Yes: a PLC's second serial port runs an HMI panel while port 1 stays on the programming computer. Confirm the port's signal interface from its pin table, because a 5 VDC supply pin does not name the data standard. Use a cable path documented for the port, set the same baud rate, parity, data bits and stop bits at both ends, and give a second panel its own start, stop and reset bits with a shared display register.
Yes: a compact PLC's second serial port can run an HMI panel while port 1 stays on the programming computer, and that split is the point. The panel goes online and the programming link stays live, with nothing unplugged mid-job. The work is three checks. Confirm from the PLC manual what the port's data pins actually speak, because the supply pin on the 15-pin connector identifies a power feed, not a signal standard. Use a cable path documented for that port. Set the same baud rate, parity and framing at both ends, then verify the link with a read and a write before any real button does anything. The sections below cover the pin table, the settings, a worked bench example and the changes a second panel brings.
What you need
- The PLC manual's pin table for the second serial port, with a signal name printed against every pin you intend to use.
- The panel manual's serial chapter: the PLC drivers it can speak, its serial setting fields, and its own connector's pin table.
- A cable path documented for this connection: an adapter from the PLC maker's programming and test cable kit, or the adapter the maker documents for this panel line.
- If you build the cable: soldering iron, connector shells, and a multimeter for a continuity check against both pin tables before anything is plugged in.
- The panel's own power supply and its programming cable, which programs the panel from a PC and has nothing to do with the PLC link.
- The programming computer, left connected to port 1 throughout.
What the 5 VDC pin tells you
The PLC maker puts 5 VDC on the port connector so its own cables and adapters can draw a small supply current. That is the whole story of the pin. The data pins beside it are a separate line in the same table: RS-232 swings between -3 and -15 V for a mark and +3 to +15 V for a space (the TIA/EIA-232 levels), on single-ended wires referenced to signal ground. RS-422 and RS-485 use differential pairs instead, and on compact PLCs the same 15-pin shell can be documented for any of the three, selected by jumpers, DIP switches or the port's mode. The connector shape identifies nothing; the pin table identifies everything.
So read the pin table first and write each pin number beside its signal name. If the manual documents the port as RS-232 with transmit, receive and signal ground, a three-wire link to the panel's matching pins is the whole cable. The interface also sets the cable's reach; the distances per interface are worked out in HMI to PLC serial cable maximum distance by interface.
Do not wire the supply pin to the panel side. A panel draws more current than a port supply pin is meant to source, and the panel has its own power terminals. The failure mode, a panel that will not start from a serial cable, has its own article: HMI panel not powering from a PLC serial port cable.
Keep port 1 on the programming computer
Leave the working programming connection exactly as it is. The panel goes on port 2 with its own cable, its own driver and its own settings, so panel testing never interrupts the programming session. Each serial port holds its own configuration; setting port 2 leaves port 1's rate and protocol untouched.
On most compact PLCs there is no rung to add: ladder logic does not assign a port to an HMI. You connect the panel, select the panel project's driver for the PLC's protocol, and match the serial values. If the controller's system configuration exposes a mode or protocol choice for the port, pick the entry the driver speaks; if it exposes none, the driver must match the port's fixed protocol. Getting driver and protocol to agree, and what breaks when they do not, is laid out in Check the PLC serial protocol before replacing a legacy HMI.
Wire and configure the link
- Write the two pin tables side by side: the PLC port's pin numbers with their signal names, and the panel jack's with theirs.
- Buy or build from those tables. Use the documented path from the list above; if no ready-made path exists for your pair of connectors, build from the pin tables and keep the supply pin entirely out of the data wiring.
- Set the PLC port: its mode or protocol per the manual, then baud rate, parity, data bits and stop bits. Write the values down.
- Set the panel: select the driver for the PLC's protocol, enter the same baud rate, parity, data bits and stop bits, and the station address if the protocol numbers its stations. Connect a Modbus RTU HMI to a legacy PLC RS-232 port covers the station-numbered case end to end.
- Power the panel from its own supply terminals, then connect the link.
- Open the panel's communication status page, then judge the link by the read check in How to check it worked below.
Example: a bench panel beside a live programming link
A packaging bench runs a compact PLC whose manual documents port 2, on the 15-pin connector, as RS-232 with transmit, receive and signal ground. Port 1 stays on the programming laptop at 9,600 bit/s. The new panel, connecting through its 6-pin RJ12 modular jack, must show a job count and offer start, stop and reset buttons.
Settings: port 2 gets 19,200 bit/s, 8 data bits, even parity, 1 stop bit, and the panel's driver screen takes the same four values plus station address 1. Framing arithmetic: 1 start + 8 data + 1 parity + 1 stop = 11 bits per character, so port 2 carries 19,200 ÷ 11 ≈ 1,745 characters per second. A 12-character count read is 132 bits, and 132 ÷ 19,200 ≈ 6.9 ms of line time per poll. Port 1 shares none of that: the laptop runs on its own wires at 9,600 bit/s, so going online with the PLC while the panel polls costs neither session anything.
Cable: three wires from the pin tables, transmit to receive, receive to transmit, signal ground to signal ground, with the shield landed as the manual directs.
Checks, in order: the panel reads the count register the logic already updates; a test write from the panel reaches a spare bit watched in the programming software; each button moves its own test bit.
Result: the panel is live on port 2 at 19,200 bit/s, the programming link on port 1 is undisturbed at 9,600 bit/s, and the two never share a wire.
A second panel: separate command bits, shared registers
Adding a second panel across the floor takes no slaving and no change to the first. Give each panel its own start, stop and reset bit locations, and let the PLC logic own the decision: the logic scans both panels' command bits, latches or rejects each press, and resolves simultaneous presses in one place. The PLC stays the referee, so neither panel depends on the other.
Display data works the other way around. When both panels show the same job count, point both at the same register: one register, one truth. When the two must show different error messages, give each panel its own message registers and let the logic write each. The decision in one line: separate command bits always, shared display registers while the displayed text is identical, separate registers the moment it is not.
Two panels that must share one serial port are a different arrangement, with the port's polling time divided between them; why two HMI panels fail on one PLC serial port explains where that stalls and how to avoid it.
How to check it worked
Check in this order, because each step narrows the next:
- Read: set a register to a known value from the programming software and watch it appear on the panel.
- Write: change a non-critical test value from the panel and watch the same location in the programming software.
- Commands: press each panel button wired to a test bit and watch that bit, not a machine output.
- Two panels, if fitted: run both through the first three checks, then press start on one and stop on the other at once, and watch the logic take the action you wrote for that case.
Reads failing sends you back to the pin table and the interface, because wrong wiring and wrong settings produce the same silent panel. Reads passing while writes go missing rules the cable out: look at the write addresses in the panel project and at the logic that scans them.
One safety note: keep the machine in a safe state until the start, stop and reset bits have been checked against test bits and the logic's outputs confirmed, and make or break the serial link only with the line stopped.
Recommendation: keep port 1 on the programming computer and bring the panel up on port 2 as soon as the pin table settles the port's signal interface; power the panel from its own supply terminals; and when a second panel joins, give it its own command bits and share the display register only while both screens show the same text. If the pin table leaves the interface unstated, use the cable path the PLC maker documents and stop there.
Questions
Can the programming session and the panel be online at the same time?
Yes, and that is the point of the split. Port 1 and port 2 run independently, so the programming session and the panel's polling share neither wires nor line time. The only coordination is yours: if a download changes a register the panel displays, expect the panel to show the old value until its next poll refreshes it.
If both panels end up on one serial port, which settings change?
Both panels then run at that port's baud rate, parity and framing, and they split the port's polling time, so screens refresh slower than a panel with a port of its own. Each still writes to its own start, stop and reset bits, and the PLC logic still resolves simultaneous presses. If refresh speed matters, give the second panel its own port.
The panel reads registers fine, but button presses never arrive. What now?
Reads passing while writes go missing points at the write path, not the cable. Check the write addresses in the panel project against the bits the logic actually scans, confirm the logic latches a momentary press, and check nothing write-protects that range. Rebuild the cable only after the addresses and the logic check out.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.