1784-U2DHP: why RSWho stays empty, and the two settings it only reads at power-up

Most 1784-U2DHP problems I end up looking at are not a dead cable. Somebody moved the rate switch with the USB still plugged in, or changed the station number in RSLinx and clicked OK, and the cable carried on with the old values. It doesn't re-read either one until it loses power. So the DH+ light blinks, RSWho shows an empty driver, and a good adapter gets the blame.

Below: what the cable reads and when, which RSLinx driver it wants, what its three lights are saying, and how else to get onto DH+ when there isn't one in the truck.

What the 1784-U2DHP connects to

It's a small box with two leads. On one side, 0.61 m (2 ft) of USB. The other side carries 2.44 m (8 ft) of DH+ cable ending in an 8-pin mini-DIN plug. It took over from the 1784-PCMK card and 1784-PCM6 cable once laptops stopped coming with PCMCIA slots, and the PCM6 was discontinued in July 2017. The cable itself is still in Allen-Bradley's active lineup.

Processor or device	Plugs in directly?	Note
PLC-5, all except Classic	Yes	8-pin mini-DIN DH+ port
Classic PLC-5: 5/10, 5/12, 5/15, 5/25	No	Needs a third-party adapter
SLC 5/04 (1747-L541, -L542, -L543)	Yes	Channel 1, DH+
SLC 5/03, SLC 5/05	No	Neither one has a DH+ port
Node with only a 3-pin Phoenix DH+ terminal	Not at that node	Plug into any 8-pin mini-DIN port on the same link and reach it over DH+

 

The USB side can't run longer than 3.0 m (9.84 ft), so a long extension back to a desk is out and the laptop goes to the panel. Which SLC 5/04 port is which, and the cable mix-ups around them, I went through in the 1747-L541 port and cable article.

Two settings the 1784-U2DHP only reads at power-up

Everything else on this cable you can change on the fly. Not these two.

The rate switch

DH+ runs at 57.6, 115.2 or 230.4 Kbps, and every node on one link has to run the same rate. The cable matches the network, never the other way around. Rate also caps how long that network can be.

DH+ rate	Max network cable length
57.6 Kbps	3,048 m (10,000 ft)
115.2 Kbps	1,524 m (5,000 ft)
230.4 Kbps	762 m (2,500 ft)

 

The switch is the little slider on the side of the box, labeled DH+ baud rate. The cable reads it once, when USB power comes on, and never again. Slide it with the cable plugged in and the 1784-U2DHP keeps running at the old rate. The light keeps blinking, the switch looks right, and you can stare at it for an hour.

To change it: stop the driver in RSLinx, unplug the USB, move the switch, plug back in, start the driver.

Station number

Default is 0, range 0 to 77 octal. Octal, so 8 and 9 never show up: 10 is the address after 7, and 18 isn't a valid address at all.

The 1784-U2DHP doesn't store a station number of its own. It takes one from the first packet RSLinx sends through it after power-up and holds it until power goes away. Edit the number in the driver configuration all you like; until the next power-up the cable answers on the old one.

Say the DH+ light goes blinking red as soon as you start the driver. Duplicate node: something on the link already owns 0. You stop the driver, type 17, start it again, and it's still red. Of course it is, nothing told the cable to drop its address. The order that works:

1.    In RSLinx, go to Communications > Configure Drivers, select the driver, click Stop.

2.    Click Configure, enter the new station number, click OK.

3.    Unplug the USB end and plug it back in.

4.    Click Start.

5.    Open RSWho and browse the driver. That traffic is what sets the new address.

And stop the driver before you pull the USB, every time, not only here. Unplug with the driver running and RSLinx can shut down or stop responding, and now you're restarting the laptop too.

What the 1784-U2DHP reads at power-up, and the order for changing the rate or the station number.

 

Which RSLinx driver the 1784-U2DHP needs

That depends on the RSLinx Classic version. Only the newer driver has the cable's own name in it.

RSLinx Classic	Driver type	Settings
2.53 and later	1784-U2DHP for DH+ devices	Comm Port; Station Number, 0-77 octal
2.52 and earlier	RS-232 DF1 Devices	Comm Port; Device 1770-KF2/1785-KE/SCANport; Station Number; Error Checking CRC; Protocol Full Duplex. Baud rate, parity and stop bits are not used

 

From 2.53 on, the Windows driver for the cable installs together with RSLinx, so the cable should just show up under Ports in Device Manager. On an older setup you load the 1784_u2dhp.inf file by hand. If Windows throws code 10, pull the cable and reconnect it.

Each 1784-U2DHP gets its own COM port, and a different cable gets a new one. That bites the day you borrow a coworker's cable: Windows calls it COM10, your driver still points at COM9, and RSLinx has nothing to open. Check the COM number in Device Manager and fix the Comm Port field, or set up one driver per cable.

Windows versions are the open question. I've never seen a supported Windows list for the cable's USB driver, so I plug it into the exact laptop I'm taking to site a day before the outage, not during it. If you're not even sure which RSLinx is on that laptop, this RSLinx Classic vs FactoryTalk Linx comparison sorts it out.

Firmware is worth a glance too. The current one is V1.008, out since 2014. Before it, the cable could occasionally make RSLinx retry a packet and deliver a duplicate message. ControlFLASH loads the update.

What the three lights on the 1784-U2DHP mean

Top of the box, left to right: USB, PWR, DH+. The first two are rarely the problem.

Light	State	Meaning	What I check
USB	Solid green	Configured, no traffic yet	Nothing, that's normal before RSLinx talks
USB	Blinking green	Traffic	Nothing
USB	Off	Can't transfer data: not enumerated, suspended or disconnected	Device Manager, replug, code 10
PWR	Solid green	Powered	Nothing
PWR	Off	No power from the USB port	Another USB port
DH+	Off	No station number yet	Start the driver, open RSWho
DH+	Blinking green	Not on the DH+ network: wrong rate, or a break or short in the wiring	Rate first (replug after changing it), then the plug and the cable
DH+	Solid green	Communicating on DH+	Nothing on the cable side, look at the other nodes
DH+	Blinking red	Duplicate node	New station number, then replug
DH+	Solid red	Internal diagnostic test failed	Replug once; if it comes back red, it's the cable

 

Blinking green is the one that eats an afternoon, because it covers two unrelated causes. I always check rate first. It costs nothing, and it's the cause that hides: if the switch got moved after power-up, it looks right and isn't. Replug and watch the light. Only after that do I go after the 8-pin plug (look for a bent pin) and the cable run itself.

No 1784-U2DHP on hand? Other ways onto DH+

Route	What it takes	The catch
SLC 5/04 DF1 pass-through	1747-CP3 serial cable to channel 0 of a 5/04 on the link; RSLinx RS-232 DF1 Devices driver, device 1770-KF2/1785-KE, settings matching channel 0	Needs a 5/04 with OS401; S:34/5 has to be set in that processor; you only reach the DH+ link that 5/04 sits on
1756-DHRIO in a ControlLogix chassis	Ethernet module in the same chassis, DHRIO channel set to DH+; RSLinx browses Ethernet, backplane, DHRIO, DH+	Discontinued December 31, 2024, so surplus only
1784-PCMK card and 1784-PCM6 cable	A laptop with a PCMCIA slot	PCM6 discontinued in 2017, and finding the laptop is the hard part
Third-party USB-to-DH+ adapter	Its own driver and its own setup	Doesn't install the way the genuine cable does

 

Third-party adapters are their own animal. The ANC-120e, for one, installs as a network adapter with its own IP address rather than a COM port, lands on DH+ through a 3-pin Phoenix connector, and RSLinx reaches it through an Ethernet-type driver. None of the COM port and station number steps above apply to it. Whatever adapter you pick, try it on a spare processor before it goes anywhere near a running line.

Four ways from a laptop onto DH+, and the RSLinx driver each one uses.

 

Questions I get about the 1784-U2DHP

Is the 1784-U2DHP discontinued?

No. It's an active Allen-Bradley product, Series A, firmware V1.008. What went away is the PCMCIA pair it replaced. The 1784-PCM6 cable went in July 2017.

Will the 1784-U2DHP work with a PLC-5/15?

Not plugged straight into that processor. Classic PLC-5s (5/10, 5/12, 5/15, 5/25) need a third-party adapter. Every other PLC-5 and the SLC 5/04 take the 8-pin plug directly.

Can I use two 1784-U2DHP cables on one laptop?

Yes. Each cable shows up as its own COM port. Two drivers in RSLinx, one per COM port, is the tidier way to run it.

Does it work on Windows 11?

There's no official Windows list for the cable's driver that I know of. RSLinx Classic 2.53 and later brings the driver with it, and whether Windows 11 takes it on your particular laptop is something to find out at your desk, not at the panel.

Before you order a 1784-U2DHP

Run the checks above first: the rate switch, a replug, the station number, the COM port. Plenty of "dead" cables are back online after that. If yours really is gone, or the second laptop needs its own, we keep the 1784-U2DHP in stock as new, never-used surplus with IQElectro's 2-year warranty. For the Ethernet route there's the 1756-DHRIO. A spare SLC 5/04, a 1747-L541 or 1747-L542, makes a good test bench for any DH+ adapter before it touches production. And if what you really need is a permanent EtherNet/IP link into a DH+ network, the ProSoft AN-X4-AB-DHRIO gateway is discontinued but still on our shelf.