1756-DHRIO status codes: display messages and LEDs for DH+ and Remote I/O, MSG error D0 hex and fixes

A 1756-DHRIO or 1756-DHRIOXT module shows its state on the alphanumeric display on its front and on the CHA, CHB and OK indicators (1756-UM514C-EN-P, p. 154). In normal operation the display keeps cycling through each channel's network type, DH+ node address and status. Status messages such as DUPL NODE, CNFG FALT or MUTE LINK name the problem on that channel; OFF_ LINE on a Remote I/O channel can be normal (p. 155). The full tables are in chapter 13, "Troubleshooting the Module" (pp. 153–156). MSG errors and Remote I/O connection faults show up in the controller. Rockwell lists the 1756-DHRIO as discontinued; see its end-of-life page.

Reading the display

At power-up the display shows, for channel A and then channel B: the channel and its network (DH for DH+, IO for Remote I/O), the node address for a DH+ channel or SCAN for a Remote I/O scanner, then the channel status. The cycle repeats while the module runs normally (pp. 153–154). Example: channel A on DH+ at node 14 and channel B on Remote I/O, both healthy, give A DH, A#14, A OK, B IO, SCAN, B OK. If the display never starts cycling, or stops, use the tables below (p. 154).

DH+ node addresses are set on switches and shown in octal, 00 to 77 (p. 17). The network type switch sets each channel: 0 = DH+ 57.6K, 1 = RIO scanner, 2 = DH+ 115K, and on channel A only 3 = DH+ 230K, which disables channel B (p. 17). The programming terminal connector shares the link of channel A and works only when channel A runs DH+ (p. 23).

DH+ channel status messages

Source: p. 154.

How a configuration fault arises: on insertion, the module compares the configuration in its non-volatile memory with the slot and serial number of the chassis, and any mismatch gives a configuration fault (p. 32). A wrongly programmed routing table, or a missing or wrong controller slot value for a DH+ channel, does the same (pp. 81, 90). Both are applied with RSLinx (pp. 32, 81); the controller slot defaults to 0 (p. 51).

Remote I/O channel status messages

Source: p. 155.

Network limits behind RACK OVER and MAX_ DEV_ (pp. 102–103, 113): channels A and B cannot scan the same rack address; a channel takes at most 32 rack numbers and 32 physical devices. An 82 Ω terminator allows 32 devices and 32 racks; 150 Ω limits the network to 16 and 16 and is required if it contains a 1771-AS, 1771-ASB/A, 1771-ASB/B, 1771-DCM or 1771-AF.

OK and channel indicators

A steady green channel indicator is normal. If the power supply indicator is off, turn on the power switch and check the power wiring and the fuse. Source: pp. 154–156.

MSG and block-transfer errors

These appear in the MSG instruction or the PLC that sent the message.

Application timeout. An error on a remote link reaches the sender as an application timeout, because error replies are not routed back; the sender counts an error and can retry. The manual's example is a target PLC-5 with full buffers (p. 33). A remote module with no free connection for the reply also causes it (p. 31).

Block transfers. The BT message timeout is fixed at 4.5 seconds. If the I/O module does not answer on Remote I/O, the BT response times out after 4 seconds (p. 145). A DH+ "BT Pass-Through" message needs a routing table in the module even when its channels run Remote I/O (p. 146). The manual lists no other MSG error codes.

How the controller sees a Remote I/O fault

The module and its owner controller exchange scanner status continuously; only one ControlLogix controller can own its Remote I/O channels (pp. 105–106). If that exchange stops for four times the RPI, the controller drops the scanner configuration, the channel goes offline and waits for new configuration. The controller takes a major fault only if "Major Fault on Controller if Connection Fails in Run Mode" is set; otherwise it keeps trying to reconnect (pp. 110, 116).

An adapter fault is a connection timeout at four times the RPI: with a 25 ms RPI the controller learns of it after 100 ms. Studio 5000 shows a nonzero Rack Status tag, a fault icon in the controller organizer, or the fault type on the connections page (p. 111).

Checks, step by step

1. Before any restart, record the display cycle of both channels, the OK, CHA and CHB states, the switch settings, any MSG error and the Rack Status tag.
2. DH+ channel (DUPL NODE, ONLY NODE, flashing channel indicator): node address and cables (pp. 154, 156).
3. CNFG FALT: the slot (p. 154). If the module or chassis was changed, the stored slot and serial number no longer match (p. 32); check the routing table and controller slot in RSLinx (pp. 32, 81, 90).
4. Remote I/O channel (flashing green or red): power at the other racks, then the cables (p. 156).
5. D0 hex: MSG fields, controller slot, routing tables on the path (pp. 27, 31).

Restarting safely

Removing or inserting the module under power can draw an arc: in a hazardous location, remove power or confirm the area is nonhazardous first (p. 19). Restarting the module or clearing a Remote I/O fault can bring outputs on remote racks back on: follow the plant's permit and lockout rules.

When it is hardware

A steady red OK indicator that returns after a restart, or a channel indicator still steady red after a restart: replace the module (pp. 155–156). Not a fault: with a 1784-KT or 1784-PCMK card on the DH+ link, the module can get the error message "Received Frame with Bad CRC". It does not affect DH+ operation; proper termination keeps it to a minimum (p. 156).

Planning the replacement

The 1756-DHRIO end-of-life page records the module as discontinued on 31/12/2024, with no replacement catalog number named by Rockwell; the migration direction is moving DH+ and Remote I/O to EtherNet/IP. See PLC-5 to ControlLogix migration and the PLC-5 and SLC 500 migration service. For an engineer on site: breakdown support.