PLC RS485 Connect Two HMIs: Wired & Wireless Modbus RTU Guide

In many industrial automation scenarios, there is a need to control a single PLC from multiple operator interfaces. A common challenge arises when one HMI is installed locally at the control cabinet, while a second portable HMI is required for remote operation—sometimes up to 1 kilometer away. This article explores a practical solution using RS485 and Modbus RTU to connect two HMIs to one PLC, with one wired and one wireless link.

Understanding the Hardware Setup

Consider a valve control application: a local control box houses a PLC (e.g., a rugged wide-temperature model) and a touchscreen HMI. The operator can adjust the valve opening on site. Additionally, a mobile portable control box with another HMI, powered by a 24V battery, is needed to control the same valve from a distance of up to 1 km. The PLC typically has a single RS485 port, and the question is how to share it between two HMIs.

The initial idea might involve Ethernet-based wireless modules, but for 1 km range, standard Wi-Fi or Ethernet radios often fall short without expensive directional antennas. A more robust and cost-effective approach is to use serial communication over RS485 with Modbus RTU protocol, which is designed for multi-drop networks and can be extended wirelessly using dedicated radio modems.

RS485 Multi-Drop: The Foundation

RS485 is a differential serial standard that supports up to 32 devices on a single bus (with repeaters, even more). In a typical Modbus RTU network, one master (the PLC) communicates with multiple slaves (HMIs, drives, etc.). However, in this scenario, both HMIs need to act as Modbus masters to read and write PLC registers. This is possible because Modbus RTU allows multiple masters if they are coordinated, but a simpler method is to configure the PLC as a Modbus slave and both HMIs as masters. The PLC will respond to requests from either HMI, but careful timing is required to avoid collisions.

The RS485 bus uses two wires (A and B) plus a common ground. All devices are connected in parallel (daisy-chain). The bus must be properly terminated at both ends with 120-ohm resistors to prevent signal reflections. For a mixed wired/wireless setup, the local HMI is connected directly via a short RS485 cable, while the remote HMI is linked through a pair of wireless serial modems.

Wireless Extension Using Radio Modems

For the 1 km wireless link, industrial serial radio modems operating in the 433 MHz or 915 MHz ISM bands are ideal. These devices create a transparent RS485 tunnel, meaning whatever is sent on one end appears on the other. They support Modbus RTU natively and can achieve ranges of several kilometers with clear line of sight. Models like the DTD418M (if using RS485 version) or similar from brands like Digi, Moxa, or Advantech are commonly used.

The local wireless modem is connected to the RS485 bus alongside the PLC and local HMI. The remote modem is connected to the portable HMI. Both modems must be configured with the same RF channel, baud rate, and data format. It is crucial to set the radio modems to a low latency mode to ensure quick response times for HMI polling.

Addressing and Collision Avoidance

Since both HMIs are masters, they must not transmit at the same time. Modbus RTU does not have built-in collision detection. The simplest method is to configure each HMI with a different polling interval and a timeout/retry mechanism. For example, HMI 1 polls every 500 ms, HMI 2 every 700 ms. If a response is not received, the HMI waits a random backoff time before retrying. Some advanced HMIs support token passing or can be set to listen-only mode, but in most cases, careful timing works reliably.

Another approach is to use a Modbus gateway or a small PLC as an intermediary that coordinates requests, but this adds complexity. For a simple two-HMI system, the timing method is sufficient.

Practical Considerations and Troubleshooting

When implementing this setup, ensure the RS485 bus is not too long. The total cable length should be under 1200 meters at 9600 baud. Use twisted-pair shielded cable (e.g., Belden 3105A) and connect the shield to ground at one point only. If the local HMI is very close to the PLC, a direct RS232 connection might be an alternative, but RS485 is preferred for multi-drop.

If the PLC has only one physical RS485 port, you can use a simple terminal block to branch the A and B lines to the local HMI and the wireless modem. No special splitter is needed. However, if the distance to the local HMI is significant, consider using an RS485 repeater to boost the signal.

For the wireless link, test the signal strength and latency. Obstacles like buildings or metal structures can reduce range. In some cases, a higher-gain antenna or a repeater radio may be necessary. Always perform a site survey before final installation.

Alternative Architectures

If the PLC supports Ethernet, another option is to use a Modbus TCP to RTU gateway. The local HMI could communicate via Ethernet, while the remote HMI uses a wireless Ethernet bridge. However, for 1 km, a wireless serial link is often more reliable and cost-effective than Wi-Fi. Cellular or proprietary 900 MHz radios are also viable.

In some cases, a single HMI with a web server or VNC capability can be accessed remotely via a tablet or laptop, eliminating the need for a second HMI. But this requires a network infrastructure that may not be available in remote locations.

Conclusion

Connecting two HMIs to one PLC RS485 port is entirely feasible using Modbus RTU multi-drop wiring. By adding a pair of industrial wireless serial modems, one HMI can be placed up to 1 km away, providing flexible remote control. Proper configuration of device addresses, baud rates, and polling intervals ensures smooth operation without data collisions. This solution is widely used in water treatment, oil and gas, and factory automation where local and remote operator stations are needed.

Always refer to the manufacturer’s documentation for your specific PLC and HMI models, and test the system thoroughly before deployment. With careful planning, you can achieve a robust and scalable control system.