ControlNet 1756-CN2/CNB faults: status display messages, channel A/B indicators, keeper signature, NUT, UMAX

On a ControlNet network built with 1756-CN2, 1756-CN2R, 1756-CNB or 1756-CNBR modules, the first reading is on the module itself: the OK indicator, the status display and the channel A and B indicators. The installation instructions list every display message and indicator state: 1756-IN074C-EN-P for the 1756-CN2 family and 1756-IN066C-EN-P for the 1756-CNB family (pp. 8–9 in both). The controller shows a lost connection as a warning symbol in the Studio 5000 Logix Designer I/O tree, with the fault code on the module's Connection tab (CNET-UM001H-EN-P, pp. 64–66). The network itself is checked in RSNetWorx for ControlNet: keeper status, keeper signature and the schedule (pp. 18–20). Most problems the user manual describes come from the configuration: a network not scheduled or not rescheduled after a change, keepers that disagree, a node address above UMAX, an RPI faster than the NUT, or too many connections. The user manual still refers to the ControlNet Modules Installation Instructions, CNET-IN005, for installing these modules (p. 10); Rockwell has replaced that publication with 1756-IN066 and 1756-IN074. Rockwell lists the 1756-CNB and 1756-CNBR as discontinued; see the 1756-CNB and 1756-CNBR end-of-life pages.

Where a ControlNet fault shows up

• I/O tree in Logix Designer. A warning symbol over the I/O Configuration folder means the controller cannot communicate with a device (p. 64). Work down the tree and take the fault at the module highest in the tree. Open its Properties, Connection tab, and read the Module Fault area. The manual sends you to Logix Designer Help (search "module fault") for the meaning of the code (pp. 65–66).
• Fault bits in the I/O tags. With a direct connection, any break in the connection, such as a module fault or a module removed under power, sets fault bits in the module's data (p. 55).
• Controller major fault. Only if "Major Fault On Controller If Connection Fails While In Run Mode" is checked; this setting covers the connection to the local communication module. If it is cleared, the controller keeps running and the manual says to monitor the connection in ladder logic (pp. 30, 34).
• RSNetWorx for ControlNet. Network menu, Keeper Status (p. 20).
• MSG instructions. If all unconnected buffers are in use when an instruction leaves the message queue, the instruction errors and transfers no data (p. 80).
• Front of the module. The OK indicator with the status display, and the channel A and B indicators (1756-IN074C, pp. 8–9; 1756-IN066C, pp. 7–9).

Status display: 1756-CN2, CN2R, CN2RXT

1756-IN074C-EN-P covers the 1756-CN2, CN2R and CN2RXT and their conformal-coated K versions (pp. 8–9). Messages marked (R) appear only in ControlLogix enhanced redundancy systems. A dash means the table gives no action.

Status display: 1756-CNB, CNBR

1756-IN066C-EN-P covers the 1756-CNB and CNBR and their K versions (pp. 8–9). Here (R) means ControlLogix redundancy systems only.

The same publication gives a power-up check (p. 7). At power-up the status indicator is red while the module runs its self-test and shows INIT, or WAIT SRM in a redundant control chassis pair; if it is not red, go straight to the table. If it stays red after the self-test, the self-test failed: read the message in the table, and the chart ends at replacing the module. If it turns to flashing green, initialization is complete; channel A and B flash alternately, then show the network state. A working module displays OK and its address, A#xx.

Channel A and B indicators

Both publications have two tables: states that indicators A and B show together, and states that A or B shows on its own (1756-IN074C, p. 9; 1756-IN066C, p. 9).

Where the two differ: only the 1756-CNB table lists flashing red/green, and it adds a duplicate node address to alternating red/off. For temporary errors (flashing green/off), the 1756-CN2 table says the module corrects them, while the 1756-CNB table sends you to check the media.

Reset to factory settings

Several keeper messages call for a reset to factory settings. Both publications give the same steps (1756-IN074C, p. 7; 1756-IN066C, p. 10). Remove chassis power, take the module out, set the address switches to 00, refit it and power up. When the display reads Reset Complete—Change Switch Settings, remove power, take the module out, set the final address, refit it and power up. Address 00 is only for this reset, never for normal operation. The steps remove power from the whole chassis: follow the plant's permit and lockout rules.

Symptoms and what the manual says

Keeper, NUT, SMAX and UMAX

The keeper stores the network parameters and configures the network with them when it starts. Every ControlNet network needs at least one. The 1756-CN2, CN2R, CNB and CNBR are keeper-capable (p. 19). Among the valid multi-keeper-capable nodes, the one with the lowest node address becomes the active keeper. If it leaves the network, a valid backup keeper can take over (pp. 19–20). In the Keeper Status dialog there must be exactly one active valid keeper, all other keepers must be valid, and the keeper signature (hex) must be the same for all nodes (p. 20). Before scheduling online, all keepers must be unconfigured or must not conflict with the current network (p. 46).

The NUT (network update time) is the fastest rate for scheduled data; the minimum is 2 ms (p. 16). The node with the lowest address sends the maintenance information that keeps the other nodes synchronized (p. 17). SMAX is the highest node address that can use scheduled time; UMAX is the highest that can communicate at all, and must be equal to or higher than SMAX (pp. 20–21). The manual recommends UMAX equal to the highest node address on the network, because the default 99 wastes bandwidth, and SMAX three or four above the highest scheduled node (p. 21). RSNetWorx sets Max Scheduled Address; Rockwell recommends not changing it (p. 45). Node addresses run from 01 to 99 (p. 19).

Cable, terminators and redundant media

A segment is a section of trunk between two terminators. Its maximum length is 1000 m − 16.3 m × (number of taps − 2); the graph assumes 1786-RG6 cable. Repeaters add segments or length, and a network supports up to 99 nodes (pp. 23–24). Only the 1786-RPFRL and 1786-RPFRXL repeaters support a ring topology (p. 22). The specifications list the 1786-XT termination resistor, the 1786-TNCLXT4 IP67 termination resistor and the 1786-CP programming cable for the RJ45 port (1756-TD003R-EN-E, p. 35).

The R versions have two ControlNet BNC ports, A and B (1756-TD003R, p. 28). In RSNetWorx the Media Redundancy parameter states whether the network uses media redundancy (CNET-UM001H, pp. 45, 48). Both installation instructions warn that a reversed tap connection gives misleading status displays: check the label on the tap drop cable for its segment before connecting. The primary and partner modules of a redundant control chassis pair must be on the same network segment (1756-IN074C, p. 6; 1756-IN066C, p. 6). The channel indicator table above gives the checks for each channel state.

Module facts

Source: 1756-TD003R, pp. 27–28. All four run at 5 Mbps with up to 99 nodes. The USB port is for temporary local programming only; do not use it in hazardous locations (p. 28). The user manual adds: a 1756-CN2 or CN2R series B in the local chassis has 131 connections, 3 of them reserved for redundant control; a 1756-CNB or CNBR has 64, and Rockwell recommends no more than 40⁠…⁠48 scheduled connections (CNET-UM001H, p. 69). Each remote ControlNet module is limited to five rack-optimized connections (p. 56). An enhanced redundancy system needs a 1756-CNB, CNBR, CN2 series B or CN2R series B; series A of the CN2 and CN2R does not support it (p. 12).

Checks, step by step

1. Before changing anything, record the OK indicator, the display message and the channel A and B indicators of each ControlNet module, the faulted modules in the I/O tree, the Module Fault text of the highest one, the Keeper Status window, the network parameters (NUT, SMAX, UMAX, media redundancy) and the node addresses.
2. Display: look up the message in the table for the module family and do the action it gives (1756-IN074C, pp. 8–9; 1756-IN066C, pp. 8–9).
3. I/O tree: start from the faulted module highest in the tree and look up its code in Logix Designer Help (CNET-UM001H, pp. 65–66).
4. Keepers: one active valid keeper, all others valid, one keeper signature (CNET-UM001H, p. 20). On a 1756-CN2, a "Keeper:" message on the display says which keeper data is wrong (1756-IN074C, p. 9).
5. Node address: every node at or below UMAX (CNET-UM001H, pp. 45, 48). No two modules on the same address; 01⁠…⁠99 in a standalone chassis, 01⁠…⁠98 in a redundant chassis, where the secondary module takes the same address as its primary (1756-IN074C, p. 5; 1756-IN066C, p. 5).
6. Schedule: any connection added, removed or changed since the last schedule means rescheduling (CNET-UM001H, pp. 19, 50).
7. Media: a channel flashing red/off means checking for broken cables, loose connectors and missing terminators (1756-IN074C, p. 9; 1756-IN066C, p. 9). If the network was extended or repeaters were added, check the media configuration (CNET-UM001H, pp. 45–46).
8. Connections and RPI: RPI not faster than the NUT, connection counts within the module limits (CNET-UM001H, pp. 18, 69). If a 1756-CNB shows BW >MAX frequently, add another module and split the traffic between them (1756-IN066C, p. 8).

Rescheduling safely

The user manual's download procedure puts the controller in Program mode first (CNET-UM001H, p. 35). When saving a schedule, "Optimize and rewrite the schedule for all connections" is the preferred choice; with "Merge changes into existing schedule", controllers whose connections have not changed stay in Run mode rather than changing to Program mode (p. 50). Changing electronic keying online interrupts connections to the device and to devices connected through it, including connections from other controllers (p. 37). Rescheduling can move controllers to Program mode, and their I/O with them: follow the plant's permit and lockout rules.

Planning the replacement

The 1756-CNB end-of-life page and the 1756-CNBR end-of-life page record both modules as discontinued on 31/12/2024, with 1756-CN2 and 1756-CN2R named by Rockwell as replacements. The same pages note that Rockwell's lifecycle tool now lists the 1756-CN2 and 1756-CN2R as End of Life, with a discontinued date of 31/12/2028 and 1756-EN4TR as the Engineering Replacement (read 14/11/2026). The specifications call ControlNet a legacy network and point to the ControlNet to EtherNet/IP Migration Reference Manual, CNET-RM001 (1756-TD003R, p. 27). The user manual gives one compatible-keying example: a 1756-CN2R can replace a 1756-CNBR; release notes give the details (CNET-UM001H, p. 38). See DeviceNet to EtherNet/IP migration and the DeviceNet and ControlNet to EtherNet/IP migration service. For an engineer on site: breakdown support.