Allen-Bradley 1747-L20C Migration-Ready Processor for Legacy SLC 500

Overview

Allen-Bradley 1747-L20C Migration-Ready Processor for Legacy SLC 500 Control Systems

The Allen-Bradley 1747-L20C is a fixed-chassis SLC 500 processor module designed for legacy programmable logic controller applications where the original unit has reached end-of-life, been discontinued, or suffered an unplanned failure. As a migration-ready replacement, the 1747-L20C supports direct program upload from the original controller using RSLogix 500 or APS software, preserving existing ladder logic, data file structures, and I/O addressing without requiring a full system redesign. For facilities managing aging SLC 5/02-class control panels, this module provides a cost-effective path to extend operational life while deferring a full ControlLogix or CompactLogix platform migration.

The 1747-L20C operates within the standard SLC 500 fixed chassis and communicates natively over the DH-485 network, making it compatible with existing HMI panels, SCADA links, and programming terminals already deployed on the plant floor. Before installation, engineers should verify the power supply capacity of the existing 1746-P2 or 1746-P4 power supply unit, confirm backplane slot addressing, and review the I/O module complement — particularly any 1746-IB16 digital input or 1746-OB16 digital output modules — to ensure the replacement processor’s memory and I/O capacity matches the original configuration. The 1747-L20C supports up to 4K words of user program memory, which is sufficient for most legacy fixed-chassis applications.

During retrofit planning, technicians should document the existing program using RSLogix 500 before powering down the panel. A full program backup to a laptop or programming terminal ensures that the ladder logic, subroutine files, and data table values can be restored immediately after the new processor is seated. The 1747-L20C uses the same 1747-L2x form factor, so physical installation into the existing chassis requires no bracket modification or DIN rail re-spacing. Terminal wiring on the I/O side remains unchanged, which significantly reduces rewiring time and the risk of field wiring errors during the changeover window.

Migration Compatibility Table

Retrofit Planning for Existing Automation Systems

A successful 1747-L20C retrofit begins with a thorough audit of the existing control cabinet. Start by identifying all modules installed in the SLC 500 chassis: the 1746-P2 power supply, any 1746-IB16 or 1746-OB16 discrete I/O modules, analog input cards such as the 1746-NI4, and any specialty modules including the 1747-SDN DeviceNet scanner or 1747-KE DH-485 to RS-232 communication module. Each of these modules must be confirmed compatible with the 1747-L20C’s I/O addressing scheme before the replacement processor is installed.

For systems that include a 1747-AIC Advanced Interface Converter or a 1747-PIC Personal Interface Converter for programming access, these communication accessories remain fully compatible with the 1747-L20C and do not need to be replaced. Similarly, if the panel includes a PanelView 550 or PanelView 600 HMI communicating over DH-485, the existing HMI application screens and tag mappings will continue to function after the processor swap, provided the data file addresses in the replacement processor match the original program. Technicians should verify this by comparing the data table structure in the backed-up program file against the HMI tag database before going live.

Where the existing system uses a 1746-BAS BASIC module for custom reporting or serial communication tasks, confirm that the BASIC program stored in the module is independently backed up, as this data resides in the module itself and is not affected by the processor replacement. For facilities planning a longer-term migration toward a CompactLogix L1 or L2 platform, the 1747-L20C serves as a reliable interim solution that maintains production continuity while the migration project is scoped, budgeted, and scheduled.

Downtime Control During System Migration

Minimizing unplanned downtime is the primary concern during any SLC 500 processor replacement. The recommended approach is to schedule the changeover during a planned maintenance window, ideally with a cold standby 1747-L20C already on-site and pre-loaded with the verified program file. Before the cutover, perform a final online program upload from the running controller to confirm the backup is current, including any data table values that may have changed since the last scheduled backup.

During the physical swap, power down the chassis using the 1746-P2 or 1746-P4 power supply switch rather than disconnecting field wiring, which preserves the I/O terminal block connections and eliminates the risk of rewiring errors. Seat the 1747-L20C into the processor slot, restore power, and download the verified program. Perform a controlled I/O force test on critical output channels before releasing the system to automatic mode. For processes where even a brief interruption is unacceptable, consider deploying a 1747-L20C in a parallel test chassis to validate the program and I/O behavior offline before the live cutover. This approach reduces the live changeover window to under 15 minutes in most fixed-chassis SLC 500 applications, protecting both the production schedule and the integrity of the original control logic.

Retrofit Support FAQ

Q: Is the 1747-L20C a direct replacement for the 1747-L20 and 1747-L20B?
A: Yes. The 1747-L20C is backward compatible with the 1747-L20 and 1747-L20B in the same fixed chassis. The program, I/O addressing, and DH-485 communication configuration transfer directly. Verify the firmware revision after installation and update if required using RSLogix 500.

Q: What pre-shipment testing is performed on the 1747-L20C?
A: Each unit is powered on and functionally tested prior to dispatch. The processor is verified to initialize correctly, communicate over the RS-232 programming port, and pass internal diagnostics. A support terms confirmed by quotation cover the unit from the date of shipment.

Q: Do I need to re-wire the I/O terminals when replacing the processor?
A: No. The 1747-L20C installs into the existing SLC 500 fixed chassis without any changes to field wiring. The I/O modules, terminal blocks, and backplane connections remain in place. Only the processor module is swapped.

Q: What is the lead time and stock availability?
A: The 1747-L20C is held availability confirmed by RFQ and available for immediate shipment. Orders placed before the daily cut-off are dispatched the same business day. Contact [email protected] or call +86 18359268345 to confirm current inventory and arrange expedited delivery if required.

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Product Identification

Brand / Ecosystem

Allen-Bradley
Allen-Bradley

Model / Series

1747-L20C
ControlLogix Series

Product Family

PLCs & Controllers

Availability Status

Available on request after model and quantity confirmation

Inquiry Type

B2B RFQ, replacement inquiry and project spare-part request

Pre-Quote Check

Exact SKU, brand, series, quantity, nameplate photos and destination country are checked before quotation

Specification & Inquiry Focus

Control platform

PLC, PAC and modular controllers

Common needs

CPU, rack, power and I/O expansion

Project support

Replacement, cabinet build and maintenance

Inquiry Support

• Model cross-checking before quotation
• CPU, power, rack and expansion module matching
• Support for replacement and project inquiry

What to Send Us

• Brand and exact model number: Allen-Bradley 1747-L20C
• Quantity and required delivery time
• Application, replacement requirement or nameplate photo
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