1785-L20C15 Allen-Bradley PLC-5/20C ControlNet Controller

1785-L20C15 Allen-Bradley PLC-5/20C ControlNet Processor

Product Introduction

The 1785-L20C15 from Allen-Bradley is a powerful ControlNet processor within the trusted PLC-5 family. This controller is specifically built for applications demanding high-speed, deterministic communication and real-time I/O control. Its integrated ControlNet phase 1.5 interface enables seamless connection to modern industrial networks. Meanwhile, it retains compatibility with traditional Remote I/O and Data Highway Plus (DH+) systems. For engineers upgrading legacy systems or designing new distributed control architectures, the 1785-L20C15 offers a compelling blend of performance and versatility.

Memory and Data Management

This processor provides 16 kilowords of battery-backed static RAM for user programs and data storage. The built-in lithium battery ensures program retention during power interruptions. Additionally, the 1785-L20C15 supports EEPROM program backup using optional modules like the 1785-ME16, 1785-ME32, 1785-ME64, or 1785-M100. This feature provides a permanent copy of your application for disaster recovery.

Notably, this processor does not include protected memory. Consequently, programs remain accessible for online editing and debugging without special unlocking procedures. This openness simplifies maintenance and troubleshooting.

I/O Capacity and System Expansion

The 1785-L20C15 offers two distinct I/O capacity modes, depending on network type:

Forcible I/O (via Universal Remote I/O):

• Maximum total I/O: 512 points (any mix) or 384 inputs + 384 outputs in complementary mode

• Maximum analog I/O: 512 channels

Non-forcible I/O (via ControlNet):

• Maximum total I/O: 3200 points

• Maximum analog I/O: 32000 channels

This dual capability makes the 1785-L20C15 exceptionally flexible. For small to medium systems requiring forced I/O, the 512-point limit applies. For larger, high-speed applications where forces are unnecessary, the ControlNet network unlocks dramatically higher I/O counts.

Additionally, the processor supports up to 77 I/O chassis in total. Among these, a maximum of 12 can be universal remote chassis connected via Remote I/O links. Extended local chassis are not supported.

ControlNet Communication Interface

The 1785-L20C15 features an integrated ControlNet phase 1.5 communication interface. This network provides deterministic, real-time data exchange between controllers, I/O modules, drives, and valves. Key ControlNet specifications include:

• Average I/O scan time: 0.5 ms per rack on ControlNet

• Physical media: Coaxial cable (standard) or fiber optic (with appropriate hardware)

• Network topology: Supports both non-redundant and redundant configurations

ControlNet enables multiple controllers to share I/O data and interlock with each other directly. Furthermore, multiple controllers can receive input data from a single I/O node, enabling sophisticated distributed control strategies.

Important compatibility note: The 1785-L20C15 uses ControlNet phase 1.5. Therefore, it cannot communicate with devices using phase 1.0 or phase 1.25 interfaces. Ensure all nodes on the same ControlNet network share the same phase version.

Front Panel Indicators and Mode Selection

The 1785-L20C15 provides comprehensive LED status indicators:

• BATT (Red): Low battery warning

• PROC (Green/Red): Green = RUN mode, Red = fault condition

• FORCE (Amber): I/O forces are enabled

• COMM (Green): Channel 0 communication active

• Channel 1A/1B Status: Communication activity indicators

• ControlNet I/O Status (Green/Red): Network health and I/O exchange status

• Channel 2 ControlNet Status: Dedicated indicator for the ControlNet link

A three-position keyswitch lets users select the operating mode:

• RUN: Program executes, outputs enabled, forces allowed, online edits permitted

• PROG: Outputs disabled, full file creation/modification/deletion allowed, uploads/downloads possible

• REM: Remote mode with three sub-modes (Remote Run, Remote Test, Remote Program)

Electrical and Environmental Specifications

The 1785-L20C15 draws a backplane current of 2.7A at 5V DC (typical) or 3.0A maximum. Its heat dissipation is rated at 54 BTU per hour, equivalent to approximately 15.8W. The controller operates within a temperature range of 0°C to 60°C (32°F to 140°F) and a non-condensing humidity range of 5% to 95%.

Installation and Compatibility

Installing the 1785-L20C15 is straightforward: plug it into any slot of a 1771 I/O chassis. It supports 2-slot, 1-slot, and ½-slot addressing modes for flexible I/O mapping. Moreover, it works with all standard 1771-series I/O modules, including digital, analog, and intelligent types.

Packaging List

Your shipment of the 1785-L20C15 includes:

• One Allen-Bradley 1785-L20C15 PLC-5/20C ControlNet Processor

• One factory-installed lithium battery for SRAM retention

• Product installation and setup guide (printed)

• Anti-static protective packaging

Note: Programming software, ControlNet cables, and memory modules are sold separately.

Shipping Information

We ship all orders worldwide via FedEx, UPS, or DHL. Standard delivery takes 3 to 7 business days, depending on destination. Express shipping options are available upon request.

Frequently Asked Questions (FAQ)

1. What is the memory capacity of the 1785-L20C15 processor?
The 1785-L20C15 provides 16 kilowords of battery-backed static RAM for user programs and data tables, suitable for medium-to-large automation projects.

2. What is the difference between forcible and non-forcible I/O on this controller?
Forcible I/O (512 points) applies to Universal Remote I/O networks where individual points can be forced on or off. Non-forcible I/O (up to 3200 points) applies to ControlNet networks, where forces are generally not used.

3. Can I use the 1785-L20C15 with older ControlNet devices?
No. The 1785-L20C15 uses ControlNet phase 1.5 and cannot communicate with phase 1.0 or phase 1.25 devices. All nodes on the same ControlNet network must match in phase version.

4. What programming software is required for the 1785-L20C15?
Rockwell Automation's RSLogix 5 software is the recommended programming tool. It supports Ladder Logic, Sequential Function Chart, and Structured Text languages.

5. How many remote I/O chassis can the 1785-L20C15 support?
This processor supports up to 77 I/O chassis total, with a maximum of 12 universal remote chassis connected via Remote I/O links. Extended local chassis are not supported.