Allen-Bradley 1785-LT2 Processor Module

The Allen-Bradley 1785-LT2, also cataloged as the 1785-LT2 PLC-5/25 Processor Module, operates as a dedicated hardware component for central logic processing and I/O control execution within Allen-Bradley 1771 Universal I/O chassis networks.

Hardware Specifications

Deterministic Backplane Bus & Network Architecture

The 1785-LT2 processor resides in the leftmost slot of a 1771 chassis, interfacing via the 1771 backplane bus to scan up to 1024 digital or analog I/O points across localized and distributed racks. Featuring integrated Data Highway Plus (DH+) and Remote I/O (RIO) channels, the unit maintains deterministic communication velocity across peer-to-peer processor networks and remote extension chassis. Non-volatile battery-backed SRAM provides 13k words of memory storage for ladder logic routines, register tables, and system status configuration, ensuring firmware execution continuity through power cycles.

Frequently Asked Questions

Q: What is the backplane current draw for the 1785-LT2 processor module?

A: The 1785-LT2 draws a maximum of 2.5 A at 5 VDC directly from the 1771 chassis power supply backplane, which must be factored into total chassis power supply loading calculations.

Q: How is program retention handled during main system power outages?

A: Internal 13k word SRAM memory is preserved during power disruptions via an onboard lithium battery, preventing program corruption and retaining register states until backplane power is restored.

Field Installation Guidelines

1. Power-Off Installation: Ensure 1771 chassis backplane power is completely isolated before inserting or removing the 1785-LT2 processor module to prevent backplane pin damage and electrical arc transients.
2. Chassis Placement: Slide the module into the left-most slot (Slot 0) of the 1771 Universal I/O chassis, applying firm pressure on the top and bottom card levers until fully seated in the backplane connector.
3. Communication Cable Grounding: Terminate DH+ and Remote I/O twinaxial shield wires strictly at one single point on the cable shield ground bar to prevent high-frequency noise loops from interfering with communication signals.
4. Battery Initialization: Verify the onboard lithium battery connection prior to keying the processor into RUN mode; a low-battery status bit will activate if voltage drops below standard operational limits.
5. Key Switch Position: Set the front panel key switch to the appropriate state (REM, RUN, or PROG) to establish local programming access levels before connecting terminal devices.