DeviceNet scanner codes 72, 77, 78, 91: 1756-DNB, 1769-SDN

Every 1756-DNB status code in 1756-IN566D (65–99, including 72, 78, 91, 92) with its action, where DNET-UM004E differs on codes 65, 77 and 78, the MOD/NET, I/O and OK indicators and the Logix status tags.

A DeviceNet scanner links a Logix controller to the devices on a DeviceNet network: the 1756-DNB in a ControlLogix chassis, the 1769-SDN in a CompactLogix system (DNET-UM004E-EN-P, p. 17). The 1756-DNB shows its state on a four-character display. When a node has a problem, the display alternates the scanner's address and mode (A#01, RUN) with the problem node's address and the error code, for example N#33 and E#72 (1756-IN566D-EN-P, p. 27). The codes come from the status code table in the 1756-DNB installation instructions, 1756-IN566D (Series C and D): 0–63 is the scanner's own node address, and 65–99 are status and error codes (pp. 29–35). The controller gets the same information through the scanner's status tags (DNET-UM004E, p. 88). The configuration manual DNET-UM004E names only three codes, 65, 77 and 78, and for these the two publications do not say the same thing (see below).

What the display tells you first

Scanner	Display	Normal reading in Run mode
ControlLogix 1756-DNB	Four characters	RUN (DNET-UM004E, p. 82); 1756-IN566D shows it alternating with the node address, A#01 / RUN (p. 27)
CompactLogix 1769-SDN	Two characters	The scanner's own node number (DNET-UM004E, p. 82)

At power-up the 1756-DNB display runs through TEST, PASS, the firmware revision (Rev xx.xxx) and the node address (A#xx). The OK indicator is red during the self-test, then green (1756-IN566D, p. 24). The display flashes each code at about one-second intervals and keeps toggling until the error is corrected (p. 27).

The 1756-DNB can also show these messages (p. 28):

Message	Meaning
Run	Scanner in Run mode
Idle	Scanner in Idle mode
Auto	AutoScan enabled, scanner in Idle mode
Flash In Progress	ControlFlash is transferring a flash image to the scanner
Duplicate Node Failure	The scanner's node address is already used by another device on the network
Bus Off Detected	The scanner detected errors on the network and was taken offline
No Network Power	No DeviceNet network power reaches the scanner
NoRX	The scanner does not contact a scan list, or has received nothing from any other device
NoTX	The scanner failed to transmit a message
Reset Complete - Change Switch Settings	Factory defaults restored: set the data rate and node address switches, then restore power

1756-DNB status codes (1756-IN566D)

Every code in the table of 1756-IN566D, with its meaning and the recommended action. The table has no codes 64, 66, 93 or 94. It covers the 1756-DNB; the publication does not cover the 1769-SDN.

Code	Meaning	Recommended action	p.
0–63	The scanner's DeviceNet node address	None	29
65	AutoScan is on and the device is in Idle mode	None	29
67	The scanner is the secondary scanner	None	29
68	The primary scanner detects no secondary scanner	Configure another scanner as the secondary	29
69	Primary and secondary configurations do not match	Check the secondary scanner's configuration	29
70	The device's address is already used by another device; the scanner failed the duplicate node address check	Change the device's address to an unused one	29
71	Invalid data in the scan list	Reconfigure the scan list in RSNetWorx for DeviceNet	29
72	A slave device stopped communicating; if it does not recover during the next scan, the code changes to 78	Check the device's power and communication connections; for a polled device, check that the interscan delay leaves the device time to return data	29
73	The device's identity does not match the electronic key in the scanner	Make sure the right device is at this address and that it matches the key (vendor, product code, product type)	29
74	Data overrun on the DeviceNet port	Modify the configuration and check for invalid data; check network traffic	30
75	The device has no scan list, or has received nothing from any other device	Check that the device has a configured scan list and a properly wired connection to the network	30
76	No traffic directed to the scanner: it hears other traffic on the network but none for itself	None	30
77	During initialization, the data size the device expects does not match the scan list entry	Check the slave device and the scan list in RSNetWorx for the correct input and output sizes	30
78	The device is in the scan list but not communicating: it failed during the scanner's second scan, after code 72	Check power and communication connections; for a polled device, make the interscan delay long enough; if needed, in RSNetWorx add the device to the network, delete it from the scan list or inhibit it	31
79	The scanner failed to transmit a message, usually right after the duplicate node check at power-up	Make sure the scanner is on a valid network; check for disconnected cables; check the communication rate	31
80	The scanner is in Idle mode	Put the controller in Run or Remote Run (keyswitch or RSLogix 5000), then turn on `O.CommandRegister.Run`	31
81	The controller put the scanner in faulted mode; this also shows a network fault state	`O.CommandRegister.Fault` is on: correct the condition that made the controller set it, then turn it off	31
82	Error in the sequence of fragmented I/O messages from the device	In RSNetWorx, check the device's input and output sizes in the scan list and the device configuration	32
83	The device returns error responses to the scanner	In RSNetWorx, check the scan list and the device configuration (the device may be in another scanner's scan list); check in the device's documentation that it supports the scanner's message type, and change the type in the scan list if not; cycle power to the device	32
84	The scanner is initializing the network	None; clears once the scanner has tried to initialize all devices	32
85	At runtime, the data size the slave sends does not match the scan list entry	Variable-length poll data is not supported: check that the slave works properly	33
86	The device is in Idle mode or not producing data while the scanner is in Run	Check the device's configuration and status; with an interlock between two scanners (controllers), make sure both are in Run	33
87	Shared inputs cannot be read: the owning scanner has no communication with the slave	Check the primary scanner's connection and configuration; check that the slave produces data	33
88	Shared inputs cannot be read: I/O parameters (polled or strobed, electronic key, data size) differ from the owning scanner	In this scanner, set the shared-input scan list entry to match the owning scanner	33
89	The scanner failed to configure a device with the Automatic Device Recovery (ADR) parameters	Make sure a compatible device is installed; if the offline configuration differs from the device's online configuration, change the offline one to match	33
90	The controller put the scanner in Disabled mode	If wanted, turn off `O.CommandRegister.DisableNetwork`	33
91	Bus-off, likely from cable or signal errors	Cycle power to the device; check that all devices use the same rate; check for shorts between the CAN wires (blue, white) and power or shield (black, red, shield); look for noise: a device near high-voltage power cable, wrong or missing terminating resistor, poor grounding, a device sending noise or bad data	34
92	The DeviceNet cable does not supply power to the device's communication port	Check the network's 24V DC power supply, the cable condition and the cable connections to the device	34
95	Device firmware update or configuration download in progress	None; do not disconnect the device during the update, or the data in its memory is lost	34
96	Communication port in test mode	None	34
97	The controller put the scanner in halt mode	If `O.CommandRegister.HaltScanner` is on, turn it off, then cycle power to the scanner	35
98	General firmware error	Replace the device	35
99	System failure	Replace the device	35

Where DNET-UM004E and 1756-IN566D differ

Code	DNET-UM004E-EN-P (03/2022)	1756-IN566D-EN-P (06/2008)
65	AutoScan Active, a scanner-only status: with AutoScan enabled and the scanner in Run mode, the display alternates 65 and the scanner's node address (p. 81)	AutoScan on and the device in Idle mode, no action (p. 29); the "Auto" message also means AutoScan with the scanner in Idle (p. 28)
77	Shown on the 1769-SDN for a device's node when, with AutoScan, the device produces more data than the per-node I/O allocation size: the scanner cuts the produced size to the allocation size, makes the I/O connection and reports the error. Raise the allocation size to fit the largest device (p. 68); change it only in Idle with AutoScan disabled (p. 79)	During initialization the device's expected data size does not match the scan list entry: check the input and output sizes in RSNetWorx (p. 30)
78	Communication with a device that sends or receives large amounts of data, such as a PanelView terminal, on polled I/O stops from time to time: increase the interscan delay (p. 135)	The device in the scan list has stopped communicating, after code 72: check power and connections, interscan delay if polled, add, delete or inhibit the device (p. 31)

DNET-UM004E also gives an AutoScan case without a code: a device that consumes more data than the allocation size is rejected and left out of the scan list (p. 68). It points to DNET-IN001 for installing and setting up the 1756-DNB and 1769-SDN (p. 12), and to DNET-UM072 for designing and troubleshooting the DeviceNet cable system (p. 13).

Status indicators on the 1756-DNB

Indicator	State	Meaning (1756-IN566D, pp. 35–37)
MOD/NET	Off	Not powered or not online; the duplicate MAC ID test is not finished yet
MOD/NET	Green	Online with connections established
MOD/NET	Flashing green	Online, no connections established; or configuration missing, incomplete or incorrect
MOD/NET	Flashing red	Minor fault or connection time-out, recoverable; one or more I/O connections timed out
MOD/NET	Red	Critical fault, may need replacing; or the device detected a duplicate MAC ID or bus-off and cannot communicate
I/O	Off	Scanner not online: check network power
I/O	Green	Run mode, outputs under control, inputs being consumed
I/O	Flashing green	Idle mode, outputs not under control, inputs being consumed
OK	Off	No power: apply chassis power, check that the module is fully inserted
OK	Green	Operating normally, at least one controller connection
OK	Flashing green	Operating, but no controller is controlling it: check the scanner in the controller's I/O configuration
OK	Red	Unrecoverable fault (repair or replace), or self-test at power-up

For a device on the network, DNET-UM004E says a solid red network status indicator typically means an address conflict or a baud rate problem (p. 73).

How the controller sees the scanner

When the scanner is added to the controller's I/O configuration, Logix Designer creates tags for the input, output and status data of the network. The tag name has the form location:type.Data[element].bit, where type is I for input from a device, O for output to a device and S for network status. For a 1756-DNB in the local chassis and for a 1769-SDN, location is Local:slot_number_of_scanner; for a 1756-DNB in a remote chassis it is the name of the remote bridge module followed by the slot (DNET-UM004E, pp. 90–91).

The status structure depends on the status size set for the scanner (p. 88):

Status size (DINTs)	Members	What they hold
10	ScanCounter	Count of I/O scans
10	DeviceFailureRegister, SINT[8]	One bit per address 0–63; bit = 1: the device at that address has failed
10	AutoverifyFailureRegister, SINT[8]	Bit = 1: the device's data size does not match the memory allocated for it in the scanner
10	DeviceIdleRegister, ActiveNodeRegister, SINT[8]	Bit = 1: the device is idle / the device is online
10	StatusDisplay, SINT[4]	ASCII copy of the scanner status display
11	ScannerAddress, ScannerStatus	Scanner address and scanner status code
11	ScrollingDeviceAddress, ScrollingDeviceStatus	Scroll through the addresses with errors and their status codes, once per second; 0 = no faults (1756-IN566D, p. 42)
24 / 32	DeviceStatus, SINT[32] / SINT[64]	Status code of each of the lower 32 devices / of all devices, one byte per device

The default configuration gives the full status data. If the status size was reduced to save EtherNet/IP or ControlNet bandwidth, only the members for that size exist (pp. 87–88).

The 1756-DNB input structure starts with a 32-bit module status register (1756-IN566D, pp. 40–41):

Bit	Name	1 means
0	Run	Run mode (0 = Idle)
1	Fault	Network is faulted
2	DisableNetwork	Network is disabled
3	DeviceFailure	A device failure exists: look in the status structure
4	AutoverifyFailure	At least one device failed to be initialized by the scanner
5	CommFailure	A communication failure exists
6	DupNodeFail	Failure from a duplicate node address
7	DnetPowerDetect	DeviceNet power failure
8	NetworkWarning	Many receive or transmit errors: check the integrity of the network

When a device stops communicating, its input tag keeps its last value. DNET-UM004E recommends buffering the input data and checking the device failure register: use the input only when the device's failure bit is 0 (pp. 92–93).

The controller runs the scanner through the CommandRegister bits of its output structure (DNET-UM004E, p. 82, p. 94; 1756-IN566D, p. 39):

Bit	Set to 1	Set to 0
`⁠…⁠O.CommandRegister.Run`	Run the network	Idle mode
`⁠…⁠O.CommandRegister.Fault`	Fault the network	Not fault the network
`⁠…⁠O.CommandRegister.DisableNetwork`	Disable the network	Enable the network
`⁠…⁠O.CommandRegister.HaltScanner`	Halt the scanner, all operation stops	Unhalt
`⁠…⁠O.CommandRegister.Reset`	Reset the scanner	Resume after the reset

To run the network, put the controller in Run or Remote Run mode and set the Run bit to 1 (DNET-UM004E, p. 82). A scanner halted by the HaltScanner bit restarts only after its power is physically cycled (1756-IN566D, p. 39). Only one Logix controller at a time can send outputs to a 1756-DNB (p. 38).

Common causes the manuals name

Duplicate node address. DeviceNet devices ship set to address 63. Adding more than one new device to an online network before giving each its address causes duplicate address conflicts (DNET-UM004E, pp. 28, 31). The manual recommends 0 for the scanner, 1–61 for devices, 62 for a computer interface and 63 left open; Auto-Address Recovery also needs 63 open (p. 20). On the 1756-DNB a duplicate shows as code 70 or the message "Duplicate Node Failure" (1756-IN566D, pp. 28–29).
Hardware address differs from the configuration file. An address set by switch takes precedence over the RSNetWorx for DeviceNet file. The manual's example is a 1756-DNB set to 2 by hardware while the file says 0: change the file to 2 (DNET-UM004E, pp. 36–37). After changing an address by switch, cycle power to the device (p. 29).
Mixed baud rates. All devices must use the same rate; mixed rates cause communication errors (DNET-UM004E, p. 20). The 1756-DNB ships at 125 Kbps and node address 63 (1756-IN566D, p. 9). Do not change its rate on an active network; a new rate takes effect only after a power cycle of the scanner (p. 10).
Device data larger than the space the scanner gives it. AutoScan allocates 4 bytes per node by default; a device such as an E3 overload relay that sends more needs a larger allocation (DNET-UM004E, p. 69). In the manual's ControlLogix example the 1756-DNB input table holds at most 124 DINTs and the output table 123 DINTs; devices outside the scanner's I/O image are rejected (p. 80).
Polled timing. Polled and strobed connections time out after 300 ms by default (expected packet rate 75 × 4 ms); a background poll ratio other than 1 may need a longer timeout (DNET-UM004E, p. 138). Keep the interscan delay, default 10 ms, at 5 ms or more (p. 139).

Checks, step by step

Record before any reset: the display text (A#, N# and E# values or the message), the three indicators, the controller mode, and the values of ScannerStatus, ScrollingDeviceAddress, ScrollingDeviceStatus and DeviceFailureRegister.
Find the node: N#xx on the display (1756-IN566D, p. 27), the bit set in DeviceFailureRegister or the address in ScrollingDeviceAddress (DNET-UM004E, p. 88).
Look up the E# code in the table above and do only the action given for that code.
Browse the network in RSLinx: every device should appear. No network at all: check the computer connection; some devices missing: check their connection (DNET-UM004E, pp. 50–51).
Code 70: change the device's address to an unused one (1756-IN566D, p. 29).
Codes 72 and 78: check the device's power and communication connections; for a polled device, check the interscan delay, keeping it at 5 ms or more (1756-IN566D, pp. 29, 31; DNET-UM004E, pp. 135, 139).
Codes 73, 77, 82: compare the device (electronic key, input and output sizes) with its scan list entry in RSNetWorx (1756-IN566D, pp. 29–32); for 77 with AutoScan, also compare the data size with the I/O allocation size (DNET-UM004E, p. 68).
Code 91: cycle power to the device; check the same communication rate on all devices, no shorts between CAN and power or shield wires, correct terminating resistors, no device near high-voltage power cable, correct grounding (1756-IN566D, p. 34).
Code 92: the 24V DC network power supply, the cable condition and the cable connections to the device (p. 34).
Codes 80, 81, 90, 97: each points to the controller and a CommandRegister bit. For 80, put the controller in Run or Remote Run and turn on the Run bit; for 81, correct the condition that made the controller set the Fault bit, then turn it off; for 90, turn off DisableNetwork if wanted; for 97, turn off HaltScanner, then cycle power to the scanner (pp. 31, 33, 35).

Changing the scanner safely

Download a new configuration only with the scanner in Idle mode: put the controller in Program or Remote Program mode, or turn off the Run bit (DNET-UM004E, p. 48). Turning AutoScan on clears the scanner's existing configuration and can change the tag addresses of the devices (pp. 70, 75). Editing the scanner in RSNetWorx turns AutoScan off; turning it back on clears the configuration just entered (p. 84). Enable Auto-Address Recovery on only one scanner per network (p. 146). Setting the 1756-DNB switches to 888 at power-up restores factory defaults and clears the scan list, including the ADR recovery files; do not use 888 in normal operation (1756-IN566D, p. 15). Switching the scanner between Run and Idle, resetting it or downloading a scan list can stop or restart machines on that network: follow the plant's permit and lockout rules.

When it is hardware

1756-IN566D gives "Replace device" as the action for codes 98 (general firmware error) and 99 (system failure) (p. 35). A red OK indicator means an unrecoverable fault, unless the module is in its power-up self-test (p. 37). A solid red MOD/NET indicator can be a critical fault that may need replacement, or a duplicate MAC ID or bus-off; check the code on the display before deciding (p. 37).

Planning the replacement

Rockwell lists the 1769-SDN as discontinued from 31 December 2025 and names the 1788-EN2DNR as its replacement. The ways to move a DeviceNet system to EtherNet/IP are in DeviceNet to EtherNet/IP migration. For an engineer on site: breakdown support.

Frequently asked questions

What does 65 mean on a DeviceNet scanner display?

DNET-UM004E calls code 65 AutoScan Active, a scanner-only status shown alternating with the scanner's node address when AutoScan is enabled and the scanner is in Run mode (p. 81). The 1756-DNB installation instructions, 1756-IN566D, describe 65 as AutoScan on with the device in Idle mode; neither needs any action (p. 29).

What is error 77 on a DeviceNet scanner?

In DNET-UM004E, code 77 appears on the 1769-SDN with AutoScan when a device produces more data than the per-node I/O allocation size; raise the allocation size to fit the largest device (p. 68). For the 1756-DNB, 1756-IN566D says the data size the device expects does not match the scan list entry; check the input and output sizes in RSNetWorx (p. 30).

What do status codes 72 and 78 mean on a 1756-DNB?

Code 72 means a slave device stopped communicating; if it does not recover during the next scan, the code changes to 78, a device in the scan list that is not communicating. For both, check the device's power and communication connections and, for a polled device, the interscan delay (1756-IN566D, pp. 29, 31).

What does code 91 mean on a DeviceNet scanner?

Code 91 is bus-off, likely from cable or signal errors. 1756-IN566D lists the checks: cycle power to the device, the same communication rate on all devices, no shorts between the CAN wires and power or shield, and noise sources such as missing terminating resistors or poor grounding (p. 34).

How does the Logix program know that a DeviceNet device has failed?

The input tag of a failed device keeps its last value. Check the DeviceFailureRegister bit for the device's address in the scanner status tags, and use the input only when that bit is 0 (DNET-UM004E, pp. 88, 92–93).

Sources

Rockwell Automation, DNET-UM004E-EN-P, DeviceNet Network Configuration User Manual, 03/2022 — pp. 12–13, 17, 19–20, 28–31, 36–37, 48, 50–51, 68–84, 87–94, 135–139, 146
Rockwell Automation, 1756-IN566D-EN-P, ControlLogix DeviceNet Scanner Module Installation Instructions, catalog numbers 1756-DNB, Series C and D, 06/2008 — pp. 9–10, 15, 24, 27–42

Read 15 November 2026. This article paraphrases the sources and does not quote them; read the original documents before designing or working on a system. Links open the latest edition in Rockwell's literature library; page numbers refer to the edition cited.