Can you program a PLC through an HMI pass-through

Why pass-through will not carry a PLC download

Pass-through exists for panel-to-panel data. In the setups where HMI makers document it, one panel shares runtime values with a second panel of the same family. A program download is a different kind of session: the PC software connects to the controller's own network address, confirms the controller's identity, then transfers the program. Pass-through offers no controller address to the computer, so a download started at the panel has nowhere to go. Plan the programming path separately from the panel and the job becomes simple.

If the reason for trying pass-through was that the PLC's communications are already used up, that is a ports problem with its own fix: add another operator interface when the PLC ports are full walks through it.

What you need

• An Ethernet interface card for the PLC, or a controller with an Ethernet port built in. The card's own manual is the authority on how its address is set.
• A desktop switch with at least three free ports. A basic 5-port unit covers the three devices and leaves two spares.
• One twisted-pair Ethernet cable per device, each run kept inside the length limit IEEE 802.3, the Ethernet standard, sets for a copper run between a switch port and a device (see the worked example below for the figure).
• The PLC programming software on the computer, and the panel maker's own software for the screen project.
• The PLC's serial programming cable, kept within reach until the network path has carried a few downloads.

A card fitted for programming can carry the panel's communications too. If it will, how many HMI panels one PLC Ethernet card supports gives the sizing.

Steps to program the PLC over one switch

1. Fit the interface card with the PLC powered down, and set any jumpers or slot positions the card's manual calls out. Nothing changes at the panel.
2. Give the card a network address in the same private range as the computer. Many modules power up with BOOTP enabled, and the manual for a module in that state says to turn BOOTP off before the programming software will accept a hand-entered address, so read that page before typing anything. Example values: 192.168.1.10 for the card, 192.168.1.50 for the computer, 192.168.1.60 for the panel, all with subnet 255.255.255.0.
3. Run one cable from each device to the switch: computer, PLC card, panel. Keep every run inside the standard's length limit for a copper twisted-pair run.
4. Confirm the link lights on all three switch ports, then ping the card's address from the computer and watch for replies.
5. In the PLC software, create or select the Ethernet path to the card's address and connect. Read back the controller identity the software reports and check it is the PLC you mean to program.
6. Put the controller in its program or transfer mode and download. Accepting a program normally takes the controller out of RUN and stops its outputs, so pick a moment when the machine can stand still. That is the one safety step in this job, and it is worth saying plainly: stop the machine on purpose.
7. Load the screen project with the panel's own software through the same switch. The panel and the controller still need one protocol they both support; how to pick one protocol when connecting an old operator interface covers that choice.

Worked example: one 5-port switch, three devices, a 45 m longest run

The inputs, with where each comes from:

• Three devices: computer, PLC with the newly fitted Ethernet card, operator panel. That is the setup the steps above build.
• A 100 m maximum per twisted-pair copper run, the limit IEEE 802.3 sets between a switch port and a device. This is the figure every run below is checked against.
• Run lengths of 10 m from the computer, 45 m out to the PLC cabinet, and 6 m to the panel. These are assumed values for a typical cell; measure your own runs and apply the same arithmetic.
• A 5-port switch, chosen for three devices plus spare ports.

The rule in words: the limit applies to each run, not to the total path. A switch regenerates the signal, so a two-run path through it is legal as long as both runs sit inside the limit.

The arithmetic:

• Longest single run: 45 m against the limit, so 100 - 45 = 55 m of margin. Passes.
• Computer-to-card path: 10 + 45 = 55 m across two runs of 10 m and 45 m. Both runs are inside the limit, so the path is allowed even at 55 m end to end.
• Switch ports: 3 devices need 3 ports, and 5 - 3 = 2 ports spare.

Result: one 5-port switch serves the whole cell, every run inside its allowed length, with two ports free. The decision this drives: if any single run measures over the limit, put a second switch at the far end of that run or use a fiber link for it; if the cell grows past five devices, step up to an 8-port unit, still one switch.

How to check it worked

• Ping replies from the card's address confirm the network path end to end.
• The programming software reports a controller identity and revision. Check both against the PLC you intended before any download; a network that reaches several controllers will download the wrong one just as easily as the right one.
• The screen project loads and the panel shows live values from the controller.
• If the panel download fails over the same switch, why a panel project download fails through the network switch lists the causes in the order to check them.
• If the PLC software cannot find the controller, check that computer and card sit on the same switch, that their addresses share the subnet, and that the card's BOOTP state matches what its manual requires.

Recommendation

Recommendation: Program the PLC directly. Fit the controller's Ethernet interface card, hang the computer, the PLC and the panel off one switch with every copper run inside the standard's length limit, set the card's address before the first download, and keep the serial cable in the toolbox until the network path has carried a few clean downloads. Read the panel maker's pass-through documentation before relying on it for any other job, and expect it to say data between panels.

When the same cell also needs a production notice board beside the controls, that is a separate screen with a separate job: the shop-floor screens section shows what we build for it.

Questions