S7-1200 FBD Programming: Converting S7-1500 Function Blocks

Overview

Siemens S7-1200 and S7-1500 controllers both support Function Block Diagram (FBD) programming in TIA Portal, but the available instruction set, firmware features, and library blocks differ between the two families. Programs originally authored in S7-1500 FBD must be re-mapped to the S7-1200 instruction catalog because the S7-1500 includes additional data types, optimized blocks, and arithmetic/logic extensions not present on every S7-1200 CPU firmware version.

This reference documents the exact procedure for converting an S7-1500 FBD block into an equivalent S7-1200 FBD block using the TIA Portal editor, identifies the S7-1200 firmware constraints that block direct porting, and provides working FBD patterns for the most common conversion cases: string comparison, length optimization, IEC timers/counters, and arithmetic blocks.

Prerequisites

• TIA Portal V15.1 or later (V17 or V18 recommended for current S7-1200 firmware coverage). Download: TIA Portal - Siemens Industry Online Support.
• STEP 7 Basic (S7-1200 only) or STEP 7 Professional (S7-1200/S7-1500).
• CPU firmware: S7-1200 CPU firmware V4.2 or later is required to support the FBD language set described here. Earlier firmware (V4.0, V4.1) supports FBD but lacks several instructions introduced for S7-1500 parity. Reference: S7-1200 programmable controller system manual.
• Source project containing the S7-1500 FBD block exported as a TIA Portal archive (.zap) or opened in the same TIA Portal instance.
• Target S7-1200 CPU selected in the project (CPU 1211C, 1212C, 1214C, 1215C, 1217C, or 1214C-F/1215C-F).

FBD Language Availability in S7-1200

The S7-1200 supports the following programming languages defined in IEC 61131-3:

Function Block Diagram in the S7-1200 implements the IEC 61131-3 graphical representation. Blocks are connected with signal flow lines; each block executes left-to-right, top-to-bottom. Unlike the S7-1500, the S7-1200 does not support GRAPH sequential programming, motion control FBD blocks, or several extended instruction extensions introduced in TIA Portal V16+ for the S7-1500 platform.

Method 1 - Switch the Block Language in TIA Portal

The fastest path to an S7-1200 FBD block is to copy the S7-1500 block and switch the editor language. TIA Portal automatically inserts an IEC-conformant equivalent in the target language.

1. Open the S7-1500 project containing the source FBD block.
2. In the project tree, right-click the block (FB, FC, or OB) and select Copy.
3. Right-click the target S7-1200 Program blocks folder and select Paste. TIA Portal prompts: "Convert and insert as new block?" Choose Yes.
4. Open the pasted block. The editor opens in FBD if the source was FBD.
5. If the block must be in LAD/FBD interchangeably, right-click the block → Properties → General → Language and select FBD.

This method works for blocks that use only instructions present in both catalogs. The compiler reports unsupported instructions in the Information window when the S7-1500-only instructions are referenced. Each error identifies the network and instruction so you can substitute the equivalent S7-1200 instruction manually.

Method 2 - LAD Equivalence Then Switch Back to FBD

For blocks written in FBD where the engineer is more familiar with LAD, switching the block to LAD often reveals which IEC elementary functions and function blocks are used. The resulting LAD network is then switched back to FBD, and TIA Portal re-renders the connections graphically. This two-step conversion exposes implicit enable inputs (EN/ENO) that are not always visible in the FBD view.

1. Open the S7-1500 FBD block.
2. Right-click the block → Properties → change Language to LAD → OK.
3. Verify the LAD rendering. EN/ENO behavior is shown explicitly.
4. Switch Language back to FBD → OK. The FBD is regenerated from the LAD representation.
5. Copy the regenerated block into the S7-1200 project (Method 1, step 3).

S7-1500 Instructions That Do Not Port Directly to S7-1200

String Handling: SCL to FBD Conversion

String instructions in S7-1200/S7-1500 FBD are located under Basic Instructions → String + Char. The following SCL example illustrates the typical input - the FBD equivalent uses the dedicated string function blocks.

SCL Source (S7-1500)

// Compare two strings and copy max-length to output
IF LEN(IBN_Enter) > LEN(IBN_Confirm) THEN
   IBN_Out := IBN_Enter;
ELSE
   IBN_Out := IBN_Confirm;
END_IF;

Equivalent S7-1200 FBD

The FBD implementation uses LEN (S_CONV / string length), GT (greater-than comparison), and MOVE_BLK or S_MOVE (string copy). Insert the following nodes from the Instructions task card:

Implementation pattern in FBD:

1. Insert LEN block, connect IBN_Enter to IN, output to a temporary INT tag iLenA.
2. Insert a second LEN block for IBN_Confirm, output to iLenB.
3. Insert GT (greater-than), IN1 = iLenA, IN2 = iLenB, output = bLongerA.
4. Insert two S_MOVE blocks (one for each source). Wire bLongerA to the EN of the first S_MOVE and its negation (NOT) to the second S_MOVE EN. The outputs both connect to IBN_Out.

String Length Optimization

Declaring HMI tags as the default STRING (254 characters) wastes PLC work memory. Tighten the declared length to the largest value the field can ever contain. This reduces per-instance memory cost across the data block.

IEC Timer and Counter Blocks in FBD

The S7-1200 uses simplified IEC timers. Each timer is a single FB instance (TP, TON, TOF, TONR, RT) in FBD:

Each instance occupies a separate Instance DB when the timer is dropped onto a network. To share a multi-instance, open the parent FB and drag the timer into the FB's Static section; the compiler generates a multi-instance inside the parent DB.

Program Flow Control (OBs) Compatibility

Organization blocks differ between families. Verify the following OBs exist in the S7-1200 target before copying FBD logic that calls them:

Verification Procedure

1. Compile the S7-1200 block. Open Project tree → Program blocks → right-click → Compile → Software (rebuild all blocks). The Information window must show zero errors.
2. Cross-reference tags via Options → Cross-reference. Every tag used in the S7-1500 source must be declared in the S7-1200 DB or symbol table.
3. Download to the target CPU using Online → Download to device. Select Stop all on first download if the controller is in RUN.
4. Monitor the FBD online. Click the glasses icon to enter Monitor mode. Verify EN/ENO at each block - a missing EN indicates the ladder/contact was not wired.
5. Force and watch tags via Project tree → Watch and force tables. Confirm that the S7-1200 string comparison produces the same result as the S7-1500 source for three sample pairs (equal length equal content, equal length different content, different length).
6. Cycle time: open Online → Diagnostics → Cycle time / memory. The S7-1200 FBD block should run within the configured OB1 maximum cycle time. If the source FBD used S7-1500-optimized arithmetic, the S7-1200 may take 5-15% longer per scan.

Troubleshooting Matrix

Related Standards and References

• Function Block Diagram - ScienceDirect Topics
• Function block diagram - Wikipedia
• Siemens S7-1200 system manual: S7-1200 Programmable Controller
• Siemens S7-1500 system manual: S7-1500 Automation System
• TIA Portal installation and use: TIA Portal - Industry Online Support