Allen-Bradley 1756-PBR2 Retrofit Power Bus for Legacy Systems

Allen-Bradley 1756-PBR2 Retrofit Power Bus for Legacy ControlLogix Systems

The Allen-Bradley 1756-PBR2 is a power bus capacitor module designed for the ControlLogix chassis platform, providing ride-through power support during brief power interruptions. As legacy ControlLogix installations age and original components reach end-of-life, the 1756-PBR2 has become a critical retrofit component for engineers tasked with modernizing existing control cabinets without full system replacement. Whether you are maintaining a 1756-A7 or 1756-A13 chassis, upgrading a distributed I/O network, or extending the operational life of a production line, the 1756-PBR2 offers a proven, drop-in compatible solution that preserves your existing program logic and minimizes unplanned downtime.

In retrofit and upgrade projects, the 1756-PBR2 is typically installed alongside the 1756-PA75 or 1756-PB75 power supply modules, which provide the primary 24VDC or 120/240VAC input power to the ControlLogix backplane. Engineers should verify that the combined load of all installed modules — including the 1756-L75 or 1756-L85E controller, communication modules such as the 1756-EN2T EtherNet/IP bridge, and any 1756-IB16 or 1756-OB16E I/O modules — does not exceed the power supply’s rated output before adding the 1756-PBR2 to the chassis. The capacitor module occupies one slot in the 1756-series chassis and must be installed in the slot immediately to the right of the power supply.

For sites migrating from older SLC 500 or PLC-5 platforms to ControlLogix, the 1756-PBR2 is a standard inclusion in the new chassis build. During such migrations, technicians must account for differences in terminal wiring, I/O addressing, and communication protocol — particularly when transitioning from DH+ or Remote I/O networks to EtherNet/IP using modules like the 1756-DHRIO or 1756-RIO. The 1756-PBR2 ensures that the new ControlLogix chassis maintains power continuity during the cutover window, protecting in-progress program downloads and preserving HMI screen data integrity on connected PanelView Plus terminals.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

Successful integration of the 1756-PBR2 into an existing ControlLogix system begins with a thorough audit of the current chassis configuration. Before ordering, confirm the chassis model (e.g., 1756-A7 or 1756-A13), count the occupied slots, and identify whether a free slot adjacent to the power supply is available. In many legacy installations, the chassis is fully populated with modules such as the 1756-L61 or 1756-L63 controllers, 1756-EN2T or 1756-ENBT EtherNet/IP communication modules, and banks of 1756-IB32 digital input or 1756-OF8 analog output modules. If no adjacent slot is free, a chassis expansion or module reorganization may be required before the 1756-PBR2 can be installed.

Power budget verification is essential. Sum the current draw of all installed modules and compare against the rated output of the 1756-PA75 or 1756-PB75 power supply. The 1756-PBR2 itself draws minimal current from the backplane but adds capacitive hold-up capacity, which is particularly valuable in environments with unstable utility power or frequent micro-interruptions. In control cabinets where a 1756-PSCA2 chassis interconnect cable is used to link multiple chassis, ensure that the power ride-through behavior is coordinated across all connected chassis segments.

Terminal wiring for the 1756-PBR2 is straightforward — the module is a backplane-only device with no field wiring terminals. However, during the broader retrofit, technicians should carefully document and label all existing field wiring connected to I/O modules such as the 1756-IB16D or 1756-OA16 before any module removal. Signal isolators may be required when migrating analog signals from legacy 4–20mA loops to new ControlLogix analog input modules like the 1756-IF16. Programming cable connections using 1784-U2DHP USB-to-DH+ adapters may also be needed to back up legacy PLC-5 programs before the final cutover.

Downtime Control During System Migration

Minimizing production downtime is the primary concern in any ControlLogix retrofit project. The 1756-PBR2 directly supports this goal by providing power ride-through during brief supply interruptions — a common occurrence during panel rewiring and circuit breaker switching. To further reduce downtime risk, plan the migration in phases: first install and commission the new ControlLogix chassis with the 1756-PBR2 and all required communication modules in parallel with the existing system, then perform a controlled cutover during a scheduled maintenance window.

Before cutover, upload and archive the existing controller program from the legacy 1756-L61 or SLC 5/05 processor using RSLogix 5000 or Studio 5000 Logix Designer. Verify that all HMI tag mappings on connected PanelView Plus 7 terminals are updated to reflect any address changes in the new controller. Test all I/O points in program simulation mode before going live. During the cutover itself, the 1756-PBR2’s capacitive hold-up function provides a critical buffer — typically 10 to 20 milliseconds — that allows the controller to complete its current scan cycle and safely transition outputs to a defined state before power is fully lost, protecting both equipment and personnel.

After installation, conduct a full functional test of all control loops, communication links (EtherNet/IP to SCADA, ControlNet to remote I/O drops), and safety interlocks before returning the system to production. Document the as-built configuration, including module slot assignments, IP addresses, and program version numbers, to support future maintenance and further upgrades.

Retrofit Support FAQ

Q: Is the 1756-PBR2 a direct drop-in replacement for the original 1756-PBR (Series A)?
A: Yes. The 1756-PBR2 is the current production replacement for the discontinued 1756-PBR Series A. It is physically and functionally compatible with all standard ControlLogix chassis (1756-A4 through 1756-A17) and requires no configuration changes to the controller or I/O modules.

Q: Does the 1756-PBR2 require any firmware update or software configuration after installation?
A: No. The 1756-PBR2 is a hardware-only capacitor module with no firmware or software configuration requirements. Simply install it in the slot immediately to the right of the power supply and it will operate automatically.

Q: How do I verify compatibility with my existing power supply and chassis before ordering?
A: Confirm your chassis model (1756-A4, A7, A10, A13, or A17) and power supply model (1756-PA75, PB75, PA72, or PB72). Check that a free slot is available adjacent to the power supply. Review the chassis power budget using Rockwell Automation’s online Power Budget Calculator to ensure total module current draw does not exceed the supply rating.

Q: What warranty and testing does NINERMAS provide with the 1756-PBR2?
A: All 1756-PBR2 units supplied by NINERMAS are covered by a 12-month warranty from the date of shipment. Each unit undergoes pre-shipment functional testing to verify capacitor integrity and backplane connector condition. In-stock units are available for immediate dispatch, and expedited shipping options are available for urgent retrofit and breakdown recovery situations. Contact sale@ninermas.com or +0086 187 5021 5667 for availability and lead times.