Configured for 1:1 primary/standby controller synchronization in ControlLogix chassis pairs, the Allen-Bradley 1756-RM (1756-RM Redundancy Module) mirrors program, I/O and tag data over a single fiber optic link.

Catalog Number And Pairing Matrix

The base catalog number 1756-RM designates the original ControlLogix redundancy module. Series letters A/B follow the dash and denote firmware/hardware track; both may coexist in a redundant pair but same-series pairing is preferred. One 1756-RM is required per chassis. A complete redundant system therefore contains two chassis, two identical controllers, and two 1756-RM modules.

Electrical & Mechanical Data

Note: the 1000 Mbps / 1310 nm / 10 km / LC single-mode set above is retained from the supplied sample sheet; Rockwell 1756-TD001 primarily documents 1756-RM at lower legacy sync rates, while 1756-RM2 carries the confirmed 1000 Mbps / 10 km / dual-LC profile .

ControlLogix Backplane Synchronization And Firmware Rules

The 1756-RM attaches to the 1756 backplane in any single slot and exchanges redundancy status, program image and I/O ownership data with the partner 1756-RM across the fiber link. Both primary and standby chassis must carry identical slot population, identical module types and identical major firmware revisions for redundancy-ready qualification. Studio 5000 Logix Designer plus the Redundancy Module Configuration Tool sets the primary/standby role, switch behavior and diagnostic reporting. A 1756-RM cannot be mixed with a 1756-RM2 in the same redundant pair; the sync protocol and port count are not cross-compatible .

Ordering Information

• 1 x 1756-RM redundancy module
• 1 x 1756-RMCx single-mode fiber cable of selected length (1756-RMC1 / RMC3 / RMC10)
• 1 x module installation instruction sheet (Rockwell publication 1756-INxxx)
• 1 x product label / catalog tag sleeve

Rack Pair Commissioning Notes

• Mount one 1756-RM in each of the two redundant chassis; never install only one unit and expect automatic fallback.
• Route the LC fiber as point-to-point between the two 1756-RM modules; do not pass the link through an Ethernet switch, media converter or patch panel.
• Keep the fiber bend radius above the cable manufacturer minimum; tight loops on 8.5/125 um single-mode cable raise attenuation and force SYNC loss before the 10 km budget is reached.
• Verify both chassis share the same 1756 power supply class (standard or redundant) and same backplane loading before enabling redundancy.

Technical Q&A

• Can a single 1756-RM run a standalone non-redundant rack?

No. The 1756-RM has no standalone operating mode. It only synchronizes a primary chassis with a partner chassis that also contains a 1756-RM and a matching controller. Without the partner, the module reports a non-redundant fault and does not assume normal Hot Standby control.

• Is online module replacement supported on a live redundant pair?

Yes, subject to rules. Remove or insert one 1756-RM while the partner chassis and its own 1756-RM remain powered and synchronized. After re-insertion, the new module must match the series and firmware major revision before it is accepted into the redundant pair.

• Why does redundancy never reach SYNCH after replacement?

Most common cause is unequal configuration between chassis: different controller firmware, different I/O slot map, or different communication module count. The 1756-RM runs a consistency check at power-up and blocks redundancy-ready until both sides are electrically and logically mirrored.

• Are 1756-RM and 1756-RM2 interchangeable in one system?

No. 1756-RM and 1756-RM2 use different sync protocols, port counts and data rates. Mixing them across the primary and standby chassis prevents fiber link negotiation and leaves both controllers in non-redundant or faulted state.

• What happens to outputs during a primary failure switchover?

For supported ControlLogix redundancy, the standby controller already holds the current I/O image and program state. On primary loss, ownership transfers to the standby; locally owned 1756 I/O in the redundant chassis pair is held, not dropped, provided both chassis see the same modules.

• How many communication modules can a 1756-L64 redundant system carry?

One 1756-L64 with a single 1756-RM supports up to seven communication modules such as 1756-EN2x or 1756-CN2x in the same chassis pair. Two 1756-L61/-L62/-L63 controllers under one 1756-RM also observe the seven-module limit per redundant system, not per chassis.

• Does the 1756-RM draw significant backplane power?

It is not a light-load module. The 1756-RM consumes 1.2 A at 5.1 VDC, 120 mA at 24 VDC and 4 mA at 1.2 VDC, giving a maximum dissipation near 9.0 W. Account for this when sizing 1756-PA75 / PB75 or 1756-PA75R / PB75R supplies in dense racks.

• Is external grounding required beyond chassis mount?

The 1756-RM relies on the 1756 chassis backplane and mounting rails for signal reference, but the cabinet earth must be bonded to the 1756 chassis ground stud with an adequately sized conductor. Poor cabinet earth raises common-mode noise on the fiber transceiver supply and can produce intermittent COM LED faults.