Is Your Legacy Control System Holding Production Hostage?
When a critical PLC fails on an aging oil and gas line, every hour of downtime cuts into output and margins. The Allen-Bradley 1785-L20B/E answers that challenge for plants still running the Bulletin 1785 PLC-5 platform. This PLC-5/20 processor delivers 16K words of battery-backed SRAM and communicates over DH+ and Remote I/O networks. It installs as a single-slot module in a 1771 I/O chassis, so you can restore a stalled line without redesigning the whole control architecture.
A Practical Replacement for PLC-5/20 Installations
The 1785-L20B/E belongs to the PLC-5 family, a chassis-based controller line that many oil, gas, and heavy industrial sites still depend on. The base catalog is 1785-L20B, and the Series E suffix marks a hardware revision of that base unit. Note that the Ethernet-enabled PLC-5/20E is a separate catalog, 1785-L20E, so verify your exact model before ordering. Rockwell Automation lists this processor as discontinued, with a discontinuation date of 2014-12-31. There is no direct Rockwell replacement; the recommended path is migration to ControlLogix. For operations that need to keep existing machinery running now, sourcing a tested 1785-L20B/E remains a practical option.
Key Specifications That Matter on the Plant Floor
Memory and Program Capacity
The 16K words of battery-backed SRAM hold your ladder logic and data through power interruptions. That capacity suits mid-sized control tasks common in wellhead panels, compressor stations, and pipeline valve control.
I/O and Rack Support
This processor supports up to 512 inputs and 512 outputs, with up to 4 local rack addresses and 13 chassis total. That flexibility lets you scale a single controller across several racks without adding another CPU.
Communication and Programming
DH+ and Remote I/O handle network traffic to drives, panels, and remote racks. A serial RS-232/422/423 port supports programming and peripheral connections. Engineers program the unit with RSLogix 5 software alongside RSLinx.
Environmental Range
The processor operates from 0 to 60 C, covering typical control room and sheltered cabinet conditions in industrial sites.
Where the 1785-L20B/E Fits Best
Oil and Gas Wellhead Control
A wellhead automation skid often runs a PLC-5/20 for pump and valve sequencing. When the original processor fails, a drop-in 1785-L20B/E keeps the skid online while the site plans a longer-term migration.
Pipeline and Compressor Stations
Remote I/O lets this processor manage distributed I/O across a compressor station. The 512 input and 512 output capacity handles pressure, flow, and temperature signals without a controller upgrade.
Legacy Manufacturing Lines
Plants with 1771 I/O hardware can replace a failed processor and resume production quickly. Fast dispatch of spares matters most when a line is down and revenue is lost by the hour.
Installation and Setup Notes
Mount the processor in a 1771 I/O chassis and power the chassis with a compatible 1771-series supply such as the 1771-P4S, P6S, or P7. Confirm the chassis slot assignment and rack addressing before applying power. Connect DH+ and Remote I/O cabling per your existing network layout, and use the serial port for programming with RSLogix 5 and RSLinx. Follow the manufacturer’s installation manual for all wiring and grounding details.
Maintenance and Reliability Guidance
Battery-backed SRAM depends on a healthy backup battery, so include battery condition in scheduled inspections. Check chassis ventilation and keep the cabinet within the 0 to 60 C operating range. Inspect network cabling and connectors periodically for wear. For safety-related functions, follow the manufacturer’s safety manual and inspect periodically without disassembling the unit.
Compatibility and Integration
The 1785-L20B/E works with 1771 I/O modules, 1771-series power supplies, and DH+ and Remote I/O networks. It programs through RSLogix 5 with RSLinx. Integration Note: wiring into DH+ or Remote I/O networks may require specific cables or connection modules; confirm cable and gateway requirements with your supplier before installation.
What to Verify Before You Buy
Confirm your base catalog is 1785-L20B with the Series E suffix, not the separate 1785-L20E Ethernet model. Check your chassis type, power supply model, and network type. Since the product is discontinued, verify current lifecycle status and availability with Oil AutoTech before ordering. Oil AutoTech offers hands-on technical selection help and fast dispatch, worldwide, for oil, gas, and industrial automation spares.
Frequently Asked Questions
Is the 1785-L20B/E still available?
Rockwell Automation discontinued this processor on 2014-12-31. Availability now depends on surplus and refurbished stock. Contact Oil AutoTech for current inventory.
What replaces the 1785-L20B/E?
Rockwell lists no direct replacement and recommends migration to ControlLogix. For existing PLC-5 installations, a like-for-like 1785-L20B/E keeps your line running during transition planning.
How much memory does the 1785-L20B/E provide?
It offers 16K words of battery-backed SRAM for program and data storage.
Which networks does this processor support?
It communicates over DH+ and Remote I/O, with a serial RS-232/422/423 port for programming.
What software programs the 1785-L20B/E?
Use RSLogix 5 software together with RSLinx for programming and communication.
Contact Oil AutoTech today for pricing and availability.
