A USB 2.0 port supplies 5 V at up to 500 mA, which is 2.5 W, and a 6-inch class HMI needs its own 24 VDC supply. When the setup screen never appears, meter the power input terminals first: the programming cable carries data, not operating power.
The panel showed your logo yesterday and shows nothing today, and the only cable attached is the USB programming lead. That cable carries data, plus the 5 V a USB port offers. A 6-inch class HMI expects a 24 VDC input at its power terminals, and a PC that went into power-save overnight took even that borrowed 5 V away. This page walks the failure in the order that saves bench time: read the symptom, meter the power input, then retry the setup keys. The short version: a programming cable is not a power supply, and the smaller panel that runs on the same cable is telling you the cable is fine.
Why did the panel stop overnight when it worked yesterday?
Because the PC went into power-save and took the panel's only power source with it. A panel running from the programming cable draws everything from the USB port, so when the port suspends overnight, the panel loses power mid-run and comes back dark or confused.
The timeline is the tell. The logo was up when you left, the PC slept, and the panel was dark by morning. A panel with its own supply rides through a sleeping PC and keeps running its project. A panel living on the USB port loses everything the moment the port suspends. Some panels recover after a clean cycle on proper supply power; none of them will open a setup screen while starved. Before you touch the project or the keys, answer one question: is anything other than the programming cable connected to this panel?
What does each symptom point to?
A blank screen with no backlight points to power; a frozen logo points to a panel that started and then lost power; a running screen with a failed transfer points to the cable or the PC link. Match your symptom before you change anything.
- Dark screen, no backlight, no key response. No operating power. Go straight to the power input terminals.
- Logo frozen since the morning after. The panel started, ran, then lost power, the signature of a PC sleep event on a bus-powered panel.
- Project runs, but the PC will not list the device. A link fault. Re-seat both ends, try a port you have seen enumerate a device today, and check the programming cable power pin before blaming the panel.
- Project runs, keys pressed, no setup screen. An access question, and it only becomes real once power is confirmed; why taps never open a hidden setup screen covers that side.
Can a USB port power a 6-inch HMI panel?
No. A USB 2.0 port is specified for 5 V at up to 500 mA, which is 2.5 W, and USB 3.0 raises the allowance to 900 mA, 4.5 W. A 6-inch class panel starts on its 24 VDC input, not on bus power.
The arithmetic is short: 5 V x 0.5 A = 2.5 W on USB 2.0, 5 V x 0.9 A = 4.5 W on USB 3.0. Even the larger figure feeds a peripheral, and it is the wrong voltage entirely for that input. Smaller panels in these families are sometimes built to run from bus power, which is why a 4-inch unit lights up on the same cable. The 6-inch class is built around its own DC input. Your panel's documentation prints the requirement; if it lists a 24 VDC input with a current figure, that input is the power path.
The 5 V line in a programming cable is the same one behind why 5 VDC never appears on a USB RS-232 adapter terminal block: it exists to feed small adapters, and pushing a display start through it asks the port for more current than it can grant.
What should I measure first, and where?
Set a meter to DC volts, put the leads across the panel's power input terminals, and read the panel's rated voltage with the panel connected, 24 VDC on this class. That single reading separates a power fault from a project fault.
If the reading is correct with the wires off and collapses when the panel tries to start, the limit is the supply or the feed: check terminal torque, wire gauge and any fuse in the path. Verify polarity before you apply power, because a reversed DC input can destroy the panel. Use a supply with safe electrical isolation, SELV or PELV, the requirement panel connection documentation writes against IEC 61010-2-201. When you fit or resize that supply, sizing the DC power supply for a small HMI panel covers the margin to leave.
What does the power budget look like in an example?
A 24 V input fed from a 1.5 A supply can draw 24 V x 1.5 A = 36 W; the USB 2.0 port behind the cable tops out at 5 V x 0.5 A = 2.5 W, about one fourteenth of that.
Scenario: a 6-inch panel sits on the bench with only the programming cable in. It showed its logo at 5 pm because the port was supplying just enough current to run, barely. The PC slept at midnight, the port suspended, and the panel lost its only source mid-run. At 7 am the screen is dark and the keys do nothing. The 1.5 A supply here is an example input, a common DIN-rail size; your panel's documentation prints its own maximum, and you size to that number.
The fix, in numbers: wire a supply rated to the panel's printed maximum, here 1.5 A. The meter reads 24.0 V at the terminals with the panel connected. The panel starts within seconds, the cable enumerates on a woken PC, and the setup keys open the screen on the first press. Bench time is about 15 minutes, against the hour you would spend reloading a project that was never the problem. Decision: any panel whose documentation lists a 24 VDC input gets wired to that input before any further troubleshooting, on any bench.
Why does the smaller panel work on the same cable?
Because the 4-inch panel's draw fits inside what a USB port can renegotiate after enumeration, so the test clears the PC, the port and the cable as a set. The open item it leaves you is the 6-inch panel's power input.
Run the comparison once, with notes. Put the same cable and the same PC port on the 4-inch panel, watch it enumerate, and write down whether that panel was on its own supply at the time. Then go back to the 6-inch, wire its supply, meter the terminals and retry. The comparison has done its job the moment the cable is cleared; repeated swaps cost bench time and add nothing.
How do I get the setup screen back once power is stable?
Wire the supply, confirm the panel completes its own start with the cable unplugged, then reconnect the cable direct to a woken PC and press the setup key combination once; on the family in question that is F1 and F5 held together.
- Power everything down and wire the supply to the panel's input, following the label for voltage and polarity.
- Apply power with the USB cable out and watch the panel complete its own start. Give it 30 seconds.
- Reconnect the cable directly to a woken PC, no hubs, no extenders.
- Confirm the PC lists the device. If it does not, stop here, because this is now a link fault.
- Press the setup keys once, F1 and F5 together on this family, wait, and make one controlled retry.
- Write down the voltage you read, the port you used and the result, so the next person starts from evidence.
If the keys still do nothing while the panel runs on its own supply, the fault has moved off power and onto link or access, in that order.
How do I stop it happening again?
Give the panel its own supply and keep the programming cable for transfers. Set the programming PC to stay awake with its USB ports never powering down, so an overnight sleep costs you a link, never the screen.
Label the supply with its voltage and the panel it feeds, and keep the PC awake through commissioning weeks. The same rule holds across the shop-floor screens that live next to a programming station: power each one from its rated supply, and let the PC own nothing the screen needs to run. A screen that survives a PC reboot, an operating system update and a Friday night is a screen you stop driving in to see.
Recommendation: wire the panel's own 24 VDC supply and meter its terminals before you press any setup keys or reload any project, whenever a 6-inch class panel goes dark with only a programming cable attached. If the panel starts on its supply and still will not communicate, treat it as a link fault next; only a panel that stays dark on confirmed supply voltage goes back for service.
Questions
Will a powered USB hub let the panel run from the programming cable?
No. A powered hub still regulates its ports to 5 V, up to 500 mA each on USB 2.0, and the total is shared across everything plugged in. That stays far below a 6-inch panel's start draw, and it is the wrong voltage for a 24 VDC input. Use hubs for adapters and memory sticks, never as a panel supply.
Is it safe to commission the panel on USB power if it started once?
Treat it as an accident, not a method. A port that renegotiates its current allowance can light a logo for a while, then the panel browns out the moment the PC sleeps or the port resets. Commission on the panel's own supply and keep the cable for project transfers, so a reproducible start is part of the handover.
How do I stop the programming PC from taking the panel down overnight?
Set the PC's power plan to stay awake, set its USB ports so they never power down, and wire the panel to its own supply so the PC controls nothing the panel needs to run. After that, an overnight sleep costs you a reconnect, and the screen keeps its power.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.