Configuring Block References in LOGO! 8 Mathematical Functions

Overview: Why Block References Exist in LOGO! 8

The LOGO! 8 logic module (Siemens part numbers 6ED1052-xxx08-0BAx, firmware class OBA8) is the successor to the OBA7 line. The programming environment LOGO! Soft Comfort compiles ladder-style and FBD-style logic and downloads it to the base module, the pure variants, or the SIPLUS variants. When you build a numeric signal chain — measure a frequency, scale an analog input, multiply by a gain, and display a derived value — you typically need one numerical block to consume the output of another numerical block without routing the value through a digital wire. This is exactly what the reference connector does: it lets an Analog Amplifier, Math function, PI Controller, Threshold Switch, or PWM output block "look across the diagram" and pick up a value computed elsewhere in the same program.

On OBA7 firmware the reference tool was exposed directly inside the block properties dialog. On OBA8 the same function is still present, but the UI surface changed: the connector is hidden behind a + symbol that appears directly under every block that supports references, and the wire-drawing behavior is identical to a normal electrical connection. This is the change that catches most users coming from an OBA7 project.

The official Siemens KB entry that documents the workflow is How can you configure references between blocks in LOGO! 8? (entry ID 109471094, Siemens Industry Online Support, valid for LOGO! Soft Comfort V8.0 and later).

OBA7 vs OBA8 — What Changed in the Reference UI

Prerequisites

1. PC with LOGO! Soft Comfort V8.0 or later installed. Siemens publishes the current build in the Siemens Industry Online Support portal under entry ID 109751205 for Soft Comfort V8.4. Earlier builds (V8.0–V8.3) all expose the + workflow.
2. A LOGO! 8 base module with firmware OBA8 (all 0BA8 revisions: FS:01, FS:02, FS:03, FS:04, FS:05). Open LOGO! → Diagnostics → Device Info on the module, or read the type plate ("6ED1052-1xx08-0BA8").
3. A project that contains a block capable of consuming a reference. The catalog in the right-hand pane must include at least one of: Mathematical functions (B001–B006), Analog amplifier (B002), PI controller (B013), Ramp (B005), Threshold switch, Analog filter, Max/Min (B004), or Analog watchdog (B003).
4. For an RPM display example you also need: one digital input wired to a digital input block, plus a Threshold trigger (B007) or Frequency trigger (B008) to convert pulses-per-period to Hz before scaling.

Step-by-Step: Enabling References on a Mathematical Function Block

1. Place a Mathematical function block on the workspace. In the catalog it is the icon with the "ƒ(x)" symbol — internally a General function B001 derivative. Double-click the icon to insert it.
2. Look at the bottom edge of the placed block. You will see a small + symbol that is rendered in a slightly lighter color than the block body. Hover over it: the tooltip reads Add reference.
3. Single-click the +. The block now exposes up to four reference pins on its top edge (Ref1–Ref4) and the cursor turns into a connection tool.
4. Drag from the chosen reference pin to the output terminal of the source block — typically the Q-output of an Analog amplifier, the AQ-output of a Math block, or the output of a Threshold trigger. The line is drawn in the same way you draw a digital connection. The reference becomes valid as soon as the cursor is released on a compatible terminal.
5. Open the Math function's properties (double-click the block body, not the +). The Reference tab now lists the connected source as an available input, and you can map it to the operator's operand slot (Ax, Ay).
6. To hide the reference lines, click the − tab at the bottom of the block, or use the toolbar View → Display Reference Lines toggle. The references themselves remain stored; only the visual representation is suppressed.

Mathematical Function Block — Operator and Reference Catalog

LOGO! 8 implements the standard FBD math set defined for the LOGO! family. Each block accepts references on its operand pins so you can build compound expressions without intermediate wires.

The catalog of math functions themselves — from elementary arithmetic to gamma, hypergeometric, and prime-counting functions — is well classified in external references such as List of mathematical functions (Wikipedia) and the runtime libraries in Visual Basic for Applications math functions (Microsoft Learn). LOGO! 8 implements only the elementary subset in firmware; if you need a transcendental function such as logarithm, square root, or exponent, you must reach for the Analog Math block (0BA8 ES2 and later) or build the approximation out of table lookups.

Practical Example: RPM Display Using Reference Connections

Goal: read pulses from a single proximity sensor and drive the LOGO! TD text display with an RPM value between 0 and 3000. The reference connector is what lets you avoid chaining a dozen wires through the workspace.

1. Wire digital input I1 to a Threshold trigger (B007) configured as a frequency-to-period converter, measurement range 1 Hz to 10 kHz. The block's Q output is the measured frequency in Hz.
2. Add a Mathematical function block (B001) configured as multiplication. The Hz value comes from the reference pin, not from a wire — connect Ref1 to the Threshold trigger Q output as described in Step 4 of the previous section.
3. Set the multiplication constants: Ax = 60 (seconds-to-minute), Ay = reference (Hz). The result of the multiplication is RPM.
4. Add a second Mathematical function configured as a gain-and-offset correction (or use an Analog amplifier) to linearize the sensor: Gain = 1.02, Offset = −3. Connect its input as a reference from the previous block's AQ output.
5. Drop a Message text block (B009) onto the workspace. In its properties, the En input is a normal wired input, but the numeric bar's value source is set to Reference and pointed at the second Math block's AQ. The display therefore shows the corrected RPM live.

Reference Connector Visibility and Edit Workflow

The reference connector concept in LOGO! 8 was designed to keep the FBD diagram visually clean. The behavior is governed by the following rules in the Soft Comfort editor:

• Default state: Reference lines are hidden. Blocks display only the + tab at the bottom.
• Hover state: The + becomes darker; a single click toggles into edit mode.
• Edit mode: All reference lines in the project become visible in a thin dashed style. The active block shows the − tab to exit.
• Selection: Clicking a reference line selects it; pressing Delete removes only that connection, not the others. Right-clicking exposes Properties, which lets you re-map the reference to a different source block.
• Cross-program copy/paste: Reference IDs are stored as global identifiers inside the .lsc project. Pasting a block into a different project without its referenced source will surface a broken-reference warning in the consistency check.

Verification and Simulation

1. Press F5 to start the on-screen simulation in Soft Comfort. Status indicators on every block turn green when the source reference is valid; yellow indicates "no signal at the reference"; red indicates a short-circuit or a type mismatch (e.g., a Boolean reference fed into an analog operand).
2. Force the input that drives the source block to a known frequency (1 kHz from a function generator, or a manual toggle for a slow input). The display block should now show 60 000 in the message text when the constants are set as in the example above.
3. Open Tools → Consistency Check. The report lists any unresolved reference targets with the line number of the consuming block. Resolve each entry before downloading to the module.
4. Download with LOGO! → PC → LOGO!. After the transfer, on the physical module, the ESC + > menu exposes the active references and their current values — useful when the on-device display is empty but Soft Comfort shows the value correctly.

Troubleshooting Matrix

Field-Proven Tips

• Name every reference with a meaningful comment in the block properties. The OBA8 editor stores these comments in the .lsc XML, and they appear in the diagnostic export.
• Avoid more than three hops of references per chain (Ref → Math → Math → Display). LOGO! 8 supports it, but the on-device diagnostic output truncates long chains and the simulation tool can be slow to refresh.
• If a reference is consumed by a PI controller, set the controller's PV source explicitly to "Reference" in the block properties — the default is the wired input, which is why so many closed-loop control loops behave as if the setpoint and process value are swapped.
• Use LOGO! → Project → Properties → Statistics to inspect the count of references and the memory footprint; each reference consumes four bytes in the compiled runtime image on OBA8 modules.