AB | 1747-L20C | Processor Unit

Allen-Bradley 1747-L20C Processor Unit – SLC 500 Series

When a processor module fails on a running SLC 500 rack, the consequences are immediate and measurable: production lines halt, safety interlocks go into undefined states, and maintenance teams face the pressure of sourcing a compatible replacement without disrupting validated I/O configurations. The 1747-L20C is not a generic controller — it is the specific processor that governs the memory map, scan cycle, and communication stack of an entire SLC 500 chassis. Substituting an incompatible unit means re-engineering ladder logic, re-validating I/O addressing, and potentially re-certifying the machine. There is no shortcut around the correct part number.

Within the SLC 500 family, the 1747-L20C occupies the entry-level fixed-chassis tier, supporting up to 20 I/O points and 4K words of user program memory. This positions it as the preferred controller for standalone machines, auxiliary skids, and retrofit applications where a full modular rack would be over-engineered. Globally, the SLC 500 platform remains one of the most widely deployed small-to-mid-range PLC architectures in discrete manufacturing, with millions of installed units across North America, Europe, and Asia-Pacific — making reliable access to genuine replacement processors a persistent operational requirement for plant engineers and MRO procurement teams alike.

Technical Specifications

Electrical

• Supply Voltage: 24V DC (from SLC 500 fixed chassis backplane)
• User Program Memory: 4K words
• Data Table Memory: 4K words
• Maximum I/O: 20 (fixed chassis, local)
• Communication Port: RS-232C (DH-485 compatible via 1747-PIC)
• Scan Time: Approximately 1 ms/K of logic (typical)
• Battery Backup: Lithium battery (1747-BA), retains program and data on power loss

Mechanical

• Form Factor: Fixed chassis processor (SLC 500 fixed platform)
• Compatible Chassis: 1747-L20C integrates with the SLC 5/01 fixed chassis family
• Weight: Approximately 1.8 kg (module + chassis assembly)
• Mounting: DIN rail or panel mount via chassis

Environmental

• Operating Temperature: 0°C to 60°C (32°F to 140°F)
• Storage Temperature: -40°C to 85°C
• Relative Humidity: 5% to 95% non-condensing
• Vibration: IEC 68-2-6 compliant
• Enclosure Rating: Open type (requires installation in suitable enclosure)

Certifications & Standards

• UL Listed, CE Marked, CSA Certified
• Compliant with IEC 61131-2 (Programmable Controllers — Equipment Requirements and Tests)
• RoHS compliant (where applicable to production revision)

Key Features & Benefits

• 4K Word User Program Memory — Sufficient for ladder logic programs with up to several hundred rungs, covering typical machine sequencing, interlocking, and analog scaling routines without memory overflow risk.
• DH-485 Network Compatibility — Connects to Allen-Bradley DH-485 networks via the 1747-PIC interface converter, enabling multi-drop communication with up to 31 nodes at up to 19.2 Kbps; eliminates the need for a dedicated communication module in small network topologies.
• Battery-Backed RAM Retention — Program and data table contents are preserved through power interruptions using the 1747-BA lithium battery, reducing restart commissioning time by eliminating the need to reload programs from an external programmer after unplanned outages.
• Drop-in Replacement for SLC 5/01 Fixed Chassis Applications — Maintains full I/O address compatibility with existing 1747-L20 and 1747-L20A units, allowing processor swap without modifying ladder logic or I/O configuration files; compliant with Rockwell Automation’s backward compatibility guidelines for the SLC 500 fixed platform.
• Compact Fixed Chassis Architecture — Integrates processor, power supply, and I/O in a single chassis footprint, reducing panel space requirements by approximately 40% compared to equivalent modular rack configurations for ≤20 I/O applications.
• RSLogix 500 / RSLogix Micro Compatible — Fully programmable with Rockwell Automation’s RSLogix 500 and RSLogix Micro software environments, supporting standard IEC 61131-3 ladder diagram programming with offline simulation capability.

Industry & Equipment Applications

• Automotive Assembly — End-of-line test fixtures, torque tool controllers, and part-present detection systems on body-in-white and powertrain lines where compact I/O counts and deterministic scan times are required.
• Food & Beverage Processing — Conveyor sequencing, filling machine control, and CIP (clean-in-place) cycle management in environments requiring washdown-rated enclosures with the processor mounted externally.
• Water & Wastewater Treatment — Pump station control, valve sequencing, and level monitoring at remote lift stations where the fixed chassis form factor minimizes panel size in confined utility enclosures.
• Pharmaceutical Manufacturing — Auxiliary equipment control on validated production lines where the 1747-L20C’s established platform history supports change-control documentation and equipment re-qualification with minimal scope.
• Material Handling & Packaging — Palletizer infeed control, label applicator sequencing, and reject gate logic on packaging lines with discrete I/O counts within the 20-point fixed chassis capacity.
• HVAC & Building Automation — Chiller sequencing, air handling unit control, and demand-response load shedding in commercial and industrial facilities where legacy SLC 500 infrastructure is maintained alongside newer BAS systems.

System Integration & Compatible Parts

Upstream / Power & Chassis:

• 1747-L20C is integral to the SLC 500 fixed chassis — the chassis, power supply, and processor are a combined assembly. Ensure chassis revision compatibility when sourcing replacement units.
• 1747-BA — Replacement lithium battery for program memory retention.

Downstream / I/O Expansion:

• Fixed chassis I/O is built-in; for applications requiring additional I/O beyond 20 points, migration to a modular SLC 500 rack (1746 series) with a 1747-L40, 1747-L40C, or higher processor is the recommended upgrade path.

Communication Accessories:

• 1747-PIC — RS-232 to DH-485 interface converter for programming and network connectivity.
• 1747-UIC — USB to DH-485 interface for use with modern programming workstations lacking RS-232 ports.

Equivalent / Substitute Models:

• 1747-L20 / 1747-L20A — Earlier revisions of the same fixed chassis processor; functionally equivalent for most applications, but verify firmware revision compatibility with existing RSLogix project files before substitution.
• Migration to 1747-L40C (40 I/O, 4K memory) is appropriate when I/O expansion is required and a modular chassis is acceptable.

Our Quality Assurance Process

Every 1747-L20C unit sourced and supplied by Konmask.com passes through a structured four-step verification process before shipment:

1. Visual & Physical Inspection — Examination of PCB condition, connector integrity, label legibility, and absence of physical damage, corrosion, or unauthorized repair marks.
2. Functional Power-On Test — Unit is powered and processor status LEDs (RUN, FLT, BATT) are verified against expected behavior per Rockwell Automation diagnostic specifications.
3. Communication Port Verification — RS-232C port is tested for correct signal levels and DH-485 handshake response using calibrated interface equipment.
4. Battery & Memory Check — Battery voltage is measured and memory retention is confirmed; units with low-battery condition are flagged and a replacement 1747-BA is offered.

Available Documentation: Test report, product photos (pre-shipment), original manufacturer datasheet, certificate of conformance (on request), and packing list with serial number record.

How to Order

1. Submit Your Inquiry — Contact us via the form on this page, email support@konmask.com, or call +0086 19859288691 with your required quantity, delivery timeline, and destination country.
2. Receive Quotation — Our technical team will confirm stock availability, unit condition (new/surplus/refurbished), lead time, and pricing within 24 hours of receiving your inquiry.
3. Confirm & Ship — Upon order confirmation and payment arrangement, units are packed to IEC 60068-2-32 drop-test standards and shipped with full tracking. Export documentation (commercial invoice, packing list, COO) is provided for international shipments.

Why Choose Konmask.com?

• Specialized Industrial Automation Inventory — We focus exclusively on PLC, DCS, and industrial control hardware, meaning our team understands the technical context of your requirement — not just the part number.
• Verified Stock, Not Catalog Listings — We confirm physical availability before quoting. You will not receive a lead time estimate based on a third-party broker’s unverified inventory.
• 24-Hour Inquiry Response — All technical and commercial inquiries receive a substantive response within one business day, with emergency sourcing support available for critical production downtime situations.
• Export-Ready Documentation — Full commercial documentation package for international buyers, including country of origin certificates, packing lists, and customs-compliant invoicing.
• Long-Term MRO Partnership — We maintain records of your sourced parts and can proactively alert you to end-of-life notices, recommended spares, and compatible upgrade paths as your installed base evolves.

Frequently Asked Questions

Q: What is the difference between the Allen-Bradley 1747-L20C and the 1747-L20 or 1747-L20A — are they interchangeable as direct replacements?
A: The 1747-L20, 1747-L20A, and 1747-L20C are successive hardware revisions of the SLC 5/01 fixed chassis processor with identical functional specifications (4K memory, 20 I/O, DH-485 communication). In most applications they are drop-in interchangeable at the hardware level. However, if your RSLogix project file was created with a specific processor revision selected, you should verify that the firmware revision of the replacement unit is compatible with your project. Contact us with your existing unit’s catalog number and series letter for a compatibility confirmation before ordering.

Q: Can the Allen-Bradley 1747-L20C processor be programmed with RSLogix Micro, and what cable or interface is required to connect a modern laptop without an RS-232 port?
A: Yes, the 1747-L20C is fully compatible with both RSLogix 500 and RSLogix Micro programming software. For laptops without a native RS-232 port, the recommended interface is the 1747-UIC (USB to DH-485 converter), which provides a direct USB connection and is recognized by RSLogix software without requiring a separate RS-232-to-USB adapter. The 1747-PIC remains the standard option for workstations with RS-232 ports.

Q: What is the expected battery life of the 1747-BA lithium battery in the 1747-L20C, and how do I know when it needs replacement?
A: Under normal operating conditions (processor powered for the majority of each day), the 1747-BA battery typically provides 3 to 5 years of service life. The processor’s BATT LED will illuminate when battery voltage drops below the threshold required to maintain memory during power loss. Rockwell Automation recommends proactive replacement every 3 years in critical applications. We can supply 1747-BA replacement batteries alongside your processor order on request.

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Contact: support@konmask.com | +0086 19859288691