Siemens LOGO!Soft V8 CTU Counter for Batch Mixer PLC

Overview

Implementing a batch mixer control program on a Siemens LOGO! 8 using LOGO!Soft Comfort V8 requires three functional building blocks that do not have a one-to-one graphical match with the Allen-Bradley RSLogix 500 / LogixPro blocks used in most textbook coursework:

• B5 — Pulse generator (German: Impulsgeber) used to emulate the open-collector pulse output of a flow meter.
• B6 — Up/down counter (German: Vor-/Rückwärtszähler) that accumulates flow-meter pulses and is the LOGO! equivalent of the RSLogix CTU instruction.
• Threshold comparators / digital flags used as the high/low count indicator that replaces the mechanical float switches on the mixer's tank.

The original Allen-Bradley solution uses B5: pulse generator and B6: up/down counter blocks; the LOGO!Soft Comfort V8 toolbox exposes the same functionality, but the icons, parameter dialog, and on-screen help are deliberately compact. The blocks live in the same FBD library tree, so once you locate one, the other is next to it.

Prerequisites

Where the CTU / Up-Down Counter Lives in LOGO!Soft Comfort V8

The block the source question is looking for is the Up/down counter. In every screenshot, it is grouped with the other counters under the "Special functions" (Sonderfunktionen) section of the FBD toolbox. The default block number ranges from B1 onward; a freshly inserted counter will appear as B6 if four prior special-function blocks already exist, which matches the B6 reference from the Allen-Bradley solution.

1. Open the LOGO!Soft Comfort V8 project editor.
2. Switch to the FBD editor mode (View → FBD). Ladder view is not supported in V8 — LOGO!Soft Comfort uses FBD exclusively.
3. In the right-hand toolbox, click the SF tab (Special Functions).
4. Locate Counter group → drag the Up/Down Counter symbol into the editing canvas.
5. With the block selected, press F1 to open the integrated Siemens help — this is the same manual shipped with the MLFB, no external download needed.

Replicating a Flow Meter with the Pulse Generator

Two interpretations of "replicate a flow meter" are valid; choose deliberately:

• Generate pulses (simulate a flow-meter output): use the LOGO! Pulse generator block (B5). This block emits a periodic 1/0 pulse train on its output (Q). It replaces a hardware pulse generator for bench testing.
• Read pulses from a real flow meter: wire the open-collector pulse output of the meter to a fast digital input on the LOGO! 8 BM (I1–I4 on the base module support up to 5 kHz counting). No software block is required to read the pulses; the pulses are routed into the counter's Cnt input directly.

The textbook batch mixer project needs both: the pulse generator to drive the simulation, and the counter to totalise the pulses.

Step 1 — Configure the Pulse Generator (B5)

The block diagram of a pulse generator wired into the counter looks like this:

[ Pulse Generator (B5) ]──Q──▶[ Up/Down Counter (B6) ]──Cnt

Recommended pulse-generator parameters for a flow-meter simulation:

Step 2 — Configure the Up/Down Counter (B6)

The up/down counter block has the following inputs and outputs in LOGO!Soft Comfort V8:

Set the threshold parameters inside the counter block:

• On threshold = 10 pulses — equivalent to the tank "full" float switch in the original Allen-Bradley ladder.
• Off threshold = 1 pulse — equivalent to the tank "low" float switch. Below 1, the block clamps at 0.
• Start value = 0 so the counter begins empty every cycle.

Building the Batch Mixer FBD Network

The complete network, expanded from the source's B5/B6 minimum, is:

1. Network 1 — Flow simulation: Pulse generator (B5) drives the count input of the up/down counter (B6).
2. Network 2 — Mixer fill pump: Pump contactor is energised when B6.Off = 1 (tank low). Latches via RS flip-flop if a true hand-off-auto is required; otherwise tie directly to the Off flag.
3. Network 3 — Mixer high-level trip: Pump contactor is dropped when B6.On = 1 (tank full). This is implemented with a NAND gate or an AND-with-NOT over the pump output to break the seal-in.
4. Network 4 — Drain valve / counter reset: When the operator pushes the "Drain & refill" pushbutton, the drain valve opens and the counter's R input is asserted simultaneously, returning the count to zero.
5. Network 5 — Display message: Use a LOGO! display text block to show the current counter value; this gives the student visual feedback equivalent to a tank-level bar graph.

Concrete Block Diagram (FBD)

 +-----------+      +-------------------+      +-----------------+
 | B5: Pulse |──Q──▶| B6: Up/Down Cnt   |──On──▶| NAND ──▶ Pump   |
 | Generator |      | On=10, Off=1      |      | (with Off pin   |
 +-----------+      |                   |──Off─▶|  inverted)      |
                    +-------------------+      +-----------------+
                            ▲
                            │ R (reset)
                            │
                    +-------------------+
                    | Drain pushbutton  |
                    +-------------------+

Step-by-Step Implementation in LOGO!Soft Comfort V8

1. Create the project. File → New → "LOGO! 8" project. Give it a name (e.g., BatchMixer_B5B6.lsc).
2. Pick the FBD view. The default is FBD in V8; if your installation opens ladder, change it via View → FBD.
3. Place B5 — Pulse generator. Drag "Pulse generator" from the SF toolbox. Double-click to open the parameter dialog and set TH = 00:00:00.500 and TL = 00:00:00.500 for a 1 Hz pulse.
4. Place B6 — Up/down counter. Drag the counter next to B5. Double-click and enter On = 10, Off = 1, start value = 0.
5. Wire B5.Q to B6.Cnt. Click the Q output pin on B5, then click the Cnt input pin on B6. A green link appears.
6. Constant on the Dir input. Drag a "Constant" from the Co (Constants) toolbox onto B6.Dir and set its value to 0 (count up only).
7. Reset input. Place a digital input flag (I1 or an F flag for a simulation button) on B6.R. This is the refill reset.
8. Output flag for On-threshold. Click B6.On and route it to a digital marker Q1 (pump). Or, more safely, route to a NAND gate together with the manual start so the seal-in breaks at high level.
9. Output flag for Off-threshold. Route B6.Off to the pump-on permissive (or to the same NAND inversion shown above).
10. Simulate. Press F3 (Start simulation) or click the play icon. The counter should increment once per second, the On flag should rise at count 10, and the Off flag should rise at count 1 after reset.
11. Download to the LOGO! 8 BM. PC → LOGO! → Ethernet → Transfer. Select "Run LOGO!".

Verification

Before declaring the program complete, run the following acceptance test on the simulator:

Use the integrated Tools → Online test in LOGO!Soft Comfort V8 to step through the program and watch every flag in real time without breaking the scan.

Parameter Cheat Sheet

For quick reference when porting between RSLogix and LOGO!Soft Comfort V8:

Troubleshooting Matrix

Performance & Timing Considerations

The LOGO! 8 base module executes its program in a deterministic scan cycle. With a typical mix of 30–50 blocks the scan time is ~1–4 ms. The pulse generator block is software-emulated, not a hardware PWM channel, so its accuracy is bounded by the scan period. For metering applications requiring better than ±2 % pulse timing, use:

• Fast counter mode on LOGO! 8.3 / 8.4 BM digital inputs I1–I4 (up to 5 kHz) wired directly to the counter Cnt input.
• External hardware pulse source (e.g., Siemens SITRANS F M MAG 5000) for the simulation instead of the software pulse generator.

Migration Notes: From RSLogix to LOGO!Soft Comfort V8

The source's frustration — "RSLogix seems easier to use than the Siemens software" — is a common observation that comes from the I/O numbering convention rather than from a functional gap. RSLogix uses file-style addressing (B5:1, B6:2), while LOGO! uses instance numbers (B5, B6). The mapping is one-to-one; only the visual idiom differs. Two practical adjustments:

1. Bookmark F1. The integrated help on every LOGO!Soft Comfort V8 block is the equivalent of the RSLogix instruction tooltip. Press F1 once, leave the help window open, and refer to it as you wire.
2. Work in FBD. LOGO!Soft Comfort V8 dropped native ladder logic; if your coursework expects ladder, plan to convert it manually or use LOGO!Soft Comfort's "Ladder view" plugin (only available on legacy V7).

Related Resources

• LOGO!Soft Comfort V8 product support page — MLFB 6ED1058-0BA08-0YA1.
• LOGO! Software overview — current LOGO!Soft Comfort V9 reference.
• LOGO!Soft Comfort V8.4 demo download — free trial, 14-day unrestricted simulation.
• Current LOGO! software and firmware bundle — V8.4.1 service pack and matching BM firmware.
• LOGO!Soft Comfort V8.3 crash resolution — repair install procedure.