A small panel that stays dark on a PLC serial cable has a power problem, not a settings problem: the port's 5 VDC supply is landing on a pin the panel ignores. Trace the cable's power conductor against both hardware manuals, then fit the documented cable assembly for that panel and port pair.
A small HMI panel that runs from a PLC serial port takes its power through that cable. When the panel stays completely dark on one cable and lights up on another, port settings are the wrong place to look: no setting turns the port's 5 VDC supply on, off, or over to a different pin. The supply is fixed wiring on the connector, and the cable decides which pin delivers it to the panel. In the documented failures of this kind the whole fault is one pin: the cable carries the supply to a pin the panel ignores, and a cable built for an older panel model is the usual culprit. Here is how to trace the power conductor, measure where it lands, and finish with a cable you can trust.
Why no port setting controls the power
Where a PLC serial port supplies power, that supply sits on specific pins of the connector whenever the PLC is powered. Two documented examples show the pattern. One PLC specification states that both of its RS-232 ports supply 5 VDC so a small panel can run without a supply of its own. A different PLC manual documents +5 VDC on pin 1 of its programming port, placed there to power a handheld programmer. Neither arrangement lives in the communications configuration, so no baud rate, protocol or station number setting can create it, move it or switch it off.
Panel makers move the power pin too. One documented panel family accepts power on pin 2 or pin 5, and its successor in the same family requires the supply on pin 5. A cable assembled for the older panel can carry the supply to pin 2 only, and the newer panel stays dark even though the plug mates cleanly. The plug fitting is a mechanical fact. The wiring behind it is a separate fact, and the two hardware manuals are where the electrical answer lives.
A panel that receives no power cannot be configured back to life. Work the physical path first: cable, adapters, connector pins.
Symptoms and what each points to
- Dark on the port cable, lit on another cable or port. The power conductor is missing or lands on the wrong pin. Start the trace below and leave the port settings for last.
- One cable powers an older panel and not its newer replacement. The two models take power on different pins, and that generational swap is the documented case behind most of these calls.
- Dark on every cable, including its own rated supply. The fault sits past the cable: the panel's supply input, an internal fuse or the panel itself. Test the panel on its own documented feed, away from the port.
- Powers up, screen stays blank. Power is solved. The startup screen setting is the usual next stop; HMI panel shows no power up screen covers it.
- Powers up, no data on the link. Power is solved again. Work through HMI no communications when one PLC model fails, and when the run between panel and PLC is long, RS-232 to RS-422 cable distance for an HMI panel link shows where the distance limits sit.
Trace the power pin before touching settings
- Confirm the port supplies power at all. Read the connector pin table for that port in the PLC hardware manual. If no pin lists the 5 VDC supply, the port is data only: no cable will power the panel, and the panel needs its own supply. Stop here.
- Read the panel's power pin from its manual. The hardware manual's connector view gives the pin numbers and the viewing direction. Some views are drawn from the mating face, some from the wiring side, and reading the wrong one mirrors the pin positions.
- Unplug both ends and power everything down. Continuity testing happens on a cable disconnected at both ends, with the PLC and the panel off.
- Trace the supply conductor on continuity. It must run from the powered pin at the PLC end to the power pin the panel manual names, and the ground conductor must run from the port's ground to the panel's documented signal ground. One open or crossed conductor keeps the panel dark.
- Reassemble and measure voltage live. Meter on DC volts between the panel-end power pin and the documented signal ground. Standard 5 V logic tolerates about ±5%, so 4.75 to 5.25 V at the panel end is the healthy band. 0 V on the panel's power pin means the conductor is open or lands somewhere else.
- Record what you found. Pin numbers at both ends, the readings, and the markings on the cable. That record decides the repair.
One safety note belongs here: never bridge pins with a jumper or a meter lead to see whether the panel starts. A slip that lands the supply on a data pin can damage the transceivers at one or both ends of the link.
Example: tracing the supply pin on a dark panel
Scenario: a 5.7-inch text panel was replaced last month with a newer model in the same family, and it stays dark on the PLC port cable. The old cable powered the previous model without trouble. Both hardware manuals are on the bench.
The PLC manual's table for this port lists 5 VDC on pin 2 and signal ground on pin 5. The new panel's manual names pin 5 as power and pin 3 as signal ground. Cable unplugged at both ends, meter on continuity:
- PLC pin 2 to panel pin 2: beep, under 1 ohm. The supply conductor reaches pin 2 at the panel.
- PLC pin 5 to panel pin 3: beep. Ground is continuous.
- PLC pin 2 to panel pin 5: open. Nothing reaches the pin the panel needs.
Reassembled and powered, meter on DC volts at the panel end:
- Pin 2 to pin 3: 4.97 V. Inside the 4.75 to 5.25 V band, so the port supply and the conductor are healthy.
- Pin 5 to pin 3: 0 V. The power pin this panel names carries nothing.
The cable is electrically sound and built for the older panel's power pin arrangement, which accepted power on pin 2 or pin 5. The newer model needs the supply on pin 5, and this cable never delivers it there. The repair is a documented cable for this panel and port pair, or a rebuilt cable that runs the supply from PLC pin 2 to panel pin 5, checked conductor by conductor before it touches the panel.
The fix, and keeping it fixed
- Fit the documented cable for that panel and port pair. When the panel documentation's cable chart names a cable and adapter combination for your port, that combination is the specification.
- If you build or rewire a cable, verify every conductor end to end against both pin tables before it connects to the panel, and label it with the device pair and the power pin at each end.
- Keep any arrangement that works, such as the other port or the old cable, labeled as temporary if the line runs this shift on it. Pull the mis-wired cable out of circulation; an unlabeled shelf of serial leads is how this fault returns.
Recommendation: Treat a dark panel as a power-path fault until a live voltage measurement says otherwise. Read the powered pin from each hardware manual, trace that conductor on the unplugged cable with a meter on continuity, fit the documented cable assembly for that panel and port pair, and label every serial cable on the shelf with its devices and power pin.
When the port supplies no power at all
Many serial ports are data only, and the connector pin table in the PLC manual settles the question: if no pin lists a supply, no cable and no setting will power the panel. The panel then runs from its own supply, sized for the input its manual states. Even where a port does carry a supply, it is built for a small panel: the documented offer above covers a small monochrome unit, and a larger display belongs on its own feed.
An HMI and a notice board are separate jobs too. If the line needs a status or andon screen that plays its own content offline, that is a shop-floor signage display, not something to hang off a serial port. And a panel with its own supply that freezes when solenoid coils switch has a different power fault: a shared DC supply with solenoid coils can freeze an HMI even when the wiring is otherwise correct.
Questions
Can I measure the power pin while the system is running?
Yes, for voltage. Meter on the DC volts range, black probe on the documented signal ground, red probe on the power pin, reading taken at the panel end of the cable. Use a breakout connector or back-probe the pin so the probe tips cannot bridge adjacent pins. Continuity is the different test: it needs both cable ends unplugged and the PLC and panel powered down.
Can I rewire the cable to move the supply to the pin the panel needs?
Only if you can verify every conductor, ground included, against both manuals' pin tables on the unplugged cable. Move the supply conductor and nothing else, leave the data pins untouched, and label the finished cable with the device pair and power pin at each end. When any path is uncertain, order the documented assembly for that panel and port pair instead of experimenting.
Will the port's supply also run a larger panel?
The documented cases cover small panels. One PLC specification offers its serial-port supply for a small monochrome unit, and the port's output is built to that scale. A larger display needs its own feed, sized for the supply input its manual states and wired to the terminals the manual names. Check both documents before expecting a serial port to carry more than its table lists.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.