Pass values between two PLCs through an HMI tag copy

Can an HMI panel pass values between two PLCs?

Yes. A repeating Tag Copy action in the Event Manager reads a tag from one connected device and writes it to a tag on another, and you can add one event per value in each direction, up to a practical ceiling of about 3 to 5 tags.

Each connected device keeps its own address space in the project, and the copy names a source tag on one device and a destination tag on the other. Nothing moves that you did not list, and nothing moves unless the panel project is running. That makes the panel a data bridge you configure by hand, tag by tag.

Treat it as a convenience, close to the edge of what an operator panel is for. It copies values; it runs no logic of its own beyond the events you define. If what you actually want is to reach the far PLC's programming port through the panel, that is a different feature, and programming a PLC through an HMI pass-through covers it.

How many tags can I move, and how fast?

Plan on 3 to 5 tags refreshed every 2 to 5 seconds: the event system checks its triggers once per panel task cycle, so every copy competes with the panel's other work in that cycle, and a small list keeps that load light.

Put a number on it before you build. Tag operations per minute equals tag count times 60 divided by the repeat interval in seconds. Five tags at a 2 s repeat is 5 x 60 / 2 = 150 tag operations a minute, and that is the top of the envelope for this method. Two things push you past it: more tags, or a shorter interval. Either one means the job belongs to a link between the controllers.

The other limit is staleness. A value crossing the bridge is at least one repeat interval old when the receiving PLC reads it, plus the panel's own polling of both devices. If the receiving logic must react inside one second, a panel bridge is the wrong tool. For the architectures that keep the panel out of the middle, see polling both PLCs or linking them directly.

Can the two PLCs run different protocols?

On the current generation of these panels, yes: the port setup screen accepts a different protocol on each port, so one PLC can sit on Ethernet while the other sits on a serial port, each as its own device connection.

The software's help text still describes one protocol per project, a line that predates the current panels; the setup screen is what governs, and your panel generation decides what it offers. Prove the pairing before you build the bridge: configure both device connections, then read one live tag from each.

Keep the two address spaces apart by device. Every tag in the copy list belongs to a named device, which is what stops two identical register numbers on two PLCs from colliding. Per-device tags on one HMI panel shows that setup with four controllers. The wiring side, one PLC on serial and one on Ethernet, is covered in connecting one operator panel to two PLCs over DF1 and Ethernet.

How do I set up the repeating tag copy?

Build one Event Manager event per value, with a Tag Copy action that names a source tag on the first device and a destination tag on the second, set the action to repeat, then copy and paste that event for each additional value.

What you need: the project software on a PC, both PLCs wired and addressed, and a one-line-per-value sheet listing source tag, destination tag, data type and direction.

1. Configure each PLC as its own device connection, on its own port, with its own protocol.
2. Confirm the panel reads a live tag from each device before you build any event. Put one tag from each PLC in a data view and watch it move; watching several device values in one data view shows the technique.
3. In the Event Manager, create an event whose action is a Tag Copy. Name the source tag on the first device and the destination tag on the second.
4. Set the action to repeat at your interval. Start at 3 s for values a person reads or a slow loop consumes.
5. Match the data types at both ends, integer to integer, and write the scaling of each side on your sheet.
6. Copy the event and paste it to build the next pair. The paste lands at the end of the Event Manager Database list, and you edit its two addresses. One event per value and direction.
7. If you fire the copy from an event tag, know that a Tag Copy is edge-triggered: it fires when the trigger tag changes state and stays quiet while the tag holds steady. Use the repeat setting for continuous refresh and a trigger tag for one-shot moves. An event can combine up to four trigger tags.
8. Download the project and test with a value that hurts nothing if it lands wrong.

How do I check each copy is working?

Change the source value by a known amount at the first PLC and watch the destination tag at the second PLC move within one repeat interval, then repeat the test for every event you built, one at a time.

Test one direction first. Change the source by a known step, 40 to 52 on an integer target for instance, and watch the destination take the new value inside one repeat interval. Then test the reverse direction with its own event.

Add one heartbeat per direction. The sending PLC increments a free-running counter once per second, the bridge copies it like any other tag, and the receiving PLC flags the bridge as stale after three unchanged samples, 9 s at a 3 s repeat. That counter tells you the event and both device connections are alive, and it keeps telling you while the machine runs.

How many tag copy events does a real example need?

Four events cover a typical case: three values forward from the filler PLC to the case packer and one count back, which at a 3 s repeat is 80 tag operations a minute, comfortably inside the 150-operation envelope.

Scenario: a filler PLC and a case packer PLC share one panel. Forward go the target in cases per hour, a speed setpoint in cases per minute, and a run command; back comes the cases-counted total. The tag names below are the example's own.

Four data events plus the heartbeat makes five events. At a 3 s repeat each event copies 60 / 3 = 20 times a minute, so the bridge runs 5 x 20 = 100 tag operations a minute. The envelope top is 150, so this bridge fits with room to spare.

Staleness: the packer acts on a target that is at most one repeat old, 3 s, plus the panel's polling of each device. Its changeover logic must tolerate that 3 s. If the packer's control loop needed the target within 200 ms, the arithmetic would fail this design before you wired anything, and the fix is a link between the two controllers, with the panel reading tags for display.

Scale check on the same arithmetic: a 60-value production table at a 500 ms refresh is 60 x 60 / 0.5 = 7,200 tag operations a minute, 48 times the envelope top. That job never goes through an operator panel.

What breaks a tag copy bridge?

Four things cover nearly every failure: a device that stopped talking, a second writer on the destination tag, an event left disabled or pointed at the wrong address, and mismatched data types; each has a test that names it.

• A device stops talking. The heartbeat freezes and the panel flags the connection, and the last copied value sits in the destination tag. Judge freshness at the receiver; keeping an HMI usable when one PLC goes offline covers the screens side of that failure.
• A second writer. Stop the project and watch the destination tag: if it still changes, something else writes it. Give the bridge destination tags that no other device touches.
• An event left disabled or misaddressed. Copy-paste builds drift when one pasted event keeps the first event's addresses. Open the Event Manager Database list and check every event against your sheet.
• Mismatched types or scaling. The value lands wrong or never lands; check both ends against the sheet.
• A recurring protocol error points at the connection, ahead of the events. An HMI protocol error that pops up every 30 seconds walks that trace.

When is a tag copy bridge the right choice?

Choose it when you need about 3 to 5 values moved every few seconds between two PLCs that already share the panel, and the receiving logic tolerates one refresh interval of staleness.

Recommendation: build the repeating Tag Copy bridge when the load stays under about 150 tag operations a minute and the receiving PLC tolerates one repeat interval of staleness, 3 s at a 3 s repeat. Keep the copy list to 5 tags or fewer, one heartbeat per direction, and give the bridge destination tags nothing else writes. Above those numbers, link the two controllers directly and let the panel read tags for display. Keep every stop, guard and interlock on inputs the controller reads locally, with the bridge carrying setpoints and counts.

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