In SCL every variable is declared in a section that decides where it lives. Input, Output, InOut and Static variables of an FB are stored in its instance DB and keep their values between calls; Temp variables exist only while the block runs; Constants are compiled into the code. The CPU keeps the project in load memory (memory card), runs it from work memory, and keeps Retain values in retentive memory through a power cycle.
- Static = the block's memory (instance DB); Temp = scratch space for one call; Constant = no runtime memory
- Mark only values that must survive a power cycle as Retain; everything else returns to its start value
- Convert explicitly (NORM_X, SCALE_X, TRUNC, ROUND), read bits with slices like
#ZSW1.%X2, and loop over arrays with FOR
Part 3 showed addresses and data types from the PLC tag table. This part looks at the same data the way an SCL programmer does: as declarations inside blocks. Every tool below uses the conveyor project: FB_PRB from Part 4, the G120 drive from Part 2 and the Motor_UDT array from Part 3.
SCL Declaration Sections: Input, Output, InOut, Static, Temp, Constant
The section you declare a variable in decides where it is stored and whether it survives the next scan.
Reference: declaration sections in an FB and an FC
| Section | FB | FC |
|---|---|---|
| VAR_INPUT / VAR_OUTPUT | Stored in the instance DB | Parameters of the call, no instance DB |
| VAR_IN_OUT | Copy (elementary) or reference (structured) | Passed from the caller |
| VAR (Static) | Instance DB, kept between calls | Not available |
| VAR_TEMP | Local data stack, only during the call | Local data stack, only during the call |
| VAR CONSTANT | Compiled into the code | Compiled into the code |
PLC Memory: Load Memory, Work Memory and Retentive Memory
The project sits on the memory card; the CPU runs it from work memory; only values marked Retain survive a power cycle.
| DB_PRB_1 member | Type | Start value | Actual value | Retain column |
|---|
In an optimized FB you set retentivity per variable in the interface's Retain column (Non-retain, Retain or Set in IDB). Keep the list short: retentive memory is much smaller than work memory.
Want to write SCL on a real S7-1500?
Live SCL sessions: declarations, UDTs, arrays, loops and FB design on real hardware. Pune classroom or online.
Typed Literals and Type Conversion in SCL
SCL checks types strictly. Move the raw analog value and watch it pass through NORM_X and SCALE_X from Int to Real, then try the rounding functions.
Real → whole number: 25.7
16#FFHex Byte/Word value2#0000_0101Binary bit patternINT#100Typed constant25.0Real1.5E3Real, exponentT#2STime: 2 secondsT#1M30STime: 90 secondsTOD#08:30:00Time of dayD#2026-11-04Date'Batch_01'StringReference: common conversions in SCL
| From → to | SCL | Note |
|---|---|---|
| Int → Real | INT_TO_REAL(#Raw) | No loss of precision |
| Real → DInt | TRUNC(#Level) or ROUND(#Level) | TRUNC cuts the decimals, ROUND goes to the nearest whole number |
| DInt → Int | DINT_TO_INT(#Total) | Values above 32 767 do not fit: check the range first |
| Raw analog → engineering | NORM_X then SCALE_X | 0 … 27 648 is the nominal range of S7-1500 analog modules |
| Word ↔ Int | WORD_TO_INT / INT_TO_WORD | Same 16 bits, different meaning |
Slice Access in SCL: Reading Bits Inside a Word
No masks or shifts needed: #ZSW1.%X2 reads bit 2 of the G120 status word directly. Click bits, or watch the drive start up.
Reference: ZSW1 bits used here
| Slice | Meaning |
|---|---|
| %X0 | Ready to switch on |
| %X1 | Ready to operate |
| %X2 | Operation enabled |
| %X3 | Fault present |
| %X4 | No OFF2 (coast stop) |
| %X5 | No OFF3 (quick stop) |
| %X6 | Switching on inhibited |
| %X7 | Alarm active |
Remember Part 3: Siemens stores the high byte first, so %X0 of %IW256 sits in %IB257. Slices count from the least significant bit, so you never need to think about byte order in SCL.
ARRAY and FOR Loop: One Loop for 16 Roller Beds
"Conveyor".PRB is the ARRAY[1..16] OF Motor_UDT from Part 3. One FOR loop checks every roller bed in a single scan.
Quick Quiz: SCL Data Types and Memory
Hands-On Lab: SCL Data and Memory in TIA Portal
15 minutes- TIA Portal V17 or later with an S7-1500 CPU or PLCSIM.
- The Motor_UDT from Part 3 (Run, Fault, Speed, Runtime_s).
Create FB_PRB in SCL
Add Start/Stop inputs, a Run output, Static Latch and Runtime_s, and a Temp EdgeStart. Set Runtime_s to Retain.
Power-cycle PLCSIM
Run the block, then switch PLCSIM to STOP, power off and on.
Scale an analog value
Use NORM_X and SCALE_X to convert a raw Int (0 … 27 648) into a Real in mm.
Loop over the array
Write the FOR loop from Step 05 and force one roller bed's Fault to TRUE.
Frequently asked questions
What are the variable sections in an SCL block?
An SCL block declares variables in VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR (Static, FB only), VAR_TEMP and VAR CONSTANT. The section decides where the value is stored and whether it is kept between calls.
What is the difference between Static and Temp variables?
Static variables belong to an FB and are stored in its instance DB, so they keep their value from one call to the next. Temp variables live on the local data stack only while the block runs, so write them before you read them.
What is load memory and work memory in an S7-1500?
Load memory is on the SIMATIC Memory Card and holds the whole project. Work memory holds the runtime-relevant code and data while the CPU runs. Retentive memory keeps values marked Retain through a power cycle.
How do I make a variable retentive in SCL?
In an optimized FB, set the Retain column of the variable in the block interface to Retain (or Set in IDB). In a global DB, tick Retain for the member. Non-retain values return to their start values after power OFF and ON.
How do I convert Int to Real in SCL?
Use INT_TO_REAL, or NORM_X and SCALE_X for analog values. To go back from Real to a whole number, use TRUNC to cut the decimals or ROUND for the nearest whole number.
What is slice access in SCL?
Slice access reads part of a bit-string tag directly: #Word.%X2 is bit 2, #Word.%B0 is the low byte. It works on S7-1200 and S7-1500 and replaces masks and shifts.