Siemens S7-1500 PLC Direct Addressing vs Indirect Addressing: Complete Comparison and Examples
1. Core Differences Comparison Table
| Feature | Direct Addressing | Indirect Addressing |
|---|---|---|
| Definition | Fixed specific address hardcoded in the program | Dynamic calculation of address in the program |
| Syntax | L MW10 |
L ID[AR1,P#0.0] |
| Address Source | Hardcoded (e.g., MW10) | Pointer variables/registers (AR1/AR2) |
| Flexibility | Low (requires program modification) | High (can change at runtime) |
| Applicable Scenarios | Simple fixed logic | Arrays, loops, dynamic tables |
| Execution Speed | Fastest | Slower (requires calculation) |
| Code Volume | Least | More (requires pointer calculations) |
| Debugging Difficulty | Simple | Complex (requires monitoring pointers) |
| Memory Usage | Minimum | Requires additional pointer variables |
| S7-1500 Support | Full support | Enhanced support (ANY pointer) |
2. Practical Application Scenario Comparison
| Scenario | Direct Addressing ❌ | Indirect Addressing ✅ |
|---|---|---|
| 8 Temperature Sensors | L MW0 T MW8L MW1 T MW9… |
1 line of code for loop processing |
| Recipe Table | 8 IF statements | 1 pointer to switch recipes |
| HMI Dynamics | Modify program | Change address at runtime |
| Data Movement | 8 MOV instructions | BLKMOV for batch processing |
3. 1:1 Corresponding Example Comparison
Scenario: Processing 8 Temperature Sensors (I0.0-I0.7 → MW0-MW7)
3.1 Direct Addressing Version (OB1, simple but cumbersome)
| Network | STL Code | Description | Disadvantages |
|---|---|---|---|
| 1 | A I0.0 L IW4 T MW0 |
Channel 1 | Repeat 8 times |
| 2 | A I0.1 L IW4 T MW1 |
Channel 2 | Modification is troublesome |
| 3 | A I0.2 L IW4 T MW2 |
Channel 3 | … |
| … | … | … | 8 networks! |
| 8 | A I0.7 L IW4 T MW7 |
Channel 8 |
3.2 Indirect Addressing Version (OB1, 1 network solves it)
| Network | STL Code | Description |
|---|---|---|
| 1 | OPN DB1L MW20 // Index 0-7*L 0.1 // Bit offset L P#I0.0 T AR1 + AR1 P#0.0 A I[AR1,P#0.0] L IW4 T ID[AR1,P#8.0] |
Process all channels once |
4. Complete Project Example
DB1 Data Block
| Address | Name | Type | Description |
|---|---|---|---|
| DBD0 | Temp_Direct[1..8] | ARRAY[1..8] OF INT | Direct method |
| DBD16 | Temp_Indirect[1..8] | ARRAY[1..8] OF INT | Indirect method |
| DBD32 | Index | INT | Indirect index |
Test Program (OB1)
// ========== Direct Addressing (NETWORK 1-8) ==========\n// A I0.0 L IW4 T MW0 // Channel 1\n// A I0.1 L IW4 T MW1 // Channel 2\n// ... Repeat 8 times\n\n// ========== Indirect Addressing (NETWORK 9) ==========\nNETWORK 9\nOPN DB1\nL MW100 // Loop counter 0-7\nAN 7\nJCN END\nL MW100\nT MW32 // Store index\n*L 0.1\nL P#I0.0\nT AR1\n+AR1 P#0.0 // AR1 points to I0.x\nA I[AR1,P#0.0] // Indirect read input\nL IW4 // Read analog quantity\nL MW32 // Get index\n*L 2 // INT byte count\nL P#DB1.DBD16 // Array base address\nT AR1\n+AR1 P#0.0 // Calculate target address\nT ID[AR1,P#0.0] // Indirect write\nEND:\nHere comes the valuable content;Siemens S7-1500/1200/200 Physical Simulation Software (Available for Download)
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