Allen-Bradley PLC-5 Modules – 1785-L80E/E

Technical Dossier

Product Details And Specifications

Allen-Bradley PLC-5 Series Technical Notes

The Allen-Bradley PLC-5 series, manufactured by Rockwell Automation, represents one of the most widely deployed programmable logic controller platforms in global heavy industry. Installed across petrochemical refineries, nuclear power facilities, pulp and paper mills, automotive assembly lines, and offshore oil platforms, the PLC-5 family established the benchmark for rack-based modular control architecture throughout the 1980s and 1990s. Its continued presence in critical infrastructure worldwide — with installed bases measured in the hundreds of thousands of units — makes lifecycle support and spare parts availability a primary operational concern for plant engineers and maintenance managers.

The Evolution of PLC-5 Architecture

The PLC-5 platform was introduced by Allen-Bradley in 1983 as a successor to the PLC-2 and PLC-3 families. The original PLC-5/10, PLC-5/12, PLC-5/15, and PLC-5/25 processors established the foundational instruction set and 1771 backplane standard. These early variants communicated exclusively via Data Highway (DH) and Remote I/O, with memory capacities ranging from 4K to 16K words.

The mid-generation PLC-5/30, PLC-5/40, and PLC-5/60 processors introduced expanded memory (up to 32K words), enhanced floating-point math, and support for the Data Highway Plus (DH+) network at 57.6 Kbps and 230.4 Kbps. This generation became the dominant platform for large-scale batch and continuous process control through the 1990s.

The Ethernet-capable variants — PLC-5/20E, PLC-5/40E, PLC-5/60E, and the subject of this listing, the 1785-L80E/E — introduced 10/100 Ethernet/IP connectivity, enabling integration with modern SCADA systems and MES platforms without requiring gateway hardware. The 1785-L80E/E specifically provides 100K word user memory, the largest in the PLC-5 family, and supports simultaneous DH+, Remote I/O, and Ethernet/IP communications.

Rockwell Automation announced the PLC-5 platform as end-of-life (EOL) with a final last-time-buy date of December 31, 2023. Migration paths recommended by Rockwell include the ControlLogix (1756) and CompactLogix (1769) platforms. However, the complexity and cost of full system migration in operating plants means the PLC-5 will remain in active service at numerous facilities well into the 2030s, sustaining strong demand for certified spare parts and repair services.

PLC-5 Full Catalog & Functionalities (SKU List)

Processors / Controllers

• 1785-L11B: PLC-5/11 processor, 4K word memory, DH+ and RIO communications.
• 1785-L20B: PLC-5/20 processor, 16K word memory, DH+ and RIO, standard rack mount.
• 1785-L20E: PLC-5/20E processor, 16K word memory, adds 10/100 Ethernet/IP port.
• 1785-L30B: PLC-5/30 processor, 32K word memory, DH+ and RIO, enhanced math.
• 1785-L40B: PLC-5/40 processor, 48K word memory, DH+ and RIO, high-capacity batch control.
• 1785-L40E: PLC-5/40E processor, 48K word memory, Ethernet/IP, DH+, and RIO.
• 1785-L60B: PLC-5/60 processor, 64K word memory, DH+ and RIO, large-scale process control.
• 1785-L60E: PLC-5/60E processor, 64K word memory, Ethernet/IP, DH+, and RIO.
• 1785-L80E: PLC-5/80E processor, 100K word memory, Ethernet/IP, DH+, and RIO.
• 1785-L80E/E: PLC-5/80E enhanced revision, 100K word memory, Series E firmware, Ethernet/IP + DH+ + RIO, maximum I/O capacity of 3072 discrete or 192 analog points.

I/O Modules (1771 Platform)

• 1771-IAD: 120V AC input module, 16-point, isolated, for high-noise industrial environments.
• 1771-OAD: 120V AC output module, 16-point, triac-based solid-state switching.
• 1771-IBD: 24V DC input module, 32-point, sinking, high-density discrete input.
• 1771-OBD: 24V DC output module, 32-point, sourcing, solid-state transistor output.
• 1771-IFE: Analog input module, 8-channel, 12-bit resolution, 4–20 mA / 0–10V configurable.
• 1771-OFE1: Analog output module, 4-channel, 12-bit, 4–20 mA current output.
• 1771-OFE2: Analog output module, 4-channel, 12-bit, 0–10V voltage output.

Communication Adapters

• 1771-ASB: Remote I/O adapter module, connects 1771 chassis to PLC-5 RIO scanner port.
• 1785-KA5: DH+ to DH+ bridge/gateway module for multi-network PLC-5 installations.

Power Supplies (1771 Chassis)

• 1771-P4S1: 5V DC power supply, 10A, for 1771 I/O chassis, single-slot mounting.
• 1771-P7: 5V DC / 24V DC power supply, redundant-capable, for large 1771 chassis configurations.

Sourcing Hard-to-Find & Obsolete PLC-5 Parts

With the PLC-5 platform formally discontinued, the primary challenge for maintenance engineers is sourcing certified replacement units for processors, I/O modules, and communication adapters that are no longer manufactured. DriveKNMS maintains a dedicated inventory of PLC-5 components — including the 1785-L80E/E — sourced through verified industrial surplus channels and decommissioned plant equipment.

All units are inspected upon receipt, catalogued by firmware revision and hardware series, and stored in ESD-controlled environments. DriveKNMS provides documented traceability for each unit, including source origin and inspection records, to support plant maintenance documentation requirements. For facilities operating under IEC 61511 or ISA-84 functional safety frameworks, DriveKNMS can provide additional test documentation upon request.

Emergency urgent RFQ handling is available for critical process shutdowns. Long-term supply agreements are available for facilities with multi-year maintenance contracts requiring guaranteed PLC-5 parts availability.

Quality Control for the PLC-5 Range

The PLC-5 processor architecture presents specific testing challenges due to its multi-bus backplane design, which simultaneously manages the 1771 I/O bus, the DH+ token-ring network, and (on E-series units) the Ethernet/IP stack. DriveKNMS applies a structured test protocol to all PLC-5 units prior to dispatch:

• Power-on and firmware verification: Each processor is powered in a 1771 test chassis and firmware revision is confirmed against the unit label and Rockwell's published revision history.
• DH+ network communication test: The processor is connected to a DH+ network segment and verified for correct node addressing, token passing, and MSG instruction execution.
• Ethernet/IP connectivity test (E-series): For 1785-L80E/E and other E-series processors, an Ethernet/IP connection is established to a test controller and explicit messaging is verified at both 10 Mbps and 100 Mbps link speeds.
• Remote I/O scanner test: The RIO scanner port is tested with a 1771-ASB adapter to confirm correct rack addressing and I/O data transfer.
• Memory integrity check: A structured ladder logic program is loaded to verify full user memory read/write integrity across all 100K words.
• Battery and capacitor inspection: Memory backup battery voltage is measured and replaced if below specification. Electrolytic capacitors on the processor board are inspected for signs of degradation.