Why the 1785-L20B/E Still Matters on the Plant Floor
When a PLC-5/20 processor drops out, the line stops with it. The Allen-Bradley 1785-L20B/E is the Series E revision of the 1785-L20B base catalog, a single-slot processor that installs directly in a 1771 I/O chassis. It carries 16K words of battery-backed SRAM, talks over DH+ and Remote I/O, and supports up to 512 inputs plus 512 outputs. For engineers keeping legacy lines alive, that combination is the practical difference between a quick swap and a full migration project.
What This Processor Actually Delivers
The 1785-L20B/E is a PLC-5/20 controller in the Bulletin 1785 family. Its 16K-word battery-backed SRAM holds the user program through power interruptions, so a brief outage does not wipe logic. The processor addresses up to four local rack addresses across a total of 13 chassis, giving you room to spread I/O where the machine needs it. A serial RS-232/422/423 port handles programming and peripheral links, while DH+ and Remote I/O move data across the plant network. It operates from 0 to 60 C, which suits most control rooms and enclosures.
Note that the Ethernet-enabled PLC-5/20E is a separate catalog, 1785-L20E. If your application depends on Ethernet, this is not that unit.
Technical Specifications
| Specification | Value |
|---|---|
| Part Number | 1785-L20B/E |
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Series | PLC-5 / Bulletin 1785 |
| Product Type | Processor / Controller |
| User Memory | 16K words battery-backed SRAM |
| Networks | DH+ and Remote I/O |
| I/O Capacity | Up to 512 in + 512 out |
| Local Rack Addresses | Up to 4 (13 chassis total) |
| Serial Port | RS-232/422/423 |
| Chassis | 1771 I/O chassis |
| Operating Temperature | 0-60 C |
Where It Fits in Your Architecture
The 1785-L20B/E pairs with 1771 I/O modules for digital and analog signals, and chassis power comes from 1771-series supplies such as the 1771-P4S, P6S, or P7. Programming uses RSLogix 5 with RSLinx. In oil, gas, and industrial automation settings, these processors often run pump stations, compressor controls, and legacy process skids where the installed base is still productive. If you are planning a replacement or upgrade, treat this as a bridge: it keeps the line running while you scope a ControlLogix migration.
Integration Note
Wiring into DH+ or Remote I/O networks may require an external gateway or connection module. Confirm cables and gateways with your supplier before installation.
Lifecycle Status and Sourcing
The 1785-L20B/E is discontinued. The 1785-L20B/L20E were discontinued December 31, 2014, and the Bulletin 1785 family reached final EOL on July 31, 2017. Rockwell Automation lists no direct replacement; the engineering path is migration to ControlLogix. That makes ready-to-ship spares the realistic option for existing installations. Please verify current lifecycle status and availability with Oil AutoTech before ordering.
Selection Checklist Before You Buy
Confirm your chassis is a 1771 series, your power supply matches the rack load, and your network is DH+ or Remote I/O rather than Ethernet. Check that your program fits within 16K words and that your I/O count stays within 512 in and 512 out. If you need hands-on technical selection help, Oil AutoTech can match the processor to your existing hardware. Contact Oil AutoTech today for pricing and availability.
