AB-CSP - Allen Bradley CSP Protocol Notes
The CSP (Client Server Protocol) is a proprietary Ethernet-based protocol developed by Allen-Bradley, Inc. It is used by several Allen-Bradley controller families, including SLC 5/05 and PLC-5, as well as software-based controllers and tools such as SoftLogix 5, RSLinx, and third-party SoftPLC implementations.
Allen-Bradley does not publicly publish the full specification for the CSP protocol. As a result, most integrations rely on documented behavior, reverse engineering, or vendor-supported interfaces rather than an openly available protocol standard.
Relationship to DF1 and PCCC
CSP is commonly referred to as “DF1 over Ethernet”. While the transport mechanism differs from serial DF1, CSP reuses the same addressing model and function code structure as the DF1 protocol.
The shared protocol component used by both DF1 and CSP is known as PCCC (Programmable Controller Communication Commands). PCCC defines the command set, data access methods, and register addressing used to interact with controller memory.
From an application perspective, this means that devices and software familiar with DF1 concepts can often map those concepts directly to CSP when communication is performed over Ethernet.
Read and Write Capabilities
CSP can be used to read from and write to registers within compatible Allen-Bradley controllers. In PLC implementations, MSG (message) blocks are commonly used to initiate these read and write operations over CSP.
The same approach applies when communicating with external devices that support CSP/PCCC semantics. Message blocks encapsulate PCCC commands, which are then transported over Ethernet using CSP.
Additional implementation details for using Allen-Bradley controllers with message blocks are described in vendor documentation, such as Using Allen-Bradley Controllers via Message Block .
Supported PCCC Function Codes
Typical CSP implementations support a subset of PCCC function codes. Commonly supported function codes include:
- Diagnostic Status (06 03)
- Echo (06 00)
- SLC Protected Typed Logical Read with 2 Address Fields (0F A1)
- SLC Protected Typed Logical Read with 3 Address Fields (0F A2)
- SLC Protected Typed Logical Write with 2 Address Fields (0F A9)
- SLC Protected Typed Logical Write with 3 Address Fields (0F AA)
- PLC-5 Typed Read (0F 68)
- PLC-5 Typed Write (0F 67)
- PLC-5 Word Range Read (0F 01)
- PLC-5 Word Range Write (0F 00)
Actual support depends on the specific controller model, firmware revision, and software environment.
Addressing Model
CSP uses the same addressing format as serial DF1. This includes file-based addressing for PLC-5 and SLC controllers, as well as typed logical addressing for protected reads and writes.
Because the addressing model is identical, DF1 addressing references apply directly to CSP communication. A detailed description of DF1 addressing formats can be found here:
Practical Notes
When integrating with CSP, it is important to recognize that the protocol is proprietary and implementation behavior may vary between controller families. Testing against the target device and firmware version is essential.
CSP is commonly encountered in legacy and transitional systems where modern EtherNet/IP is not available or where backward compatibility with DF1-based tooling is required.