The 1785-L40C is a PLC-5/40 ControlNet processor from Allen-Bradley. It offers 48K words of battery-backed SRAM. This module controls up to 2048 total I/O points in distributed systems.

Product Overview
The Allen-Bradley 1785-L40C is a powerful ControlNet processor module. It belongs to the reliable PLC-5 series of control system components. This processor mounts directly into a 1771 I/O chassis. It handles field device control with deterministic network performance. The 1785-L40C supports both local and remote I/O architectures.

Technical Specifications
The 1785-L40C provides 48 Kilowords of maximum user memory. Its SRAM is battery-backed to retain programs during power loss. The processor supports EEPROM backup using 1785-ME16, -ME32, -ME64, or -M100 cartridges.

I/O capacity details:

Maximum total I/O: 2048 (any mix)

Complementary mode: 2048 inputs + 2048 outputs

Maximum analog I/O: 2048 channels

Maximum I/O chassis total: 61 units

Universal remote chassis: 32 max per I/O link

Communication ports on the 1785-L40C:

One ControlNet port via RJ45 connector

One RS-232-C/422-A/423-A serial port (25-pin D-shell)

One 8-pin mini-DIN port for programming

One 3-pin DH+ port (default)

One Control Coprocessor expansion port

Transmission and scan times:

ControlNet I/O rate: 5 Mbps

Network update time: 2 to 100 ms (user selectable)

Serial port rates: 57.6K to 230.4K bit/s

I/O scan per rack: 10 ms at 57.6K bit/s

I/O scan per rack: 3 ms at 230.4K bit/s

Program scan: 0.5 ms per K-word (bit logic)

Power and thermal data:

Backplane current draw: 3.3A at 5V DC

Maximum heat dissipation: 59 BTU/hour

The 1785-L40C requires no power derating in normal use

Environmental limits:

Operating temperature: 0 to 60 °C (32 to 140 °F)

Storage temperature: -40 to 85 °C (-40 to 185 °F)

Relative humidity: 5-95% non-condensing

Operational shock: 30g

Operational vibration: 1g at 10 to 500 Hz

ControlNet Hot Backup Support
The 1785-L40C works with the 1785-CHBM ControlNet Hot Backup Cartridge. This setup creates a redundant processor pair. If the primary controller fails, the backup takes over automatically. This feature suits critical processes that cannot stop.

Module Compatibility
The 1785-L40C supports many I/O module families. These include 1771, 1794 Flex I/O, 1746 SLC, and 1791 blocks. It handles 8-point, 16-point, and 32-point digital modules. Intelligent I/O modules also work with this processor. This wide compatibility simplifies system upgrades.

Installation Guidelines
Install the 1785-L40C into any slot of a 1771 I/O chassis. Push the module firmly until the backplane connector seats. Secure the top and bottom latches to lock it in place. Use a grounded wrist strap to prevent electrostatic discharge. Do not force the module into the chassis slot.

Keep the surrounding air temperature within 0 to 60 °C. Allow at least 50 mm of clearance around the module. This space ensures proper cooling and maintenance access. The 1785-L40C weighs about 1.42 kg (3 lbs). Its open-style enclosure suits installation in a control cabinet.

Package Contents
One Allen-Bradley 1785-L40C PLC-5/40 ControlNet Processor

One removable terminal block for DH+ port

One product label with catalog number and serial code

Battery for SRAM backup (installed)

Installation instructions leaflet

Frequently Asked Questions (FAQ)
1. What is the maximum user memory of the 1785-L40C?
The 1785-L40C provides 48 Kilowords of battery-backed SRAM. This memory stores your control program and data tables.

2. Can the 1785-L40C act as a remote I/O scanner?
Yes, the 1785-L40C scans remote I/O over ControlNet. You can connect up to 32 universal remote chassis per link.

3. Does the 1785-L40C support hot backup?
Yes, use the 1785-CHBM cartridge with the 1785-L40C. This creates a redundant ControlNet hot backup system.

4. What programming software works with this processor?
Rockwell RSLogix 5 software programs the 1785-L40C. You connect through the 8-pin mini-DIN or DH+ port.

5. What happens to the program if battery fails?
The 1785-L40C loses its SRAM program only if the battery fails. Use an EEPROM cartridge to store a permanent backup copy.