1756-RMC1 — 1756 Redundancy Module Chassis Interconnect

The 1756-RMC1 is the chassis interconnect module used in a 1756 redundancy system. Two 1756-RMC1 modules — one in each of the two redundant chassis — connect via a fiber optic cable to form the redundancy communication link. This link synchronizes program data, I/O states, and controller status between the primary and secondary chassis in real time. When the primary controller faults or is taken offline for maintenance, the secondary takes over control with minimal interruption to the process.

Redundancy in a 1756 system is a pairing: two identical chassis, each with matching processors, power supplies, and I/O modules, connected via the redundancy link. The 1756-RMC1 is specifically the module that creates that link. Without it, the two chassis cannot synchronize and controller redundancy cannot function.

Specifications

Parameter	Value
Part Number	1756-RMC1
Platform	1756 Logix chassis
Function	Redundancy chassis interconnect (one per chassis)
Communication Medium	Fiber optic (62.5/125 μm multimode)
Fiber Connectors	2 × ST-type (TX and RX)
Max Fiber Cable Length	10 m (standard); up to 200 m with appropriate fiber and connectors
Switchover Time	Typically < 25 ms (with qualified I/O and network setup)
Required per System	2 modules (one in each redundant chassis)
Compatible Processors	1756-L7x, 1756-L8x series (redundancy-capable firmware)
Backplane Current (5V)	950 mA
Operating Temperature	0°C to 60°C
Standards	UL 508, CE

What Redundancy Provides — and What It Doesn't

Controller redundancy in a 1756 system means that a failure in the primary controller — hardware fault, power loss, or firmware exception — triggers an automatic switchover to the secondary controller, which has been running a synchronized copy of the program state throughout. The secondary controller assumes control of the process with the I/O in the same state as the primary had them at the moment of switchover.

What redundancy doesn't cover: field device failures, network infrastructure failures, and I/O module failures are outside the scope of controller redundancy. Redundancy specifically protects against controller failure — the single point of failure that would otherwise be the brain of the entire system. In processes where even a brief unplanned shutdown is costly (continuous chemical processes, pharmaceutical batch reactors, water treatment systems), the cost and complexity of redundancy is justified by what it prevents.

The switchover time of under 25 ms is fast enough that most controlled processes don't notice it. PID loops, analog control, and most sequential logic resume from their synchronized states. Some applications with very tight timing requirements (high-speed servo systems, for example) are not suitable for standard controller redundancy architectures — verify the process's tolerance for switchover events before committing to a redundancy design.

FAQ

Q: How many 1756-RMC1 modules are needed per redundant pair?

Two — one in each chassis of the redundant pair. They connect to each other via the fiber optic cable. A single 1756-RMC1 without its pair serves no function.

Q: Can the secondary controller be in a different physical location from the primary?

Yes, within the fiber optic cable length limit. Standard multimode fiber supports up to 200 m between the two chassis, making it practical to locate the secondary in a different panel, room, or building section — providing some protection against localized physical events (panel fire, flooding) affecting both controllers simultaneously.

Q: What happens if the redundancy link itself fails?

A redundancy link failure is detected and reported. The primary continues controlling the process; the secondary goes into a desynchronized state where it cannot receive state updates. The system continues operating but without redundancy protection until the link is restored and the secondary is re-synchronized. A link failure should be treated as an alarm condition requiring immediate investigation.

Q: Does redundancy affect the controller's scan time?

Yes, modestly. The synchronization data transfer over the redundancy link adds some overhead to the scan cycle compared to a non-redundant system. The actual impact depends on the amount of data being synchronized and the scan rate. For most process control applications, this overhead is a small fraction of the total scan time and doesn't affect application performance.

1756-RMC1 New Original Seal 1756RMC1 Optic Cable