Annunciator display for two PLCs: HMI panel or monitor

A monitor only shows a video signal

A monitor is the last link in a video path. Something behind it has to read the controllers, hold the tag values, draw the screens and push the pixels. An HMI panel is that whole chain in one enclosure: editor, PLC drivers, connection manager and screen. A monitor on the shelf is not an annunciator missing a few features. It is a screen waiting for the rest of the system.

The two Ethernet cards do not change this. A network interface gives a device an address and a cable. It does not collect tags from two controllers or draw alarm lines. Fitting one card per PLC to a monitor produces a monitor with two cables and no software.

So the honest comparison is between two systems that can both do the job:

1. A second HMI panel from a family that allows two controller connections in one project.
2. An industrial panel PC with HMI software, driving a separate monitor.

Everything else in the decision follows from those two.

What decides it

Two facts decide this: whether your HMI software can hold two controller connections at once, and how far away the annunciator will be read from.

Start with the editor you already own. HMI projects keep a connection list, and you add one connection per PLC, each with its protocol and its tags. Panel models and editor versions differ in how many connections run at the same time; the manual for your exact panel prints the limit. If your current family allows two, most of option a is done before you shop: same editor, same symbol libraries, same tag style.

The controller side has a limit too. An HMI client is still a client. On EtherNet/IP systems, each connection uses CIP connection capacity in the controller, and a PLC already serving an existing panel can be close to its cap. The controller manual states the capacity.

Put the annunciator on Ethernet and leave each PLC's serial port, with its existing panel, untouched. That means one Ethernet option card per PLC, then two direct runs or a small switch. IEEE 802.3 sets 100 m as the limit for one twisted-pair copper segment, which covers any run inside a normal building. If serial is the only path your hardware supports, an RS-422 multidrop bus can carry several display panels on one bus where the protocol allows it; wiring one HMI panel to three PLCs on an RS422 multidrop walks through that layout.

HMI panel and panel PC side by side

Where each option wins

The second HMI panel wins most annunciator jobs. An annunciator shows a handful of alarm and status lines, so a small panel covers it, the editor is already on your bench, and the hardware list is one panel, two option cards and either a switch or two cables. One box to mount, one power feed, one project to back up.

The panel PC route wins when the annunciator is more than an annunciator. If the same screen must also carry trend history, a large-format layout no panel family sells, or a page you will later fold into plant SCADA software, run the HMI application on an industrial panel PC and feed a monitor sized for the wall. Expect more failure points: PC, monitor, video cable and OS upkeep.

A digital signage display sits on the monitor side of this line and is the wrong device for this job. It renders its own scheduled content and is not an HMI; it will not poll your controllers, and our units take no video in either, since the HDMI port is an output. If a separate project needs a plant notice board that plays its own pages, that is the product we sell, starting at the shop-floor signage category.

When part of the wall only needs to mirror states that change slowly and does not need live controller data, plan an offline sensor status display for your shop floor covers that simpler build.

Worked example: 8 alarm lines read from 40 ft

Inputs, and where each comes from:

• 40 ft viewing distance: the declared case for this example, an across-the-aisle read in a small assembly shop.
• 1 in of capital letter height per 20 ft for fast reading: the safer version of the FHWA planning figure, which is 1 in per 30 ft.
• Line spacing of 1.7 × capital height: part of the same planning figure.
• 8 alarm lines, 4 from each PLC: the declared content requirement.
• One 24-inch 1920 × 1080 monitor with 16:9 proportions: a typical screen for this job.

The rule in words: capital height in inches equals distance in feet divided by 20. Line pitch equals 1.7 times capital height. Required stack height equals line count times pitch.

The arithmetic:

• Capitals: 40 ÷ 20 = 2.0 in. On the FHWA planning figure: 40 ÷ 30 = 1.33 in.
• Pitch: 1.7 × 2.0 = 3.4 in. On the planning figure: 1.7 × 1.33 = 2.27 in.
• Stack for 8 lines: 8 × 3.4 = 27.2 in. On the planning figure: 8 × 2.27 = 18.2 in.
• Viewable height of the monitor: 24 × 9 ÷ √(16² + 9²) = 216 ÷ 18.36 = 11.8 in.
• Lines the monitor carries: 11.8 ÷ 3.4 = 3.5, so 3 lines. On the planning figure: 11.8 ÷ 2.27 = 5.2, so 5 lines.

Result: at 40 ft the 24-inch monitor shows 3 alarm lines under the fast-reading rule and 5 under the FHWA planning figure. The 8-line requirement needs 27.2 in of stack height, which no single screen of this class provides.

Decision, with its threshold: when lines × 1.7 × (distance ÷ 20) is greater than the panel's viewable height in inches, cut lines, split the display or shorten the distance. Here, moving the annunciator to the line for a 10 ft read gives capitals of 10 ÷ 20 = 0.5 in, pitch of 1.7 × 0.5 = 0.85 in, and a full 8-line stack of 8 × 0.85 = 6.8 in, which fits with margin. Keeping the distant screen means condensing to one line per PLC: 2 × 3.4 = 6.8 in, with the detail list left on the local HMI panels.

The screen size and viewing distance guide covers this arithmetic for signage generally; the annunciator version is the same sum with alarm lines.

What to confirm before you order

Work down this list with both manuals open:

• The simultaneous-connection limit for your exact panel model and editor version.
• The controller's connection capacity where the PLC already serves another client; on EtherNet/IP that is its CIP connection capacity.
• The driver for each PLC's protocol and processor family, at the firmware generation you run.
• The Ethernet option card for each PLC, or the switch and cable plan, keeping each copper run inside 100 m.
• The tag list for both controllers: address, data format and meaning, plus a source label for every block on the screen.
• The screen size, from the worked example threshold above.

One safety note: do not unplug an existing panel on a running machine to test an unconfirmed topology. Confirm ports and cable types from the manuals and the installed hardware first, and leave both live panel connections as they are until the new annunciator path has passed its checks.

For the wider rules on a screen that only shows process values, specify a read-only process display for a plant collects them.

The decision

For a display-only annunciator reading two independent PLCs, buy the second HMI panel when your editor allows two simultaneous connections and the content is a handful of alarm lines. Add one Ethernet option card per PLC and run the sizing threshold before you fix the screen size. Buy the panel PC and monitor when the same wall needs a size, a layout or a history function no HMI family gives you, and treat monitor, PC and software as one purchase. The bare Ethernet monitor is not a third option; it is the screen half of the second option.

Recommendation: reuse the HMI family you already own for the annunciator, with one connection entry per PLC and one Ethernet option card in each controller, provided its editor supports two simultaneous connections on the panel model you buy. Step up to an industrial panel PC with HMI software and a separate monitor only when the wall needs a size or a function the panel families do not sell. Size the text at 1 in of capital height per 20 ft of viewing distance with 1.7 line spacing, and when lines × 1.7 × (distance ÷ 20) outruns the panel height, shorten the distance or cut lines before you buy a bigger screen.

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