The History of the SLC-500
The History of the Allen-Bradley SLC-500 Platform: Hardware Evolution & Communication Architecture
The Allen-Bradley SLC-500 (Small Logic Controller) platform was introduced by Rockwell Automation in 1991 as a compact, cost-effective alternative to the larger PLC-5 control systems. Specifically, engineered for small-to-midsize industrial applications, the series provided reliable ladder logic execution within a substantially reduced footprint.
Initially, the release included the Fixed hardware "brick" controllers (20, 30, and 40 I/O) with 1K memory, alongside the first modular CPU: the SLC 5/01. Consequently, these controllers operated with an average scan time of 8 ms/K and communicated over DH-485 via an RJ45 physical port acting exclusively as a responder/target node.
Figure 1: Allen-Bradley 5/01 Processor (1747-L511) with native DH-485 RJ45 port.
2. Modular Architecture: SLC 5/02 Messaging & Execution
Furthermore, the release of the SLC 5/02 (1747-L524) introduced 4K of battery-backed RAM, faster program scan rates (4.8 ms/K), and support for up to 4,096 I/O points across 3 chassis. In addition, it was the first CPU capable of initiating peer-to-peer DH-485 network messages (MSG instructions), while adding user fault routines, hardware interrupts, and 32-bit signed math instructions.
Figure 2: Modular 5/02 CPU (1747-L524) with expanded math and messaging support.
3. Advanced SLC-500 Networking: The SLC 5/03 & SLC 5/04 Era
Subsequently, the family advanced with the 5/03 and 5/04 models, incorporating online ladder editing, hardware run-mode keyswitches, floating-point math, and multi-protocol connectivity:
Serial Communication and Data Highway Plus Integration
- SLC 5/03 (1747-L531, L532, L533): Provided memory options up to 32K and introduced a dedicated RS-232 DB9 port (Channel 0) supporting DF1 Full/Half-Duplex and ASCII protocols alongside the DH-485 channel.
- SLC 5/04 (1747-L541, L542, L543): Integrated built-in Data Highway Plus (DH+) networking running at 57.6k, 115.2k, and 230.4k baud. As a result, this enabled compact racks to exchange data directly across plant-wide DH+ backbones with PLC-5 systems and supervisory workstations.
Figure 3: Modular 5/03 and 5/04 Processors featuring physical mode keyswitches and dual communication channels.
4. Industrial Ethernet Integration: The SLC 5/05 Controller
Finally, the SLC 5/05 (1747-L551, L552, L553) marked the final hardware evolution of the platform. Specifically, it replaced proprietary fieldbuses with an integrated 10/100 Mbps Ethernet port supporting CSPv4 and EtherNet/IP (CIP) protocols, enabling direct plant network integration without dedicated interface cards.
Figure 4: Processor 5/05 (1747-L551) featuring native 10/100 Ethernet connectivity.
ANC-100e: DH+ to Ethernet Gateway
Dedicated gateway for SLC 5/04 and PLC-5 systems. Bridges DH+ nodes to modern EtherNet/IP SCADA and PanelView Plus HMIs.
- • Standalone DH+ gateway architecture
- • Ignition, Kepware & FactoryTalk compatible
- • 30-day evaluation & Lifetime warranty
ANC-120e: USB to DH+ Cable
Field programming cable for SLC 5/04 & PLC-5 via RSLogix 500 and RSLinx Classic on Windows 10 & 11.
- • Windows 10 & 11 native driver support
- • Auto Baud & Auto Polarity detection
- • Includes Phoenix & mini-DIN cables
Processor Port & Communication Channel Architecture
In general, the channel configurations across the platform transitioned through three primary physical network layers: DH-485 (RJ45), RS-232 (DB9), Data Highway Plus (3-pin/mini-DIN), and Ethernet (10/100 RJ45):
Figure 5: Detailed port and channel architecture across modular processors (5/01 to 5/05).
Allen-Bradley SLC-500 Evolution & Technical Specifications
Hardware Specifications by Processor Series
| Processor Series | Memory Options | Primary Communication | Secondary Communication | Modern Connectivity |
|---|---|---|---|---|
| Fixed Hardware (1747-L20/L30/L40) | 1K Instructions | DH-485 (RJ45 Port) | None | USB via 1747-UIC / ANC-125 |
| SLC 5/01 (1747-L511, L514) | 1K or 4K | DH-485 (RJ45 Target) | None | USB via 1747-UIC / ANC-125 |
| SLC 5/02 (1747-L524) | 4K | DH-485 (Peer-to-Peer MSG) | None | USB via 1747-UIC / ANC-125 |
| SLC 5/03 (1747-L531, L532, L533) | 8K, 16K, 32K | DH-485 (RJ45 Port) | RS-232 (DF1 / ASCII DB9) | USB-to-Serial / DF1 converters |
| SLC 5/04 (1747-L541, L542, L543) | 16K, 32K, 64K | Data Highway Plus (DH+) | RS-232 (DF1 / ASCII DB9) | ANC-100e (Ethernet) / ANC-120e (USB) |
| SLC 5/05 (1747-L551, L552, L553) | 16K, 32K, 64K | EtherNet/IP (10/100 RJ45) | RS-232 (DF1 / ASCII DB9) | Standard Plant Ethernet / Switches |
Hardware Obsolescence & Phased Migration Strategies
Following Rockwell Automation’s official transition of the 1746 I/O chassis and 1747 modular CPUs to discontinued status, industrial facilities often evaluate lifecycle pathways:
- Maintaining Active Racks: Continue operating functional SLC 5/04 controllers and 1746 I/O chassis without undertaking complete panel rewiring or ladder logic conversions.
- SCADA & HMI Modernization: Additionally, deploy dedicated ANC-100e gateway hardware to interface DH+ nodes directly into modern Ignition, FactoryTalk View, or AVEVA Wonderware systems.
- Inter-Platform Messaging: Therefore, enable MSG data exchange between legacy SLC 5/04 (DH+) and modern SLC 5/05 (Ethernet) architectures across the manufacturing floor.
Hardware Solutions for Plant Floor DH+ Connectivity
For this reason, for manufacturing facilities managing active Allen-Bradley industrial networks, Automation Networks offers standalone communication hardware in stock:
- ANC-100e ($1,195 USD): Standalone EtherNet/IP to DH+ gateway bridge for SCADA, DCS, and PanelView Plus modernization. Video setup demonstration available on The Automation Blog (Show #2).
- ANC-120e ($995 USD): Likewise, a USB to Data Highway Plus (DH+) programming cable for laptops running Windows 10 & 11 via RSLogix 500. Technical review available on The Automation Blog (Show #58).