optilex 390 bios A14 with corsair k55 keyboard

corsair k55 REQUIRES USB3 port with 900ma available.

USB2 ports only supply 500ma max.

You will need an add in card and a power supply cable to the card.

https://www.amazon.com/Athena-Power-CABLE-S15S15M4-Female-Adapter/dp/B003K7H1AM

https://www.dell.com/en-us/work/shop/accessories/apd/a6246530

The 390 DOES NOT HAVE any USB3 ports.

If you do not have a tower you will need a low profile card.

https://www.dell.com/en-us/work/shop/2-port-usb-30-pcie-sff-internal-card/apd/a6751443/

The Optiplex sell with six different chassis types.

MT (Mini Tower)
DT (Desktop)
MFF (Micro Form Factor)
SFF (Small Form Factor)
USFF (Ultra Small Form Factor)
AIO (All-in-One)

OP,  using the manual on the K55, turn off the LEDs,   do this on any working PC.

reset is unplug it, usb, then hold ESC key and plug in Keyboard USB and it rests.

then if leds glow push brightness down on key just above printscr. key.

the FN keys also work and can change colors . or even OFF.

official manual is here.

dp

The "k55 keyboard" has many LED's inside and uses MUCH MORE THAN 100MA.  Its not a simple keyboard its backlit.

Many of the USB ports on smaller models use PASSIVE hubs so 100MA is max power available at the port.

current of 30 ma is typical PER LED and the K55 has more than 20 of them which at peak would be 600MA.

A USB Hub current is allocated in units of 100 mA up to a maximum total of 500 mA per port for ACTIVE hubs and 100MA max for PASSIVE Hubs. self-powered hubs do not supply enough power to drive a 500 mA load.

The pc displays the issue because when this is plugged in the system DOES NOT BOOT aka its drawing too much power from the USB port.

Note the many colors and LEDS

 ok, have bad k55 here from friends that failed repaired them and have a new K55.here is my report.

k55 overloads some relic PCs for excess current on USB power buss, Vbus.
rare for sure. (below is actual test data I made just for you and others at Corsair.com)

1: new keyboard, fully upgraded firmware,
the keyboard will run on any PC or laptop. (for SURE ! back lamps turned off )

with all background lamps LED's turned off by you hitting the star DIM button (justt above prtscrn) up to 3 times and is now off.
the current NOW, is tiny less than 0.010 (10mA) (this fact tells you it runs on ANY PC! made)
the worst current is DIM set to full bright, and only WHITE COLOR SET IN static colors.
White mode ====== 340mA. (120ma on max DIM) all other modes are LESS than 340..

Here are the colors. on bright only, white uses the most power.
all in mA (milliAmps) (1mA is 0.001 amp)

Purlple 260 (mA)
Blue 180
Aqua 250
green 100
yellow 180
red 120

The dim key is marked as STAR burst (above Prtscn key)
the keyboard reset is this:

PC on,
hold ESC key and plug in keyboard to any USB port, but does not reset colors, to off. sadly.

The Magic FN+F4 same thing. no true reset to LEDs off.I filed a complaint on that. to maker..

do not have cell phones attached to the PC now, as this can overload many PC's easy. (some)
use only mouse and a keyboard now.

if you keep failing , move the keyboard to PC #2 (other PC) and hit DIM to off.
then take the keyboard back to failing PC. (as LEDS are not off for sure)
see it does work, if not the keyboard is bad.

many are , the inside guts of the keyboard has 3 ribbon connectors, one is real real poor and loves to fall off. (hinge type connector super weak quality)
or the same connector can go canted, and this shorts the pins. easy as pie. and is design weakness is this keyboard.
Disassembly is very complex. and not covered here. (ask or buy new Keyboard if bad)

related to above: (Power on tap in a PC is  called VBUS.)
Most PCs made can do 500mA on USB2 ports, 99%. for sure if you keep other current hogs off other USB ports. (they have shared ports)
Most PCs desktops have at least 4 VBUS drivers and with 8 USB ports 2 are shared in groups, so the trick is skip ports as you plug things in.
Most PCs can do 1000mA, even with USB 2... (if it has both USB2 and 3 ports in one PC the current is standardized in these PCs. (simple is best)
test the keyboard on all USB ports
Test the keyboard with all other USB ports empty, some PCs have hidden USB ports, find them and make them empty for testing.

The keyboard works ok too, in BIOS , so no OS need be present to use this keyboard or test it for keys working.
I can use BIOS to set my personal asset tag, data.
Most PCs have no keyboard tests, in firmware diagnostics, but this trick works ! even no HDD present.

last point some PC if the USB device is shorted 100% (0 ohms) the PC resets. others just shut down the port only (most common is latter)
hope my facts prove timely and helpful.

I tested your keyboard on all DELLS here, and everyone works prefect as did the iCUE software.

desktops and laptops  even micro PCs very very old worked perfectly.   but not charging external batteries, no.

Even my 790 dell works.(all PCs are MT sized so sorry no way to do those more lame SFF)

You ignore the OP who says it wont post with keyboard attached.

You ignore the fact that the ports are PASSIVE Hubs.

"why not learn to measure the current first, then tell the truth, I see you did not. "

The system is not posting due to TOO MUCH CURRENT Draw.

I do not need a meter to tell me this.

OP please post case.

size  matters.

OP tell all things connected to all USB ports, or better yet

just plug in 1 thing the K55,

also make sure the RTC coin cell (batterY) in this very old PC is not dead, below 2.9vdc IS NO GOOD.

if the cell gets near the edge of death the BIOS goes NUTS. (nvram data loss)

a $1 fix. that.

the keyboard only uses 250mA max and does work on most PC made for sure all modern PCs all work.

not sure about 390, dell fails to spec ports, correctly (zero facts but USB2 known)

the keyboard can be tested on a 2nd modern PC, not a old classing weak laptop of any kind. (100mA relics)

The forum at CORSAIR has not one complaint on power, loss, but most gamers with cash to spend on this $50 keyboard  have real gaming PCs that your is not.

nobody has this problem there, I checked for you , ok?

But for a fact will fail on 100mA old old PCs. for sure. but most times just be born dead and will not ever cause PC to fail really,

but some PC might pop up a screen saying KEYBOARD is dead, want me to continue , sure click yes.

just to prove PC is not dead. just KBD born dead.

test only the KBD and no other USB devices at the same time for sure not gaming controllers same time to do this basic simple test.

and do test all USB ports, using just the key board.

good luck in your quest !!!!

if it turns out the K55 only fails on your PC there are 2 cures, (one PCI USB card ,rated 500mA)

or this powered hub for $10

I have one of this just when things LIKE this happen.

https://www.ebay.com/itm/Black-7-Port-USB-3-0-Hub-On-Off-Switches-AC-Power-Adapter-Cable-for-PC-Laptop/163131243949?hash=item25fb613dad:g:tagAAOSwLWBbO2uv

it is never over until the Gordo lady sings, (I'm neither)

I doubled my effort, knowing  for  a fact it is not a power limit problem.  using my own advice. (but harder work)

I then tested all DELLS here and now did all ports no just one. each PC.

keep in mind I have 3 tools , as scope,  voltmeter and ammeter to find truth. (electronics) (a real lab too)

The nrew discovery is the new K55 fails only on my 790 and only fails on  8 of 10 ports.

The k55 is  current FIRMWARE burned.

the 2 outboard front USB ports work perfectly (leds full on 350ma or full off) works

but is dead on all rear USB ports.  and dead on get this 1/2 the front ports, (2 inboard dread)

All Dell real keyboards work,  BIOS VERSION A18 is full of bugs.

the problem is BIOS only. and now we know 790 and the 390 fail and I bet they share much of BIOS code.

and worse BIOS A18 hangs mid POST,  in the progress bar and now the PC power button goes dead.(ouch)

hard rock total failure.  the ports that work are farthest from the sound audio 3.5mm jacks. (front I/O panel.)

PROBLEM SOLVED! and is NOT CURRENT LIMITS causing the failure, I have hard proof  of that.

my current is WAY below 10mA per meter proof.

But my guess dell will say gee we don;t support non Dell Keyboards. LOL

Good luck to all....

250 MA draw isn't allowed with passive USB hub port.

100MA is max for the ports.

Again I do not need to do volt meter kabuki to tell me that the keyboard is drawing more current than the machine provides. 

My direct experience with these models tells me that adding a powered hub will prevent booting.  Adding a pci-e card with aux power connection inside the box will not have issues.

Berating the user that their machine is too old is not solving the basic issue.

you keep up with the passive  fib.

the VBUS chips on the mobo connect to the +5vdc standby rail and are  2amp to 3amps rated at the rail.

that magic is how USB sleep wakes up.... see? it has power from THERE.

that is fact #1 you fail to know.

then the chips used by dell VBUS  driver distribution chips (facts) can do 500mA all day long one one port.

In fact all my dells with USB2 and 3 ports all use USB3 rules on power,.(dell was smart there, make it simple use  same VBUS chip for all, using economies of scale and all that .......... and DOES 1FULL AMP. (just one port all others empty)

the typical Dell can have 8 USB ports and 4 off those powered HUB chips connected to  pairs to ports

all my dells can do 1 full amp at 4.6vdc out. (max Vdrop USB spec)

but not on all ports at once, due to the DELL Sharing.

so learn that the internal USB structures on all DELLS have VBUS hub drivers,  everyone does.

the only changes PC to PC model are the sharing ratio's  , but is moot, just plug in the keyboard only and get full power from that magical VBUS chip,  (it also has current rollback feature and IOS shut down features, if you care to read the data sheet on the chip) ask. can do.

it is powered hub, from standby rail , the only limits are the chips actual, and the sharing rules.

so OP test the keyboard on PC #2 ;and end this. (do so with no ext batteries charging or external hdd or other  huge loads, ok?) just  test the keyboard.