Configuring LOGO! PI Controller Setpoint from an HMI via VM Mapping

Siemens LOGO! 8 series logic modules include a built-in PI controller instruction block (commonly referenced as PI-Regler in the German toolchain and PI Controller in English toolchains). A frequent integration task is allowing an HMI to write the controller setpoint (parameter SP) in real time while the FBD program is running. The naïve approach — wiring an analogue flag/amplifier into SP — corrupts the value because the analogue amplifier stage rescales and offsets the raw integer. This article documents the two field-proven methods for binding an HMI-supplied value to SP on a LOGO! 8 (8.1, 8.2, 8.3, 8.4):

• Method A: Native Parameter-VM-Mapping table inside LOGO! Soft Comfort.
• Method B: A mathematical instruction that copies a VM (Variable Memory) word directly into the SP input of one or more PI controller blocks.

Both methods preserve the integer -500 … +2500 range (i.e. -50.0 °C to +250.0 °C at 0.1 °C resolution) without scaling drift.

1. System Architecture and Prerequisites

Reference documents:

• Siemens LOGO! 8 System Manual (109741041)
• LOGO! Soft Comfort V8.4 Online Help (109741040)
• Siemens LOGO! product page

2. LOGO! PI Controller Block: Parameter Model

The PI controller block exposes the following parameters on its block dialog in LOGO! Soft Comfort:

The SP value is internally a 16-bit signed word. When you bind SP to a LOGO! analogue flag (the small flag list on the left rail of the FBD editor) the platform wraps the binding in an analogue amplifier so that the flag can be scaled — but that amplifier's gain/offset are fixed at design time and will mutilate any pre-scaled integer the HMI is sending. This is the trap that consumes hours on first contact with the platform.

3. Why Analogue-Flag Wiring Fails

An analogue flag in LOGO! Soft Comfort (for example AM1, AM2, …) is the result of a previous computation and is exposed only after an Analogue Amplifier block has been inserted. When you drag a connection from such a flag onto the SP pin of a PI controller, the toolchain generates an internal Analogue Amplifier stage with default gain = 1.0 and offset = 0.0, but the value is then re-mapped into the display range. The result: an HMI value of 1250 (intended to mean 125.0 °C) can re-emerge as 12500 (1250.0 °C) depending on the display-mode toggle of the controller block.

To avoid this you must keep the HMI value in the raw VM word that the controller block reads directly — never let it pass through an amplifier. That is exactly what the two methods below do.

4. Method A — Parameter-VM-Mapping (Recommended)

The Parameter-VM-Mapping table is the cleanest way to expose SP as a writable Modbus register.

1. Open the LOGO! program in LOGO! Soft Comfort.
2. Select Tools > Parameter-VM-Mapping from the menu bar.
3. The editor lists every parameter of every function block in the program. Locate the SP row(s) of your PI controller block(s). The block is shown by its name in the FBD (default B001, B002, …).
4. Tick the VM checkbox on the SP row, then set the Start Address (e.g. VW0) and Length = 1 word. Click Apply.
5. Recompile and download the program to the LOGO! base unit.
6. From the HMI, write the setpoint to Modbus holding register 0000 (function code 0x06, single-write, or 0x10 multi-write) at the LOGO! IP address, port 502.

After download, the LOGO! internal diagnostic page (LOGO! Display / LOGO! TDE > VM Mapping) will show the live value. The Modbus register map is automatically extended; no LSC code change is required for additional controllers — simply assign a new VM address to the next controller's SP.

5. Method B — Mathematical Instruction Reference

If you cannot or prefer not to use the Parameter-VM-Mapping tool (for example, when the value must be filtered, clamped, or computed at runtime), use an Analogue Math or Math block to copy a VM word into the PI controller's SP pin directly.

5.1 Diagram example (FBD)

  +-------------------+      +-------------------+      +-------------------+
  |   VM address VW0  |      |  Math (or Analogue|      |  PI Controller    |
  |   (HMI-written)   +----->|  Math) block      +----->|  block (B001)     |
  |   int  -500..2500 |      |  Gain = 1         |      |  SP = computed    |
  +-------------------+      |  Offset = 0       |      |  PV = AI1         |
                             +-------------------+      +-------------------+

With LOGO! Soft Comfort:

1. Insert a Math block (or Analogue Math if you need floating-point interpolation). Set:

2. Wire the Ax input of the Math block to the VM address that the HMI writes to (this is the same address you would have used in Method A). On the Math block's connection dialog, type the VM name (e.g. VW0) directly — do not use an analogue flag here.
3. Wire the Math block's output to the SP pin of the PI controller.

5.2 Why a Math block is acceptable but not ideal

The Math block with gain = 1.0, offset = 0.0 is a pass-through. It does not corrupt the value because the integer-to-integer path is preserved when both gain and offset are within the integer range. However, the block consumes program memory and one execution tick, so for setpoint-only writes prefer Method A. Use Method B only when you need run-time signal conditioning (e.g. SP = clamp(VW0, 0, 2500) to prevent the operator from commanding a temperature below ambient).

6. Scaling the Setpoint for Engineering Units

LOGO! 8 has no native floating-point display for VM values. Two common scaling strategies are used in the field:

For the OP's process (-50.0 °C … +250.0 °C), the integer-×-10 convention maps cleanly to the LOGO! PI block's internal 16-bit signed representation: the integer wire value ranges from -500 to +2500, which is exactly 3000 codes across 300 °C of physical span (0.1 °C per LSB). The PI controller will treat this as a dimensionless value but the closed-loop behaviour is preserved because both SP and PV are passed through the same scaling chain on the analogue input side.

7. HMI Configuration (Kinco Example)

The OP referenced a Kinco HMI. Kinco's HMIsoftware (formerly Kinco DTools) and the current KINCO HMIware configure Modbus TCP servers on the device. To bind a numeric input object to a LOGO! VM word:

1. Add the LOGO! as a Modbus TCP Server / Modbus TCP Master device in the HMI project. Set IP = LOGO! IP, port = 502, station = 1.
2. Create a Numeric Input or Numeric Value Entry element on the screen.
3. In the element's address configuration:

4. Set the keyboard attribute to Numeric and enable the Confirm write bit. Without confirm, the HMI writes on every digit change — usually undesirable for a setpoint.
5. Add an Enter button or use the Auto-write on Enter property of the numeric object.

Write a one-time Preset value of 200 (= 20.0 °C) so the loop starts at a sensible default after a power cycle.

8. Sharing One VM Address Across Multiple PI Controllers

It is common to cascade or parallel several PI loops to the same setpoint (e.g. multi-zone furnace trim). In LOGO! Soft Comfort:

1. Map the SP of each PI block to the same VM address in the Parameter-VM-Mapping table (e.g. VW0 for controllers B001, B002, B003).
2. Compile and download. LOGO! will broadcast every HMI write to all three blocks in a single scan.

Alternatively, use Method B with a single Math block whose output fans out to the SP pin of every controller — the same integer is wired multiple times in the FBD. This consumes one Math block plus a multi-pin connection. The Parameter-VM-Mapping approach is preferred because it does not consume runtime resources.

9. Verification and Diagnostics

After download, perform the following checks in order:

1. LOGO! display check: On the LOGO! base unit or TDE, navigate to Menu > Diagnostics > VM (or use the block's SP diagnostic page) and confirm the value tracks the HMI input. Force a value of 1500 from the HMI and verify it appears within one scan (~50 ms typical for LOGO! 8).
2. Modbus poll test: From a PC running modpoll or QModMaster, read register 0 with function code 0x03 to confirm the LOGO! echoes the last-written value. If the read returns the previous value, check the IP and port in the HMI project.
3. Block-status check: On the LOGO! display, open the PI controller's block status page. The SP line should update within 1 scan after the HMI write.
4. Closed-loop response: Step the setpoint by 100 codes and observe the MQ (manipulated variable) response on a trend. The output should move smoothly toward the new operating point with no integral wind-up; if it pegs at 0 or 1000, the action direction (Dir) is inverted.

10. Common Pitfalls and Field Notes

For deeper control theory background, refer to the Wikipedia PID controller article and National Instruments' PID Theory Explained whitepaper, which cover proportional, integral, and derivative actions in detail. Note that the LOGO! block is a PI block (no derivative term) and so derivative-related tuning rules do not apply.

11. Quick Reference — HMI Address Map for the OP Application

12. FAQ