Allen-Bradley 1785-L40C PLC-5/40C ControlNet Processor

June 11, 2026 • 3 min read • wayne • Science

Product Summary

The Allen‑Bradley 1785‑L40C is a PLC‑5/40C ControlNet processor module. It delivers 48K words of battery‑backed static RAM and handles up to 2048 total I/O points. Its fast scan times and hot backup cartridge option ensure dependable, high‑speed control for demanding industrial environments.

Product Procurement & Buyer Guide

Engineers and procurement managers need a processor that balances memory, I/O density, and network speed. The 1785‑L40C meets these needs with 48K words user memory and 2048 I/O capacity in any mix or complementary mode. It supports 2048 analog I/O points.

This model integrates ControlNet at 5 Mbit/s with user‑selectable network update times from 2 to 100 ms. Such flexibility allows precise tuning for time‑critical applications like conveyor systems or batch processes. The processor also includes a dedicated ControlNet access port (RJ45).

The 1785‑L40C draws 3.3 A from the 5 V DC backplane and dissipates 59 BTU/h. Verify your chassis power budget accordingly. It supports hot backup via the 1785‑CHBM cartridge, ensuring minimal downtime during controller swaps. This feature is critical for continuous manufacturing lines.

Procurement teams should confirm compatibility with 1771, 1794, 1746, and 1791 I/O modules. The processor accepts EEPROM backup with 1785‑ME16, ‑ME32, ‑ME64, and ‑M100. No protected memory is available. Order the correct backup media based on your program size.

The unit operates at 0‑60 °C and 5‑95% non‑condensing humidity. It withstands 30 G shock and 1 G vibration at 10‑500 Hz. These ratings suit harsh factory floors. Weighing 1.42 kg, it ships in an open‑style enclosure.

Expert Technical Evaluation

The 1785‑L40C delivers a scan time of 0.5 ms per K‑word in bit logic and 2 ms per K‑word typical. These speeds enable tight loop control for fast processes. The I/O scan time per rack varies: 10 ms at 57.6 kbit/s, 7 ms at 115.2 kbit/s, and 3 ms at 230.4 kbit/s.

Its serial port supports RS‑232‑C, RS‑422‑A, and RS‑423‑A with selectable baud rates from 57.6 to 230.4 kbit/s. A 25‑pin D‑shell connector provides this interface. Additionally, the processor includes an 8‑pin mini‑DIN programming port and a 3‑pin DH+ port.

The ControlNet coprocessor expansion port allows future upgrades. The RJ45 ControlNet port offers a base configuration for network access. Engineers should set the network update time between 2 and 100 ms depending on application jitter tolerance.

Thermal dissipation remains at 59 BTU/h during operation. The module handles storage temperatures from ‑40 to 85 °C. Such margins prevent damage during off‑season downtime. The unit carries UL, CE, CSA, and C‑TICK certifications.