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. Currently, facilities modernize these nodes to plant Ethernet using the ANC-100e DH+ Gateway or connect programming laptops with the ANC-120e USB to DH+ Cable.
Figure 3: Modular 5/03 and 5/04 Processors featuring physical mode keyswitches and dual communication channels.
4. Industrial Ethernet Integration: The SLC-500 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 money-back satisfaction guarantee & 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
SLC-500 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 | DH-485 interface (e.g., 1747-UIC); no DH+ port |
| SLC 5/01 (1747-L511, L514) | 1K or 4K | DH-485 (RJ45 Target) | None | DH-485 interface (e.g., 1747-UIC); no DH+ port |
| SLC 5/02 (1747-L524) | 4K | DH-485 (Peer-to-Peer MSG) | None | DH-485 interface (e.g., 1747-UIC); no DH+ port |
| 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 |
Programming the SLC 500 with RSLogix 500
RSLogix 500 is Rockwell's ladder logic software for the SLC 500 and MicroLogix families. Instead of tags, it organizes controller memory in fixed data files: O0 (outputs), I1 (inputs), S2 (status), B3 (bits), T4 (timers), C5 (counters), R6 (control), N7 (integers) and F8 (floating point).
To go online with an SLC 5/04, RSLogix 500 connects through RSLinx Classic over the processor's DH+ channel. On a Windows 10 or 11 laptop, the ANC-120e provides that DH+ connection through a USB port. For SCADA or HMI access from the plant network, the ANC-100e bridges the same DH+ channel to Ethernet. See our RSLinx Classic driver setup guide.
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).