PLC-5® System Overview

The PLC-5® processor module in a 1771 platform provides a proven approach for industrial control. PLC-5 processors are high-speed, single-slot processors used for control and information processing. They are designed for larger sequential and regulatory control applications with specialized I/O requirements and/or the need to coordinate with other processors and devices.

PLC-5 processors are available in a range of I/O capacity and memory size, and can be connected in a variety of networks for distributed processing and distributed I/O. The 1771 I/O products offer a full range of digital and analog I/O (including intelligent I/O) in a rugged modular assembly.



    • Modularity — Modular I/O and communication interfaces provide a configurable and expandable system. A system should be configured for the required number of I/O and the communication networks. Later, when system expansion is necessary, I/O or communication interfaces can be added.
    • Speed — Fast delivery of messages between networks, between links within networks, and between modules across the backplane.
    • Toughness — Industrially-hardened hardware platform is designed to withstand the vibrations, thermal extremes, and electrical noise associated with harsh industrial environments.
    • Broad I/O options — A large selection of 1771 I/O modules available for use in the local I/O chassis, and an even larger selection of I/O available at locations remote from the processor, can be connected across ControlNet™, DeviceNet™, and Remote I/O links.


A PLC-5 processor communicates across the 1771 backplane to 1771 I/O modules in the chassis in which the processor resides. The different models of PLC-5 processors have various on-board ports for communication with other processors, computers, and remotely located I/O. Also, separate modules are available to provide additional communication ports.

Each PLC-5 processor has a built-in RS-232/422/423 communication port. Each PLC-5 processor also has one or more on-board ports that can be configured for either Data Highway Plus (DH+) or Universal Remote I/O. As a Universal Remote I/O port, it can be configured as either an I/O scanner port or an I/O adapter port. Selected models of PLC-5 processors have on-board ports for Ethernet, ControlNet, or Extended Local I/O communication.

In addition to the on-board ports available with PLC-5 processors, adding another communication port for a PLC-5 processor is as simple as adding an optional communication module.

I/O adapter modules for 1771 I/O are available for ControlNet, Universal Remote I/O, and Extended Local I/O links. An I/O adapter module in a chassis with I/O modules interfaces the I/O modules with the I/O link for communication with a scanner port of a processor at another location.


System Components

A PLC-5 / 1771 control system at minimum consists of a processor module and I/O modules in a single 1771 chassis with a power supply. Choose the processor module with the on-board communication ports needed.

On-board Universal Remote I/O scanner ports are available on all PLC-5 processors. On-board Extended Local I/O scanner ports are available on some PLC-5 processors. On-board ControlNet ports are available on some PLC-5 processors. To provide a DeviceNet I/O scanner port to the system, a DeviceNet scanner module (1771-SDN) is necessary.

Typical Configurations

In this typical configuration, a ControlNet port on the processor module interfaces the processor to the ControlNet link. In each of the two chassis remote from the processor, a 1771-ACN15 I/O adapter module provides I/O modules in those chassis with an interface to the ControlNet link. The PLC-5 processor monitors/controls the I/O in its local I/O chassis as well as the I/O in the remote locations.

The 1771 power supply module plugs into an I/O module slot, or you can connect a stand-alone 1771 power supply to the left end of each chassis.


PLC-5 Processors Overview

The PLC-5® processors are single-slot modules that are placed into the left-most slot of a 1771 I/O chassis. They are available in a range of I/O, memory, and communication capability.

PLC-5 processors have ports configurable for either a Data Highway Plus messaging communication link or a Universal Remote I/O link. As a Universal Remote I/O port, it can be configured as either an I/O scanner port or an I/O adapter port. As an I/O scanner port, it monitors and controls the I/O on the link by communicating with the I/O adapters for those I/O. As an adapter port, it communicates only with the I/O scanner port on the link, passing a maximum of one I/O rack of input and output data between the two to provide distributed processing.

All PLC-5 processors have Data Highway Plus and RS-232-C/422-A/423-A communication ports. In addition to these ports, each Ethernet PLC-5 processor has an Ethernet communication port, and each ControlNet PLC-5 processor has a ControlNet communication port.

Although each Ethernet PLC-5 processor has on-board Ethernet ports; additional Ethernet ports can be added to any PLC-5 processor system with a 1785-ENET Ethernet Interface Module. PLC-5 processors do not have an on-board DeviceNet port. A DeviceNet port can be added to the system with a 1771-SDN scanner module.


    • Ladder-logic and structured-text programming
    • Advanced instruction set including file handling, sequencer, diagnostic, shift register, immediate I/O, and program control instructions
    • Multiple main control programs for segregation of control tasks
    • Processor input interrupts and global status flags
    • Programmable fault response for reacting to a fault before the system goes down
    • Timed interrupt routine for examining specific information at specific time intervals
    • Protected memory selectable by word on selected processors
    • 512 through 3072 maximum forcible I/O in any mix
    • Up to 50,176 maximum non-forcible I/O
    • Processor-resident local I/O (1771 I/O modules)
    • Extended-local I/O on selected processors (1771 I/O modules)
    • Universal Remote I/O (1746, 1771, and 1794 I/O modules and 1791 I/O blocks)
    • DeviceNet I/O (1794 I/O modules, 1792D I/O blocks)
    • ControlNet I/O on selected processors (1771, 1734, 1794, 1797 I/O modules)
    • Universal Remote I/O ports can be configured as either an I/O scanner port or an I/O adapter port

Built-in Web Services

Ethernet PLC-5 processors (1785-L20E, -L40E, and -L80E), and the Ethernet interface module (1785-ENET) provide built-in web capabilities:

    • Web Diagnostics and Module Information lets a standard web browser access information stored in the PLC-5 processor data table, including module diagnostic information (requires RSLogix5, version 5.2 or later).
    • Web User Provided Pages allows for unique Web page creation to provide custom data table information. Any Internet user who has network access to the PLC-5 processor can view these pages, which can be either HTML pages that contain data table elements, text and images, or Custom Data Monitor pages that contain data table elements in table form.
    • Domain Name Service (DNS) lets Internet users access the built-in Web server by a specified name instead of by IP address (requires RSLogix5, version 5.2 or later).

PLC-5 Processors with Protected Memory (1785-L26B, -L46B, L86B, -L46C15)

All enhanced PLC-5 processors have memory protection. However, those processors designated as having the memory protection feature have the added capability of limiting access to individual words.

Choose a PLC-5 processor with protected memory when access must be limited to critical or proprietary areas of programs, selectively guard processor memory and I/O words, or restrict use of processor operations.

The PLC-5 protected memory feature expands system validity and security beyond what is provided by the password-and-privilege capability of the other PLC-5 enhanced processors. Custom software protection schemes can be designed for each application.

Rockwell Software programming software can be used to assign class privileges to specific user accounts or a user job function, such as system administrator, plant engineer, maintenance engineer, or operator. Using four privilege classes and associated passwords, access can be limited to critical program areas and restrict access to:

    • Communication channels
    • Remote nodes attached to a DH+ or ControlNet network
    • Program files
    • Data files
    • Individual data words

The PLC-5/26, PLC-5/46, and PLC-5/86 protected processors are equivalent to the PLC-5/20, PLC-5/40, and PLC-5/80 standard processors, respectively, plus the added memory protection feature. These processors are listed under the heading “Standard PLC-5 Processors with Protected Memories” in the PLC-5 Processor selection tables.

The PLC-5/46C15 protected processor is equivalent to the PLC-5/40C15 ControlNet processor plus the added memory protection feature. This processor is listed under the heading “ControlNet PLC-5 Processors with Protected Memories” in the PLC-5 Processor selection tables.