Why Allen-Bradley has so many protocols
Rockwell Automation's Allen-Bradley brand has been producing programmable controllers since the 1970s. Each generation of hardware introduced new communication standards — from the serial RS-232 connections of the earliest PLC-5s through to the Ethernet-based EtherNet/IP used by modern ControlLogix and CompactLogix families. Most industrial plants have multiple generations of equipment on the floor simultaneously, which means multiple protocols in active use at the same time.
This guide covers all the major Allen-Bradley communication protocols, when each was introduced, which controller families use it, and how TOP Server's AB Suite drivers connect to each one. The goal is to give engineers a clear picture of what they have in the field and what the right connectivity path looks like from each protocol family to OPC and beyond.
Rockwell Automation and Allen-Bradley: Rockwell Automation acquired Allen-Bradley in 1985. The Allen-Bradley brand name appears on hardware; "Rockwell Automation" is the corporate parent. In industrial connectivity, the two names are used interchangeably. TOP Server refers to its connectivity suite as the "Rockwell/Allen-Bradley (AB) Suite."
The major protocols at a glance
Allen-Bradley protocols fall into three broad eras: the serial and proprietary network protocols of the PLC-5 and SLC 500 generations, the Ethernet and fieldbus protocols introduced with the ControlLogix/Logix5000 platform, and the current EtherNet/IP standard used across all modern Rockwell hardware. Here is a reference overview of each.
CIP: the common application layer behind EtherNet/IP, ControlNet, and DeviceNet
EtherNet/IP, ControlNet, and DeviceNet are all transport-level implementations of the same application protocol: CIP (Common Industrial Protocol). CIP was developed by Rockwell Automation and is now maintained by ODVA (Open DeviceNet Vendors Association). Understanding CIP helps explain why these three protocols share concepts like object model, explicit messaging, and implicit messaging, while using entirely different physical layers and link protocols.
CIP defines a set of object classes that abstract device capabilities — the Identity Object, the Message Router, the Connection Manager, and application-specific objects for I/O assemblies, motor control, and more. Every CIP-capable device exposes its data through this object model, regardless of whether it is connected via Ethernet, a ControlNet coax cable, or a DeviceNet CAN segment.
For OPC connectivity, the most important aspect of CIP is its two messaging modes:
- Explicit Messaging (Connected and Unconnected): Request-response transactions used to read or write specific data points on demand. This is the mode TOP Server uses when polling a ControlLogix controller for tag values — it sends a CIP read request, receives the value, and caches it for OPC clients.
- Implicit Messaging (I/O Messaging): Pre-configured, time-scheduled I/O connections that deliver data on a fixed interval without explicit requests. This is the mode used for real-time I/O between a ControlLogix controller and remote I/O adapters. TOP Server's ControlLogix Unsolicited Ethernet driver uses this capability to receive change-of-state notifications from the controller without polling.
Logix tag-based vs. file/element addressing: One of the most practically important differences between old and new Allen-Bradley protocols is the addressing model. PLC-5, SLC 500, and MicroLogix use file/element addressing: data is organized into typed files (Output O:, Integer N7:, Float F8:, Timer T4:, etc.) with numeric element indexes. Logix5000 controllers (ControlLogix, CompactLogix) use symbolic tag-based addressing: every data point has a programmer-defined name (e.g., Reactor4_Temp_SP) rather than a numeric address. This makes Logix data more self-describing but requires that the OPC server understand the Logix tag database structure — which TOP Server does via the ControlLogix Ethernet driver's tag browsing capability.
Protocol comparison
The table below summarizes key characteristics of each major Allen-Bradley protocol to help engineers quickly identify what they have in the field and what connectivity approach it requires.
| Protocol | Transport | Speed | Addressing | Status | Gateway needed? |
|---|---|---|---|---|---|
EtherNet/IP (CIP) | TCP/UDP over Ethernet | 10/100/1000 Mbps | Symbolic tag names | Current standard | No — direct Ethernet |
CSP / PCCC Ethernet | TCP port 2222 | 10/100 Mbps | File/element (N7:0, F8:1…) | Legacy, still in use | No — direct Ethernet |
DF1 Serial | RS-232 serial | Up to 115.2 kbps | File/element | Legacy, still in use | No — direct serial / USB-serial |
DH+ (Data Highway Plus) | RS-485 token ring | 57.6–230.4 kbps | File/element | Legacy — gateway required | Yes — ETH/IP or KF2/KF3 gateway |
DH-485 | RS-485 token ring | Up to 19.2 kbps | File/element | Legacy — gateway required | Yes — 1761-NET-AIC or similar |
ControlNet | Coax / fiber, CTDMA | 5 Mbps | CIP objects | Active in large installs | Yes — CNB bridge or PC card |
DeviceNet | CAN bus | 125–500 kbps | CIP objects | Active in device-level I/O | Yes — via Logix scanner or PC card |
Micro800 Ethernet | EtherNet/IP (CIP) | 10/100 Mbps | CIP explicit messaging | Current — Micro800 family | No — direct Ethernet |
Which protocols does my controller support?
Use the matrix below to identify which protocols are natively supported by your controller family, and which require an external gateway or adapter for connectivity.
| Controller Family | EtherNet/IP | CSP/PCCC Ethernet | DF1 Serial | DH+ | DH-485 | ControlNet | Micro800 ETH |
|---|---|---|---|---|---|---|---|
| ControlLogix (1756) | ✓ | – | Via port | Via DHRIO | – | ✓ | – |
| CompactLogix (1769) | ✓ | – | Via port | – | – | – | – |
| FlexLogix (1794) | ✓ | – | – | – | – | – | – |
| PLC-5 (5/xx Ethernet) | – | ✓ | ✓ | ✓ | – | 5/xxC only | – |
| SLC 5/05 | – | ✓ | ✓ | Via KF2 | ✓ | – | – |
| SLC 5/03 – 5/04 | – | – | ✓ | ✓ | ✓ | – | – |
| SLC 5/01 – 5/02 | – | – | ✓ | – | ✓ | – | – |
| MicroLogix (all) | Via ENI adapter | – | ✓ | – | ✓ | – | – |
| Micro800 family | – | – | ✓ | – | – | – | ✓ |
✓ Native support Via… = requires adapter or gateway module – = not supported
How TOP Server connects to Allen-Bradley controllers
TOP Server's Rockwell/Allen-Bradley (AB) Suite is a set of specialized drivers bundled in a single license. Each driver is optimized for a specific protocol family and addressing model. Understanding which driver maps to which hardware is what determines whether your OPC connection is properly configured.
Free trial available: The TOP Server demo is the full product with a 2-hour runtime before the service must be restarted. Software Toolbox strongly recommends using the free trial to verify connectivity to your specific controllers before purchasing — especially for older hardware where firmware revision, communication module model, and network configuration all affect what works.
Frequently asked questions
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