining Time Calculation

For the reference circuit (5 min = 300 s period, 2 s pulse), the formula we push into the message text is:

remaining_seconds = 300 - VM20 (elapsed time in seconds)

This is implemented with one Arithmetic block (B008) wired to the TON block's elapsed-time output:

B008 Arithmetic (Function: A - B, scaling off)
    A (Input 1)  = 300          // constant: period in seconds
    B (Input 2)  = VM20         // TON elapsed-time parameter
    Q (Output)   = VM30         // drives message text line 2
    Q ranges from 0 to 300, displayed as seconds remaining

If you want the display in minutes:seconds, replace the constant with a second Arithmetic block that does integer division (LOGO! Soft Comfort V8.3+ supports / with quotient and remainder):

B009 Arithmetic (A / B)
    A = VM30 (seconds remaining)
    B = 60
    Quotient (Q1) = minutes  -> VM40
    Remainder (Q2) = seconds -> VM42

Then in the message text, two VM placeholders display VM40:VM42 formatted as MM:SS.

6. Message Text Configuration

1. Drag a Message Text block (B005) onto the diagram. The block is found under Special Functions > Message Text.
2. Set the Enable input to the Q output of the RS latch (B002). The message is only visible while the latch is set.
3. In the message text properties, enable Show actual values on the second line.
4. Click into line 2, insert a VM placeholder, and choose VM30 as the source. Set the format to Decimal, 3 digits.
5. Enter the static string Next pulse in on line 1 and add the VM placeholder + s on line 2.
6. Open Properties > Acknowledge and choose No so the text updates automatically without operator ESC.
7. Compile and download to the LOGO! base module.

The resulting on-display text reads, for example: "Next pulse in 287 s" and ticks down once per second.

7. Ladder / FBD Topology

8. Latch Reset Behavior and Edge Cases

The terminal stop button I1 is wired normally-closed so that a broken wire also releases the latch. When the operator presses the stop button:

1. RS-latch B002 resets. Its Q output goes LOW.
2. APG B003 sees a LOW on its Trg input and immediately freezes; Q returns LOW.
3. TON B006 (parallel timer) also freezes; its elapsed-time value is held.
4. The Arithmetic block B008 stops updating because VM20 stops changing; VM30 stays at the last value. This is the correct behavior: the displayed time "freezes" at the value when the stop was pressed.
5. Message text B005 hides because its enable input is the latch Q.

9. Verification Procedure

1. Download the program with LOGO! Soft Comfort in Online > PC ↔ LOGO! mode.
2. Switch the LOGO! to RUN; observe the message text. The line 2 value should count down from 300 (or 4:60 / 04:60) to 0 within one period, then jump back to 300 when the pulse fires.
3. Press I1 (terminal stop). Confirm the message text disappears within one scan and the motor Q1 stops.
4. Release I1. Confirm the system remains stopped until the next weekly timer edge.
5. In Soft Comfort, open Tools > VM Mapping. Verify VM30 matches the displayed value on line 2. Any mismatch indicates an incorrect VM address assignment.
6. Force a one-hour test by changing the APG period to 60 s and verifying the countdown behaviour and pulse duration.

10. Troubleshooting Matrix

11. Notes on the Asynchronous Pulse Generator vs. Standard Pulse Generator

The standard Pulse Generator (PG) block in LOGO! Soft Comfort is a cycle-time-dependent block; its accuracy is bounded by the LOGO! scan time (typically 10-20 ms on a LOGO! 8). The APG block uses a hardware timer interrupt, giving accuracy < 1 ms. For a 2 s pulse on a 5 min period the PG block is technically sufficient, but the APG is preferred when:

• Multiple APG blocks are running and one is gating a high-speed counter.
• The pulse must remain phase-accurate even when the LOGO! scan is momentarily extended by Ethernet traffic or S7 communication.
• The pulse is wired to a hardware interrupt input (I3-I6 on LOGO! 8) for fast capture.

Both blocks expose a Cnt value in VM; both require the parallel-TON pattern described above to derive a remaining-time display.

12. Commissioning Checklist