Touchscreens have changed the way operators work with industrial machines. A good touch interface is intuitive, shortens training and makes operators more productive. But one of the most important choices behind that screen is easy to overlook: the interface between the display and the controller board. It affects performance, durability and reliability.
Screen size, resolution and touch technology all matter when choosing a display. In this article we focus only on the interface, the link that carries information between the machine and its operator.
The choice isn’t simple. General-purpose buses like I2C, SPI and USB sit alongside display-specific links like DSI, LVDS, eDP and HDMI, and each has strengths and weaknesses. Below we group the common options and then look at each one in turn.
Ways to categorise display interfaces
Universal or image transfer
- Universal interfaces carry any kind of data at relatively low bandwidth. Examples: I2C, SPI, USB.
- Image transfer interfaces are designed specifically for video. Examples: HDMI, DSI, LVDS, eDP.
Parallel or serial
- Parallel interfaces send many bits at once over separate wires, clocked together. Example: parallel RGB.
- Serial interfaces send data as bit streams over one or a few lanes. Examples: LVDS, DSI, eDP, HDMI, I2C.
Internal or external
- Internal interfaces connect a display and controller that share one enclosure. Examples: parallel RGB, DSI, LVDS, eDP, V-by-One.
- External interfaces connect two separate devices with a cable. Examples: HDMI, DisplayPort, VGA.
The common interfaces in detail
HDMI
HDMI (High-Definition Multimedia Interface) is a serial, external, image-transfer interface that carries video and audio between devices.
Pros
- Widely supported, so it is easy to find displays that work with a given single-board computer (SBC).
- High-quality video, suitable for multimedia applications.
- Carries audio as well as video.
- High refresh rates, many supported resolutions and wide device support.
Cons
- No touch data. Touch usually needs a separate USB connection, which is less integrated.
- Relatively costly.
DSI
DSI (Display Serial Interface) is a high-speed serial interface defined by the MIPI Alliance and widely used in phones and embedded devices. It is a serial, internal, image-transfer interface.
Pros
- Designed for mobile and embedded devices, so it is power-efficient.
- The native interface of the official Raspberry Pi touchscreen displays. On those displays, touch data travels over I2C through the same connector, so no separate USB cable is needed.
- Moderate cost and high refresh rates.
Cons
- Fewer displays to choose from than HDMI.
LVDS
LVDS (Low-Voltage Differential Signalling) sends data as serial streams over differential pairs. Display links built on it use several pairs side by side to carry pixel data. It is an internal, image-transfer interface and is common in industrial displays.
Pros
- High-speed data transfer, suitable for responsive, smooth displays.
- Low power consumption.
- Good noise immunity and robust signal transmission.
Cons
- Less common in consumer displays, so compatible panels can be harder to find.
- Needs more configuration than plug-and-play options like HDMI.
- Can cost more and bring compatibility challenges.
eDP
eDP (Embedded DisplayPort) is a high-speed serial interface for connecting built-in displays, and is the standard panel interface in laptops. It is an internal, image-transfer interface.
Pros
- Supports high resolutions and high refresh rates.
- The primary interface on many compact, high-resolution panels.
Cons
- Fewer options, and harder to source, than HDMI or DSI displays in small quantities.
- SBCs such as the Raspberry Pi have no eDP output, so an adapter board is needed.
- Complexity, cost and cable-length limits need to be weighed against the application.
Parallel RGB
Parallel RGB (often called TTL or DPI) sends the red, green and blue bits of each pixel on separate lines, together with clock and sync signals. It is a parallel, internal, image-transfer interface.
Pros
- Simple and widely supported by small TFT panels and many processors.
- Low cost.
- Good image quality at small and medium resolutions.
Cons
- Uses many signal lines, which makes cabling bulkier and uses more processor pins.
- More power than serial links at higher resolutions.
- More susceptible to electromagnetic interference, so it needs careful layout and shielding, and cables must be short.
I2C
I2C (Inter-Integrated Circuit) is a two-wire serial bus that can connect several devices, each with its own address. It is a universal, internal interface. In touchscreens it is most often used for the touch controller, and it can drive only very small displays such as character or small OLED modules.
Pros
- Only two wires, so wiring is simple and easy to troubleshoot.
- Several devices can share the same bus.
- A widely supported standard.
- Low power and low cost.
Cons
- Low data rates, unsuitable for graphics-rich or high-resolution displays.
- Low refresh rates.
Wrap up
Each interface has its place. HDMI is the easiest to source and prototype with. DSI is a strong fit for compact embedded designs. LVDS and eDP suit robust, high-resolution built-in displays. Parallel RGB is simple and inexpensive for small panels. I2C is best left to the touch controller.
If you’re choosing a touchscreen and controller for a machine, get in touch. We’re happy to help.