Run one HMI panel on serial and Modbus TCP together

Yes, one panel runs both drivers

In the panel project, device connections are added one at a time, and each carries its own protocol, port and driver. Add one connection set to the PLC's serial protocol on the panel's serial connector, and one connection set to Modbus TCP on the Ethernet port, aimed at the drive's IP address. Each connection keeps its own poll interval, so the serial link can run at a relaxed pace while the drive link polls faster.

Two rules keep the combined machine honest. First, list every signal that crosses between the systems: what it is, which device sources it, which device receives it, and which device is allowed to write it. One owner per Start coil, one owner per Stop coil, one owner per shared value. Second, leave each machine's local logic untouched. With the link between the cabinets unplugged, each system still starts, runs and stops from its own panel, because that is how the two systems behave when they are sold separately. The cross link carries operator commands and display values, nothing else.

Deciding which points the panel gets is the first half of the job. The walkthrough on planning a touchscreen operator interface covers the points list and the ownership question in detail.

What you need

• The panel project software, with a driver in its list that matches the PLC's serial protocol. The check of the PLC serial protocol before replacing a legacy HMI shows how to make that match before any wiring starts.
• A Modbus TCP driver for the drive, which this class of panel carries as standard.
• The PLC serial port's settings from its manual: baud rate, parity, data bits, stop bits, and the port mode that selects the protocol function rather than the programming function.
• The drive's Modbus TCP register map: addresses, data types, scaling, and which registers accept writes.
• A serial cable built to the wiring both manuals print.
• An Ethernet switch, or two ports documented to auto-detect the cable type.
• A spare test bit in the PLC wired to nothing, and a safe test register in the drive.

Wire the serial link to the PLC

1. Use the free serial port on the PLC and the panel connector its documentation labels as the PLC port. Treat both ends as RS-232 unless a manual says one of them offers RS-422.
2. Build the cable to the wiring the two manuals print. A connector that fits both ends can still be wired wrong, so check it conductor by conductor before power goes on.
3. Set the panel driver and the PLC port to matching parameters: baud rate, parity, data bits, stop bits, and the same protocol mode. Use values both devices list as supported.
4. Confirm the PLC port is in its protocol mode. A port left in its programming mode answers nothing on the cable.
5. The 9 to 15 m separation decides this link's interface and rate. The worked example below runs the numbers for RS-232, and the per-interface figures sit in the article on HMI to PLC serial cable maximum distance by interface.

Add the drive link on Modbus TCP

1. Connect the panel and the drive to the same network through a switch, or port to port if both manuals document auto-detection of the cable type.
2. Give each device a unique IP address on one subnet and write the plan down: panel, drive, and the PLC's Ethernet module if it has one. Static addresses leave room for the next device on the network.
3. Set the drive connection's protocol to Modbus TCP, enter the drive's IP address as the target, and leave the TCP port at 502 unless the drive's manual says otherwise. Port 502 is the fixed well-known port the Modbus TCP protocol reserves.
4. Map each value through the drive's documented register map: address, data type, scaling, and read or write access. Take these from the drive manual, because a wrong data type still updates on screen and reads as nonsense.
5. Check the drive permits writes to the registers you mapped. Drives commonly separate monitoring registers from control registers, and a write aimed at the wrong side is refused or ignored.

If drive values arrive but fall behind, shorten the poll on the values the operator acts on and lengthen it on display-only values. The fix walkthrough for Modbus TCP data that is slow to update on an HMI panel takes those symptoms apart.

Worked example: the serial run at the RS-232 limit

Inputs. Your two cabinets sit 30 to 50 ft apart, and the serial link spans that gap, so it is designed to the worst case of 50 ft. Both ends are RS-232. The port rate is 19.2 kbps, the common standard rate just under the interface ceiling. The interface rules used: RS-232 supports data rates up to 20 kbps at a maximum cable length of 15 m (50 ft), with longer runs traded for slower rates; behind that sits the TIA/EIA-232-F limit of 2500 pF total load capacitance on the line, which corresponds to roughly 20 m of typical cable.

Rule. An RS-232 link is inside its limits when the cable length is at or under the length allowed at the selected rate, 15 m (50 ft) at 20 kbps, and the cable's total capacitance stays within the 2500 pF budget.

Arithmetic.

• 50 ft × 0.3048 m/ft = 15.24 m, which sits on the 15 m (50 ft) limit; the 15 m reading is the conservative one.
• 30 ft × 0.3048 m/ft = 9.14 m, which is 5.86 m inside the 15 m limit, a 39% length margin.
• 19.2 kbps sits under the 20 kbps ceiling.
• 15.24 m of ordinary serial cable stays inside the 2500 pF budget, which fills at about 20 m.

Result. At 30 ft the link passes with margin. At the full 15.24 m it passes with nothing to spare: the run sits at the rated length, so the cable must be one finished piece, connector quality matters, and extensions are out.

Decision. Run the serial link on RS-232 at 19.2 kbps or slower when the run is 15 m or less. Above that, change the interface: RS-422 for a longer copper run, or carry the link over the Ethernet network, where one twisted-pair segment is good for 100 m.

Check each direction on spare values

1. PLC read: point the panel at a PLC value that changes on its own, such as a clock or counter register, and confirm the number on the panel tracks it.
2. Panel to PLC write: write the spare bit that is wired to nothing, and watch it change in the PLC's online view while the machine ignores it. Never make the production Start coil the first write; a correctly formed write on the wrong address can start a machine.
3. PLC to panel write: have the PLC write a known test value into a panel variable, and confirm the panel displays it. Confirm first that the serial driver accepts PLC-side writes to panel variables; some panel data models only take panel-side writes, and the supported workaround is an exchange area in PLC memory that the panel reads.
4. Drive read: display a known drive value on the panel.
5. Drive write: perform one safe register write at the drive with motion not enabled, and confirm the value in the drive's own view.
6. Record a pass for each direction, and confirm the observed destination address in the PLC or the drive, not just a changing number on the panel.

The same order holds across a machine link: check writes before motion, which is exactly the sequence the operator panel PLC link setup checklist walks through.

What each failure points to

• PLC values never reach the panel: check the driver match first, then the port mode and the parameter match, then the cable wiring. The walkthrough on an HMI panel not talking to the PLC over a serial settings mismatch follows that order.
• The panel reads the PLC but the PLC cannot change a panel variable: the driver or data model lacks PLC-side writes, so use the PLC exchange area described in the check section.
• The serial link works and the drive values are missing: check the Ethernet link lights, the IP addresses and the subnet, then the register map. A duplicated IP address produces faults on both devices at once.
• Values arrive but lag: adjust the poll intervals before touching any wiring, shortening them on operator values and lengthening them on display-only values.

The decision

Recommendation: Keep the two links as two drivers in one panel project, one serial connection to the PLC and one Modbus TCP connection to the drive over Ethernet, give every Start and Stop coil and shared value a single owner, and pass the spare-value checks on both links before any real command reaches the panel. Run the serial link inside the RS-232 limit from the worked example, at the port rate both devices share; when the gap outgrows that limit, carry the link on RS-422 or over the Ethernet network. Leave each machine's logic able to run with the cross link unplugged, so the set still works as two systems when it is sold that way.

Questions