Selecting an HMI communication method

How should your HMI talk to the PLC or other controller? You have several main options with differing functionalities based on your application needs.


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If you’re a machine builder or end user, you may find yourself facing numerous challenges when connecting a human machine interface (HMI) to other industrial products. Perhaps the biggest problem is deciding which solution both best satisfies the application requirements and doesn’t break the budget. There are many good solutions to choose from that are based on serial, integrated, Fieldbus, and Ethernet communication technologies. Here is an overview of each of these solutions with some suggestions on applicability.

Serial-based connections are by far the simplest and most cost-effective solutions available. They can handle fair amounts of data over very short distances (<30 meters) and don’t require a lot of expertise or hardware knowledge to complete a configuration or implementation. Virtually all equipment manufacturers offer a serial solution, and there is a high level of commonality of communication cables and connectors (9 pin, 25 pin Dsub., twisted pair wiring, etc.), which helps extend the number of applicable devices and the overall flexibility of the solution for all point-to-point applications.

The integrated communication solution is another method to connect modular controller products to HMIs. Less universal than serial, some manufacturers can integrate the HMI directly onto the high-speed backplane of the modular controller. These connections handle larger amounts of data than a typical serial solution with a similar distance limitation (<37 meters). They are also very flexible and well suited to provide point-to-point communication and connections to multiple HMIs.  Like serial connections, they provide for uncomplicated wiring and common connectors. This solution has been very popular for many machine builders and users for a number of years.

If distance is a factor, you can use one of the common fieldbus technologies to connect HMIs to industrial devices. These typically handle a similar amount of data as a serial connection, but over much longer distances, spanning up to 1,200 meters in some cases. These solutions are usually more challenging and require more expertise and specialized fieldbus or network knowledge to implement properly. Fieldbus connections are generally a bit more complex than a serial or integrated solution but can be a very good alternative for many distributed architectures. Additional advantages for these types of solutions include common communication architecture, reduced wiring, and modest expansion capabilities.

Table: Connectivity options; Source: Mitsubishi Electric Automation

Ethernet-based connections have grown rapidly in popularity over the years and have become the second leading technology after serial-based connections. Ethernet-based solutions handle very large amounts of data quickly (10 Mb/100 Mb baud rates) over moderate distances (<100 meters). Ethernet solutions vary in complexity as does the expertise necessary to support the installation. Additional advantages for these types of solutions include low hardware costs (cables, connectors) and flexible architecture with many wiring and expansion options for additional interfaces.

Ultimately, there are many solutions to choose from, so keep some of the advantages in mind when considering a suitable HMI connection strategy. Choose a solution that best aligns with your configuration and satisfies your needs today and into the future.

Timothy Lomax is HMI product marketing manager for Mitsubishi Electric Automation.


For more information, visit:

Control Engineering: Optimizing Your HMI, April, 2011

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