Overview
The Allen-Bradley 1785-L20B is a PLC-5/20 standard processor module designed for distributed control applications in mid-range industrial automation. This modular controller offers 16K words of battery-backed SRAM, supporting complex process logic while maintaining compatibility with legacy PLC-5 architectures. The unit delivers reliable, real-time control across remote I/O networks and local rack configurations, making it suitable for process automation, material handling, and discrete manufacturing systems.
Key Features
- 16K words of user memory (SRAM, battery-backed)
- Supports 512 total I/O (any mix) or 512 input + 512 output (complementary configuration)
- Embedded Data Highway Plus (DH+) communication channel
- Configurable DH+/Remote I/O adapter or scanner channel
- Serial port (RS-232, RS-422A, RS-423) for programming and peer networking
- Supports up to 13 total racks (1 local + 12 remote I/O chassis)
- Extended ladder logic programming with subroutines, MCPs, and structured text
- Online program editing and I/O forcing capabilities
Technical Specifications
- Memory: 16K words SRAM (user program and data)
- I/O Capacity: Maximum 512 total (any combination) or 512 input + 512 output
- I/O Racks: 4 addresses (0–3) local, plus up to 12 remote chassis
- Communication Ports: 1× DH+ (Fixed), 1× DH+/Remote I/O (configurable), 1× Serial (RS-232/422/423)
- Backplane Current Load: 2.3A @ 5VDC
- Dimensions: 11.0″ W × 2.0″ H × 7.0″ D (27.9 cm × 5.1 cm × 17.8 cm)
- Weight: Approximately 1.0 kg (2 lbs 5 oz)
- Battery: 1770-XYC Lithium battery (included)
- Series: PLC-5 Family
- Mounting: Single-slot 1771 I/O chassis module
Typical Applications
The 1785-L20B excels in distributed control architectures where mid-range processing power and flexible I/O distribution are required. Common applications include:
- Process automation and continuous production control
- Material handling and conveyor system coordination
- Discrete manufacturing and assembly line control
- Multi-location remote I/O monitoring and actuation
- Legacy PLC-5 system upgrades and expansions
- Supervisory control over distributed adapter-mode I/O networks
Compatibility & Replacements
Compatible With
- 1771-series I/O chassis and backplanes
- 1771-series power supplies and I/O modules
- Remote I/O adapter modules and nodes
- Data Highway Plus (DH+) networks and devices
- Standard RS-232/RS-422/RS-423 serial devices
- 1771-SDN DeviceNet scanner module for remote I/O forcing
- 1785-RC Relay Cartridge for external relay control
Replaces / Drop-In Replacement For
Verify compatibility with your system integrator before ordering.
Works With
- RSLogix 5 programming software (ladder logic, structured text, MCPs)
- Studio 5000 (via gateway/bridge modules on hybrid networks)
- PLC-5 peer-to-peer messaging and block transfer protocols
- Remote I/O scanning and adapter-mode protocols
- Sequential function charts (SFC) and subroutine programming
