A new HMI panel that stays silent on the PLC serial port almost always has one of four mismatched settings: protocol driver, baud rate, parity or station address. Check them as a set against the PLC port, then confirm the link with one read and one safe write. The worked example matches a panel to K-sequence at 9600 baud, odd parity, station address 1.
An HMI panel that sits silent on a PLC serial port almost always has one mismatched setting, and it is rarely the one you changed last. Four parameters must agree on both ends: the protocol driver, the baud rate, the parity and the station address. The PLC keeps executing its program the whole time, so nothing on the floor looks wrong; only the panel shows timeouts. Match all four in one pass, send the project to the panel, and the link comes up. Below are the four settings, the symptom each mismatch produces, the check order, and a worked example that brings a silent panel up on a port set for K-sequence, 9600 baud, odd parity, station address 1.
The four serial settings that must match
A panel and a PLC pass data only when both ports carry the same serial parameters: protocol, baud rate, data bits, parity and stop bits, plus the station address the protocol uses. A mismatch in any one of them keeps the link silent.
- Protocol driver. The command language. If the panel's device entry selects a different driver than the port speaks, the PLC ignores every poll, and no baud value fixes that. A PLC port may accept only one protocol; the PLC manual lists what each port takes. Set the driver first, before touching any number.
- Baud rate. The speed in bits per second. Common values on PLC ports are 1200, 9600, 19200 and 38400, and both ends must carry the same one.
- Parity. The check bit inside every character. With odd parity, the eight data bits plus the parity bit hold an odd number of 1s. Odd, even and none produce different frames, so a single difference here corrupts every byte on the wire.
- Station address. The number a poll names. The address rides inside each message even on a two-device cable, so a PLC set as station 1 stays silent when the poll asks for station 2.
Data bits and stop bits complete the character frame and must match as well; read their values from the two manuals instead of assuming them.
Symptoms and what each points to
| Symptom | What it points to |
|---|---|
| Timeouts from the very first request, nothing readable back | Wrong protocol driver, or a poll naming the wrong station address |
| Garbage characters, framing or checksum errors | Baud rate or parity mismatch; data bits or stop bits if those were edited |
| Link worked before, dead after a project transfer or firmware update | The device entry fell back to its defaults; see HMI panel will not communicate after a firmware update |
| One panel talks, a second panel on the same port times out | A shared port or a duplicated address; see why two HMI panels fail on one PLC serial port |
| Panel dark, or it drops out when it transmits | Power, not settings; see HMI panel not powering from a PLC serial port cable |
The check order below separates these causes faster than guessing at them.
Check the settings in this order
- Read the PLC port first. Open the port configuration in the PLC programming software, or check the DIP switches on older models, and write down protocol, baud rate, parity, station address, data bits and stop bits. On a CS1 or CJ1, that job is walked through step by step in set Host Link serial settings on a CS1 or CJ1 PLC.
- Open the panel project's device list. Find the PLC device entry the screens actually read and write, not a leftover from a template. The worked example below uses the tag DEV001.
- Set the driver before the numbers. If the driver name does not match the port's protocol, stop and fix it first; check the panel driver list before tuning serial settings covers the selection step.
- Enter the numbers as a set. Baud rate, parity and station address together, plus data bits and stop bits from the manuals.
- Send the project and repower the panel. Test one read of a value you can verify at the PLC before editing any screen logic.
If all parameters match and the panel is still silent, stop editing settings and move to the physical path. Serial cable wiring varies between panel models and between PLC ports, so verify the cable against the connection documents for your exact pair of devices, and try a known-good cable of the specified type. If the PLC port's baud rate is genuinely unknown, measure it: put a scope on the transmit line, time the width of one bit, and divide one second by that width. The result is the baud rate.
Example: a silent panel on a DL105 port
Scenario: a new control panel on a machine rebuild. The PLC is a DL105 whose RS-232 port is set for K-sequence, 9600 baud, odd parity, station address 1. The panel project's device entry, tagged DEV001, came from a template and still carries its defaults: 19200 baud, even parity, station address 1. Every screen request times out.
Three of the four parameters are wrong, and each produces its own failure:
- Driver. The template driver speaks a different protocol, so every poll is meaningless bytes to the PLC. No numeric change fixes this; the driver must become the K-sequence type first.
- Baud rate. At 9600 baud one bit lasts 1 ÷ 9600 = 104 µs. A panel transmitting at 19200 sends bits 52 µs wide (1 ÷ 19200), so the PLC's receiver samples mid-bit and reads garbled characters. At 38400 the bit is 26 µs wide.
- Parity. Even parity against the PLC's odd parity fails the count of 1s on every character, so even a correctly timed byte is rejected.
The fix is one edit, not three: set DEV001 to the K-sequence driver and enter 9600 baud, odd parity, station address 1 in the same pass, send the project to the panel, and repower it. The panel now polls the port and the six timer presets appear on the screens.
The station address would have bitten even with every number correct. A poll naming station 2 draws silence from a PLC listening as station 1, because the address travels inside each message even on a two-device cable.
Result: reads work. To confirm writes, change one preset from 10.0 s to 12.0 s with the machine in a safe state, and confirm the new value at the PLC and the expected machine response before touching the other five. A preset written to the wrong address can start or change a motion, so keep the machine in its safe state until every field is confirmed.
After the link is up
Verify all six fields before releasing the panel, not just the one you fixed. Two screen objects that look identical can point at different PLC addresses, so check each displayed value against the PLC data and make one controlled edit per field. Record the four serial parameters, the frame settings and the field-to-address list with the panel documentation; the next project transfer or panel swap then starts from recorded facts.
Recommendation: On a new panel install, enter all four serial parameters in one pass from the PLC port's documented values: protocol driver first, then baud rate, parity and station address, then send the project and confirm the link with one read before any write. If all four match and the panel is still silent, the settings are no longer the suspect: check the cable against both manuals, then the panel's power feed.
Questions
Why does the machine keep running when the panel cannot talk to the PLC?
The PLC executes its scan program on its own; the serial link only carries reads and writes between the panel and the PLC's data memory. Timers keep timing with the presets already stored in the PLC, so production continues unchanged. That is why a dead panel link usually appears at commissioning as timeout messages on the panel, not as a machine fault, and why restoring the link does not disturb the running logic.
All four settings match and the panel is still silent. What is left?
Move to the physical path. Confirm both cable ends are fully seated, then verify the cable wiring against the connection documents for your exact panel and PLC port, because serial wiring varies between panel models and between PLC ports. A known-good cable of the specified type is the fastest test. If the panel takes its power from the PLC port cable, a weak feed can leave it dark or resetting, which looks exactly like a communication fault.
Should I change the PLC port settings or the panel settings?
Change the panel side. PLC serial ports often serve more than one duty, their protocol choices are fixed per port, and some older models set the port with switches rather than software. Matching the panel project to the port's documented values leaves the PLC untouched and cannot disturb its program or any other device on that port. Edit the PLC port only when its own documentation calls for it, and record the change.
What should I record once the link works?
Write down the four serial parameters (protocol driver, baud rate, parity, station address) plus data bits and stop bits, keep the cable drawing, and keep a list mapping each screen field to its PLC address. Store all of it with the panel documentation. The next project transfer, firmware update or panel replacement then starts from recorded facts, and a replacement panel can be configured before its first power-up.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.