Description
Product Introduction
The Allen-Bradley 1785-L20B (PLC-5/20) is a single-slot, mid-range central processing unit designed for the 1771 Universal I/O chassis platform. It executes system control programs using up to 16K words of battery-backed SRAM memory, making it a cornerstone for medium-scale manufacturing cells, material handling systems, and utility controls.
Equipped with built-in Data Highway Plus (DH+) and Universal Remote I/O (RIO) scanner/adapter ports, the 1785-L20B coordinates real-time data exchange across remote 1771 racks, panelview HMIs, and peer processors. It mounts directly into the leftmost slot (Slot 0) of any 1771 I/O chassis and can be expanded with Ethernet capabilities using a 1785-ENET sidecar module.
Key Technical Specifications
| Parameter | Value |
| User Memory | 16K Words (SRAM, Battery-Backed) |
| Max Total I/O | 512 Max (Inputs or Outputs) |
| Max Remote I/O Racks | 4 Racks (Up to 3 Remote Racks via RIO Port) |
| Backplane Current Draw | 2.3 A @ 5 VDC |
| Onboard Network Ports | Channel 1A: DH+ | Channel 1B: DH+ or Universal RIO (Scanner/Adapter) | Channel 0: Serial RS-232/422A/423A |
| Program Scan Time | 0.8 ms per 1K memory (typical) |
| EEPROM Backup Memory | Optional 1785-ME16, 1785-ME32, or 1785-ME64 Modules |
| Replacement Battery | 1770-XYC (3.0V Lithium) |
| Compatible Chassis | 1771-A1B, 1771-A2B, 1771-A3B, 1771-A4B |
Application Scenarios & The “Trench” Experience
The Trench Story
During a scheduled weekend overhaul, a bottling line controlled by an aging 1785-L20B fails to boot up following a main power disconnect. The red “BATT” LED stays solid ON, and the processor’s active memory is completely cleared due to a dead 1770-XYC lithium battery. Because the site lacked a recent backup file on EEPROM, technicians used an RSLogix 5 program archive stored on a shop workstation, dropped in a new battery, uploaded the memory file via Channel 0 serial, and re-established the DH+ node address to get the bottling line running within 45 minutes.
Specific Applications
- Mid-Sized Machine Control: Drives automated assembly cells, stamping presses, and conveyor routing tables up to 512 I/O points.
- Remote I/O Master Scanner: Functions as the primary RIO scanner supervising multiple 1771-ASB remote chassis drops across a plant floor.
- Legacy DH+ Network Nodes: Communicates peer-to-peer with other PLC-5 controllers, SLC 5/04 processors, and supervisory HMI stations over Data Highway Plus.
- 1785-L20B
- 1785-L20B
- 1785-L20B
Transparency SOP: Quality Assurance & Testing
Every 1785- processor module undergoes systematic hardware verification and burn-in testing prior to shipment:
- Physical & Battery Check: We inspect backplane gold card-edge pins, verify mechanical keying clips, and install a fresh 1770-XYC lithium battery.
- Firmware & Boot Diagnostics: The CPU is inserted into a test chassis to verify power-up self-tests, clearing error codes, and checking internal memory registers.
- Communication Port Testing:
- DH+ Ports (1A & 1B): Verified on an active DH+ network via RSLogix 5 for active node browsing and program upload/download.
- Serial Channel 0: Tested for DF1 full-duplex/half-duplex communication stability.
- I/O Backplane & RIO Loop Test: The processor is loaded with a test program that cycles local chassis discrete/analog cards while scanning an external 1771-ASB remote drop.
- Memory & EEPROM Retain Test: Power is cycled multiple times to ensure non-volatile memory retention and EEPROM backup transfer functionality.
The Veteran’s Tech Trap Guide
- ⚠️ Replace the Battery BEFORE Turning Off Chassis Power: If the front-panel BATT LED is ON or flashing red, the backup lithium battery is depleted. Do not turn off rack power until you have replaced the battery (1770-XYC) or saved the active ladder logic program to a PC—otherwise, all CPU memory will be permanently cleared.
- ❗ Slot 0 Installation Only: The 1785- must always be installed in the leftmost slot (Slot 0) of the primary 1771 chassis. It cannot be installed in standard I/O slots or remote expansion chassis drops.
- ⚠️ Channel 1B DIP Switch Configuration: Channel 1B can function as either a DH+ node or a Remote I/O Scanner/Adapter. Its role is defined both in software and by physical DIP switches on the side of the processor. Ensure DIP switch settings match your program configuration before powering up.
- ❗ EEPROM Recommended for Critical Up-Time: To protect against unexpected memory loss during extended power outages or battery failures, always install a 1785-ME16 (or larger) EEPROM module programmed to auto-load on memory fault.
Frequently Asked Questions
Q: What is the main difference between the 1785- and the 1785-L20E?
The uses standard DH+, RIO, and serial communications. The 1785-L20E features an integrated 10Base-T Ethernet port built directly into the front panel of the processor.
Q: Which programming software is used for the 1785-?
The module is programmed using RSLogix 5 software via a DH+ interface (e.g., 1784-U2DHP or 1784-PKTX) or via Channel 0 serial using a 1784-CP10 cable.
Q: How much user memory does the 1785- have?
It features 16K Words of total SRAM memory shared between ladder program logic and data table files.
Q: What replacement battery does the 1785- use?
It uses a 3.0V Lithium 1770-XYC battery assembly.
Q: What warranty covers your surplus and refurbished 1785- processors?
Every 1785- module we supply includes our full 12-month replacement warranty. If a hardware fault, memory error, or communication port failure occurs under normal operating conditions within 365 days of purchase, we replace the unit immediately from stock or issue a full refund.
Start Chat
