Siemens LOGO! 8 PI Controller Setpoint Reference Missing: Fix

Problem Overview

The Siemens LOGO! 8 (order code suffix 0BA8) introduces a redesigned PI controller block in LOGO! Soft Comfort V8.x where the historical Reference input on the PI controller block is no longer exposed as a configurable signal terminal in the standard block-properties dialog. Engineers migrating projects from LOGO! 0BA6/0BA7 observe that:

• Dragging a new PI controller block onto the FBD/LAD editing canvas in V8 yields a block with only SP, PV, Output, Enable, Manual and Reset terminals.
• The Reference sub-dialog (used in 0BA6/0BA7 to assign an arbitrary analog block as a dynamic setpoint) is not present.
• Selecting an older hardware target (0BA6 or 0BA7) in the same V8 project re-enables the Reference selector.

The behavior is not a corruption of the project file. It is a deliberate change in how Siemens implemented the reference function in 0BA8 firmware and Soft Comfort V8. This article documents the root cause, the supported workarounds, and the parameter mapping required to recreate the missing functionality in production code.

Affected Hardware and Software Versions

Verify the installed Soft Comfort build from the menu: Help → About LOGO! Soft Comfort. The firmware version of the target device can be read online via Tools → Transfer → Read Device Identification or directly from the LOGO! onboard menu: System → Information.

Root Cause Analysis

Siemens re-architected the PI controller in the 0BA8 generation to align with the IEC 61131-3 standard PID function block pattern and to consolidate closed-loop control into a single block object. The change that most directly impacts setpoint referencing is the deprecation of the legacy "Reference" pin in favor of three distinct mechanisms:

1. SP (Setpoint) input with analog multiplexer: The dynamic setpoint is now sourced from any analog flag (AM, AM2), network input (NI), or analog block output connected directly to the SP terminal.
2. Setpoint weighting (PV/SP ratio): A new Setpoint Weight parameter in the block properties applies the classic β (beta) coefficient that the PID literature describes for two-degree-of-freedom control. β is a real number in the range 0.0 to 1.0; the effective setpoint used in the proportional path becomes β · SP while the derivative action continues to operate on the full setpoint SP − PV.
3. Auto-tuning and dead-band parameters: New parameters AutoTune (one-shot identification on enable), DeadBand (output dead-zone in engineering units), and Offset (manual output bias) replace several settings that were previously accessible through the Reference dialog.

The previous "Reference" terminal in 0BA6/0BA7 was effectively a soft-mux that pointed the controller at an analog block to use as its live setpoint. In 0BA8, that mux is implemented in the wiring itself: any analog signal can drive the SP pin, eliminating the need for a special selector. Engineers who relied on the Reference dialog were therefore seeing a feature that has been superseded, not removed, by the redesigned block.

Step-by-Step Workaround Procedure

The following procedure restores adjustable setpoint behavior for a 0BA8 target without downgrading Soft Comfort.

Prerequisites

• LOGO! Soft Comfort V8.2 or later installed.
• Target device: any 0BA8 variant (with or without display).
• An available analog source (AI1 – AI8 on the base module, AI9 – AI16 on expansion DM8 12/24R or AM2 modules, or a network variable from a LOGO! CMR/LOGO! 8 communication module).
• If the setpoint is operator-adjustable, an HMI text-display block (e.g., Message Text with a numeric value) or a connected HMI panel via the S7/Modbus interface.

Configuration Steps

1. Open the project in Soft Comfort V8 and confirm the hardware target is set to a 0BA8 device. If the hardware was previously 0BA6/0BA7, right-click the device in the network view and choose Change Device → LOGO! 8 (0BA8).
2. Remove the legacy PI block if it was carried over from a V7 project. Place a fresh PI controller block from the block catalog: Special → PI Controller.
3. Wire the dynamic setpoint by dragging the output of any analog block directly onto the SP terminal. Acceptable sources include:
   • Analog Input (AI) — raw 0-10 V or 0-20 mA scaled value.
   • Analog Flag (AM, AM2) — holds the live setpoint written from the operator panel or network.
   • Analog Multiplexer (AM) — selects between multiple setpoints based on digital inputs.
   • Mathematic Instruction (ARI) — computed setpoint, e.g., ramping profile.
   • Network Analog Input (NAI) — setpoint from a peer LOGO!, S7 PLC, or Modbus master.
4. Open the PI block properties (double-click the block). Set the parameters in the table below.
5. Compile and download to the target. Soft Comfort will validate that the SP source is a 16-bit signed analog signal (range −32768 to +32767, scaling 1 LSB = physical unit).
6. Verify online using the Online Test view; the live SP and PV values are shown above the block when monitoring is active.

PI Controller Block Property Mapping (0BA8)

Reference Function Architecture in 0BA8

For users who need the exact equivalent of the legacy Reference function (e.g., switching the active setpoint between two source blocks under program control), build a two-stage network:

Stage 1: Analog Multiplexer as Setpoint Source

1. Insert an Analog Multiplexer block (catalog: Analog → Multiplexer, part name AM).
2. Wire two analog sources to inputs X1 and X2 (e.g., fixed setpoint 50.0 and ramped setpoint from ARI).
3. Wire a digital flag to selector input SEL. The active setpoint appears on output Y.
4. Connect Y to the SP terminal of the PI controller.

Stage 2: Optional Operator Override

1. Insert a Message Text block (catalog: Display → Message Text).
2. Set the Set Value property of the message to a parameter that writes into an Analog Flag (AM2). Soft Comfort exposes this through the block's "Acquisition" mode.
3. Replace the fixed input X1 with the analog flag so the operator can edit the value at runtime from the LOGO! display or TDE.

Migration Procedure from LOGO! 6/7 Projects

The fastest path to a working 0BA8 PI controller is to not rely on the round-trip trick (author in V7, open in V8) because the resulting program still contains the legacy reference field. While Soft Comfort V8 will accept the file, online testing will report the obsolete parameter as ignored at runtime. Use the procedure below to fully migrate.

1. Open the V7 project in Soft Comfort V8. Soft Comfort will prompt: Project was created in an older version. Convert? — click Yes.
2. For each PI controller block, double-click and read the previously-assigned Reference in the V7-format block comment. The reference is shown in the migration log file ~migration.log in the project directory.
3. Delete the migrated PI block and reinsert a fresh one under the 0BA8 target.
4. Reconnect the original Reference signal to the new SP terminal.
5. Copy the legacy Kp, Tn, Sample time, Direction and Output limits into the new block property dialog.
6. Cross-check using Project → Consistency Check (Ctrl+F9) before downloading.

Verification Procedure

1. Offline check: In Soft Comfort, select the PI block, then View → Block Information. The Setpoint Source must show the connected analog block, not (unwired).
2. Simulation check: Use Tools → Simulation (F11). Force the analog setpoint to two values 10 % apart and observe the controller output tracking with the expected Kp and Tn response.
3. Online check: Connect via Ethernet (0BA8 supports the built-in RJ45 port; static IP 192.168.0.1 default). In Online → Test, the live SP, PV, Output and integrator state are visible. Step the setpoint from 30 % to 70 % and confirm the output settles within the expected rise time (0.8 × Tn for a PI-only loop).
4. Auto-tune check: Enable AutoTune, apply a step disturbance, and watch the block recompute Kp and Tn within 1-2 sample intervals. Soft Comfort displays the new gains in the Online → Auto-Tune Result window.

Troubleshooting Matrix

Sample Wiring (FBD)

                    +-------------------+
[ AI1 (Pt1000) ]----| Analog Amplifier  |----+---( PV )---[ PI Controller ]---( Q )---[ AQ1 ]
                    |  Gain=1.0, Off=0  |    |              SP=Mux_out
                    +-------------------+    |              Kp=2.5, Tn=00:30
                                            |              Dir=+, SP Weight=1.0
                                            |              Min Out=0, Max Out=1000
[ AM2 (setpoint) ]--+
                     |
[ Fixed: 50.0  ]-----+----[ Analog Mux ]---( Mux_out )
[ Ramp: ARI  ]-------+      SEL = I3
                            (I3 = 0: Fixed, I3 = 1: Ramp)

Best Practices for Production Programs

• Always scale the PV signal with an Analog Amplifier (Gain × PV + Offset) so that the engineering units match the setpoint. The PI controller uses a 16-bit signed value with 1 LSB = 1 engineering unit; mismatched scaling causes integrator wind-up.
• Anti-windup: 0BA8 implements output clamping internally; the integrator is held when the output is at the rail. Do not add external anti-windup unless driving a cascade structure.
• Retain setpoint across power cycles by setting the analog flag retention to Yes in the block properties.
• Networked setpoints: when receiving a setpoint over Modbus TCP from a master, map the holding register to a Network Analog Input (NAI) using Soft Comfort's Connection Resources dialog. Register allocation: NAI1 ↔ Holding Register 40001 by default for a single-peer Modbus connection.
• Document the setpoint source in the project comments. The block diagram in 0BA8 does not preserve a textual "Reference" field, so a comment such as // SP = AM2 (operator setpoint, engineering units °C) is essential for the next maintainer.

References to Official Documentation

• Siemens Support: How can you use an analog input of LOGO! setpoint value for a PI controller?
• Siemens Support: LOGO! 8 PI controller — setpoint reference discussion
• Wikipedia: PID controller (setpoint weighting theory)