20 Sep 2017
Fast & Straightforward Implementation of Serial Interfaces
Hans-Peter Lüdeke of Murata Europe looks at serial interfaces which are in common use and how to make their implementation more straightforward.
Serial interfaces are in widespread use in electronic devices for communication with external devices. For the majority of these interfaces, standard solutions are available today based on ICs and discrete components.
In specific cases, however, it may make sense to replace a discrete approach with an all-in-one interface module providing a turnkey solution with minimum development overhead.
Electric devices need interfaces to communicate with their environment or with other devices. Serial interfaces are prevalent for this purpose because they are often easier to handle. Depending on the application sector and the transmission capabilities required, multiple standards have entered the market over the last decades. These standards are generally based on different hardware and especially on different software protocols. Thus, it is no surprise that device designers often need to implement a variety of software protocols to enable communication with multiple devices using today’s standards and maybe to ensure compatibility with legacy devices.
Problems that may result from the spatial configuration of communicating devices and the existing electrical installation often lead to the use of a galvanically isolated interface implementation. Possible issues include potential differences between the grounding points of a large electrical installation. Devices often get their power supply from different domains of the electrical installation. This may result in communication faults in serial interfaces including RS-485. Possible ground loops impacting the signals to be processed are a general problem that galvanic isolation can mitigate significantly.
Legal regulations can be another reason for choosing an isolated interface implementation. For instance, the legislator mandates that all interfaces of medical devices meet the requirements of electrical safety, especially in case of possible patient contact.
Isolated Power Supply
If an isolated interface implementation is required, the corresponding data transfers must use a signal isolator, while the power supply must be galvanically isolated using a DC/DC converter.
Discrete Implementation or Turnkey Module?
Many manufacturers of signal isolators, which are often implemented as integrated circuits (ICs), also provide examples of the implementation of the necessary DC/DC converter. It is common to build a straightforward, discrete SMPS consisting of switches, driver logic, filters and a transformer providing galvanic isolation. Significant board area may be required because of the discrete implementation, which can be an important consideration if space is at a premium. As an example, Figure 1 illustrates a galvanically isolating transformer that is listed in the reference design of an interface IC manufacturer and is available as a standard component from Murata.
Figure 1: 78253 standard transformer series from Murata
As another widespread alternative, ready-made, isolated DC/DC converter modules are available in different packages, for different isolation voltages and for the most common combinations of input and output voltages. Advantages of these turnkey modules include reduced development overhead (by using a pre-tested and pre-characterized component instead of at least a handful of components whose interaction can only be tested after finalizing the layout) and improved procurement security, because standard footprints have established themselves in the market for these converters from 1 to 3W, which are available from different vendors.
Interface Module as an All-in-One Solution
As another design simplification enabling hardware designers to focus on their core tasks, modules can combine a signal isolator and a DC/DC converter. Manufacturers of such all-in-one modules, including Murata, can thus ensure that the components interact optimally and that integration is advanced more and more. Users therefore have a very straightforward option to implement an isolated interface reliably, quickly and at very low component count.
Cost is often cited as a disadvantage of modules and especially of all-in-one solutions. It is true that the purchase price of a module is higher than the sum of individual components. However, it is often overlooked that modules entail almost no additional costs for development, testing and any applicable safety certifications. In addition, procurement and inventory are much easier.
Considering all aspects, selecting an all-in-one module may be the better alternative especially for applications manufactured in volumes of less than tens or hundreds of thousands annually.
Figure 2: NM485 All-in-One solution for an RS-485 interface with additional remote supply voltage
Design Simplifications Enabled by Additional Functions
This applies even more if the all-in-one solution provides additional functions enabling cost reductions in other parts of the system design.
As a good example, Murata’s NM485 interface module enables the implementation of an isolated interface according to the RS-422 or RS-485 specification. Apart from the basic function (isolation of the data interface), the module provides two additional isolated supply voltages at the output side, including +5V DC and ±6V DC. The latter can be forwarded using the data cable as a kind of remote supply voltage for the electronics of the next device. Material and installation savings can result if a separate supply voltage in the slave device is no longer needed thanks to this feature.
As another example, the NMUSB interface module of the same manufacturer enables the straightforward implementation of an isolated USB interface. In particular, USB interfaces are mostly implemented without isolation because users are mostly working in office and computer environments. However, the prevalence of this interface results in an increasing number of applications that use the USB interface for data-capturing tasks, even if galvanic isolation is required for safety reasons (electrical safety or noise de-coupling). All-in-one modules like the NMUSB are the ideal choice for these applications, especially with their additional hub functionality (1 USB input, 2 USB outputs). Providing 2,5W at both outputs, they enable a seamless connection of higher-power loads directly to the interface.
Fig. 3: The NMUSB dual USB hub as a module solution
Different module options available for the implementation of isolated serial interfaces can make the design process easier and faster. As the most widespread alternative, turnkey DC/DC converter modules can be used instead of a discrete solution. Suited to a multitude of interfaces, this approach results in simplifications for the board layout, the certification of the final device regarding electrical safety, and material procurement.
Available in the market only recently, all-in-one solutions for specific isolated serial interfaces combine the data interface and the corresponding power supply in a single module, further simplifying the application and accelerating the design process. Due to their often highly compact form factor and possible additional functions including remote supply voltages, these products save board space and lead to potential savings in the rest of the electrical installation.
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About the author
Hans-Peter Lüdeke is a Senior Product Manager - Power Supply, Murata Europe. He joined Murata in 2011 in a technical sales role. He previous roles with Lineage Power and Tyco Electronics have all involved responsible for power electronics. At Lucent Technologies he was responsible for designing power supplies. Hans-Peter graduated from the University of Paderborn with a PhD in electrical engineering.
Murata was founded by Akira Murata as a personal venture in Nakagyo-ku, Kyoto-shi, Japan in October 1944. Since then the company has grown to be one of the world’s largest component manufacturers, producing everything from ceramic capacitors through to piezo-electric, ceramic and SAW filters, connectors, isolators, inductors and sensors. The company also produces a he volume of modules including power supplies in which they have gained significant experience and as a result have a major percentage of the global market.
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