Configured for peer-to-peer chassis synchronization in ControlLogix redundant systems, the Allen-Bradley 1756-RMC3/A (1756-RMC3 RM Fiber Optic Cable) transports the duplex optical redundancy link between paired 1756-RM-class modules .

Catalog Marking and Length Family

Hardware Specifications

ControlLogix Redundant Link Throughput and Firmware Pairing

The 1756-RMC3 is the physical media carrying the ControlLogix redundant chassis synchronization traffic between two 1756-RM-class modules mounted in separate 1756 racks. On 1756-RM / 1756-RMXT the optical link runs at 100 Mbps; on 1756-RM2 / 1756-RM2XT the same duplex LC path runs at 1000 Mbps, mirroring program, I/O image, tag, timer, alarm and communication buffer state between primary and secondary controllers . There is no backplane bus communication velocity licence field on the cable itself; the effective synchronization rate is fixed by the slower of the two terminated RM modules and the fiber grade. Firmware flash compatibility is enforced at the 1756-RM pair, not at the cable: a primary 1756-RM2 and secondary 1756-RMXT, or 1756-RM mixed with 1756-RM2, are rejected by the redundancy manager regardless of a clean 1756-RMC3 optical link . I/O density scaling of the overall system is unaffected by the 3 m cable length, but 3 m only closes adjacent or neighboring-cabinet rack pairs; longer RM cable assemblies (1756-RMC10 or field-terminated singlemode spans) are required before the 100 m multimode or 10 km singlemode limits are reached.

Ordering Information

• 1 x 1756-RMC3 3 m duplex LC-to-LC RM fiber optic cable (base assembly)
• 2 x LC dust caps pre-fitted on both ends on sealed NOS stock
• 1 x Cable label set for PRIMARY / SECONDARY rack identification
• 1 x Printed cleaning instruction card for LC ferrule wipe before insertion

Rack-to-Rack Optical Laydown Rules

• De-energize both redundant chassis or rely on RM module RIUP only when the exact RM generation and firmware match is already confirmed; never insert a suspected-damaged cord into a live SYNC port
• Connect 1756-RMC3 directly between the two RM module optical ports of the same generation; no patch panel, media converter or Ethernet switch is permitted in the path
• Keep the 3 m service loop away from 480 VAC tray cable, VFD output duct and welding return conductors; route through separate grounded steel conduit where cabinet separation is not achievable
• Wipe each LC ferrule with lint-free IPA swab before mating; a contaminated ferrule is the dominant cause of intermittent SYNC LED loss on a previously commissioned redundancy link
• Do not kink, staple or zip-tie the jacket below the 30 mm bend radius; a crushed jacket section raises return loss and collapses the redundant channel even when the OK LED is still lit on one side

Technical Questions and Answers

Is 1756-RMC3 a standard LC-LC patch cord usable on a switch?

No. It is a prepopulated RM synchronization cord sized, labeled and validated for the 1756-RM family optical budget. Connecting it to a non-RM port or through a coupler/switch breaks the ControlLogix redundancy handshake and leaves both chassis non-synchronized.

Can 1756-RMC3 link a 1756-RM to a 1756-RM2 across the two racks?

No. 1756-RM and 1756-RM2 are not cross-pairable; the redundancy manager faults on dissimilar module type even with a clean 1756-RMC3 optical link. Primary and secondary must be the same RM generation and firmware major revision .

Does the 3 m length limit system availability?

Only mechanically. Electrically the multimode RM link supports up to 100 m on 1756-RM / RMXT, and singlemode spans support longer distances on RM2 / RM2XT. A 3 m cord simply means the two redundant racks must sit in adjacent or near-adjacent enclosures.

Who carries the switchover delay, the cable or the RM module?

The 1756-RMC3 is passive; it adds only propagation delay over 3 m, which is negligible. Primary-to-secondary bumpless takeover is governed by the 1756-RM2 / RM2XT synchronization engine, commonly sub-20 ms to sub-50 ms depending on program size and I/O map .

Can two 1756-RMC3 cords be spliced to make 6 m?

Not recommended. Every added splice, barrel or inline adapter inserts insertion loss and reflectance into a link that has no margin headroom reporting at the cable level. Use a factory 1756-RMC10 or a qualified field-terminated singlemode span instead.

Why does SYNC stay off although both LC ends click home?

Most often the far-end RM module is not in redundancy mode, the primary/secondary role is unassigned in Studio 5000, or one LC face is dirty / partially seated. The 1756-RMC3 itself has no diagnostics; verify with RM OK / SYNC / COM LEDs and the redundancy diagnostics object.

Does XT and non-XT mix work if both use 1756-RMC3?

1756-RMXT pairs with 1756-RMXT, and 1756-RM2XT pairs with 1756-RM2XT; a wide-temp module does not form a redundant pair with its non-XT sibling merely because the optical cord is inserted. The 1756-RMC3 only closes the physical layer, not the firmware compatibility gate .

Can the cord survive cabinet door flexing over years?

Only if the service loop is not loaded by the door hinge. Repeated bending at one point, or trapping the jacket in the door gasket, microfractures the cladding and produces gradual attenuation increase that first appears as periodic COM LED flicker before hard SYNC loss.