Hardware Comparison: SLC 500 Controllers vs. CompactLogix L5370/80
The Allen-Bradley SLC-500 product line, first introduced nearly three decades ago, has long been a staple in industrial automation. Today, the SLC-500 platform is in "Active Mature" status, signaling that while it remains supported, many modules are approaching end-of-life. Allen-Bradley recommends migrating to CompactLogix processors, especially the high-performance CompactLogix 5370 and 5380 series. Below, we compare both systems in detail, covering architecture, memory, scan time, I/O capacity, and power distribution.
| Feature | SLC-500 | CompactLogix |
|---|---|---|
| CPU | 16-bit operations | 32-bit operations |
| Programming Languages | Relay ladder only | Ladder, Structured Text, Function Block, SFC |
| OS Logic Structure | Program files | Tasks, Programs, Routines |
| I/O Processing | Mapped to synchronous I/O tables | Tag-based, asynchronous I/O |
| Data Handling | Data Tables | Tag-based, global/local, produced/consumed tags |
| Timers | 16-bit, selectable bases | 32-bit, single 1ms base |
| Communications | RS-232, DH-485, DH+, Ethernet (model dependent) | USB, Dual Ethernet/IP, MSG over multiple protocols |
| Motion Control | Add-on modules required | Embedded EtherNet/IP motion support |
Memory Comparison
| Model | SLC Memory Size | CompactLogix Memory Size |
| SLC-5/01 | 1K, 4K | - |
| SLC-5/02 | 4K | - |
| SLC-5/03 | 8K, 16K, 32K | L306: 0.6MB, L310: 1MB |
| SLC-5/04 | 16K, 32K, 64K | L320: 2MB, L340: 4MB |
| SLC-5/05 | 16K, 32K, 64K | L350: 5MB, L380: 8MB, L3100: 10MB |
Note: Logix programs require ~10-12x more memory than SLC programs. A 32KB SLC program translates to ~360KB in Logix
| Processor | SLC Scan Time (typical) |
| SLC-5/01 | 8 ms/K |
| SLC-5/02 | 4.8 ms/K |
| SLC-5/03 | 1.0 ms/K |
| SLC-5/04/05 | 0.9 ms/K |
CompactLogix: Uses task-based, priority-driven execution, offering 50–80% faster scan times compared to SLC.
SLC-500 Max: 30 modules / 4,096 I/O points
| CompactLogix 5380 Controller | Max Local Modules | Max EtherNet/IP Nodes |
| L306 | 8 | 16 |
| L310 | 24 | 24 |
| L320 | 16 | 40 |
| L330 | 31 | 60 |
| L340 | - | 90 |
| L350 | - | 120 |
| L380 | - | 150 |
| L3100 | - | 180 |
I/O Expansion Options:
-
Local: Compact 5000 I/O
-
Networked: 1734 POINT I/O, 1794 FLEX I/O, 1756 I/O, 1746 SLC I/O
Example: L350 + POINT I/O
-
120 nodes x 63 modules x 4 I/O = 30,240 I/O Points
Power Considerations
SLC-500:
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Requires dedicated power supply on left side of chassis
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No shared SA (Sensor/Actuator) power bus
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Requires separate field power connections per module
CompactLogix 5380:
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Uses external 24VDC for MOD power
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No chassis required; modules connect side-by-side
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Has common SA power bus, simplifying wiring
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Can isolate power groups with Field Potential Distributor
Migration Advantages
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Performance: 5–20x faster execution and logic scan
-
Scalability: More I/O capacity, network support
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Integration: Tag-based architecture and EtherNet/IP motion
-
Maintenance: Simplified wiring and diagnostics
Whether you're looking to increase performance, add flexibility, or standardize on a modern platform, the CompactLogix 5380 family offers a powerful upgrade path from SLC-500 systems while maintaining support for legacy infrastructure during transition.
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