Nowadays, displays are an integral part of electronic devices in many industries – from home appliances to medical diagnostic devices to operator panels (HMIs) and complex embedded systems used in industry. In order for the image generated by the processor or controller to be displayed on the screen, it is necessary to use a suitable communication interface for signal transmission. Without a smoothly functioning interface, the transmission of digital image data would not be possible, so choosing the right interface is crucial for the proper operation of the entire end device.
Interfaces and connectors play a key role in the process of visualizing information on LCD-TFT screens used in industrial equipment, as their proper selection determines the compatibility of devices and the quality and smoothness of the image. In this article, we will take a closer look at the most popular interfaces used in industrial displays. We will discuss their applications, parameters, advantages and disadvantages. The popularity ranking of interfaces has changed over the years as new technologies and standards emerge. So we will look at the current state of affairs and trends for the future. We hope that the information in the article will allow you to make an informed choice of the right interface for your specific applications.
Interface vs. protocol
To begin with, it is worth clarifying the difference between an interface and a protocol. The protocol defines the rules for exchanging information and encoding data, that is, it defines the syntactic rules of communication – how data should be interpreted on both sides of the interface. The interface, on the other hand, defines the transmission medium – the type of connection and type of signals, i.e. the physical way of connecting devices and the means of data transmission. A simple portable comparison: a protocol is a language, an interface is a voice.
Interfaces fall into two categories: internal, used for communication between components within the device (including displays and touch panels), and external, used to connect separate devices (such as a computer with a monitor). Interfaces can also be divided into universal interfaces for general data exchange, as well as those dedicated strictly to the transfer of digital image information (image transfer interfaces). The latter must provide much higher bandwidth for smooth refreshment of moving images.
Internal universal interfaces
Internal universal interfaces include. SPI, I2C, RS232 and UART. The name “universal” means that they can transmit various other types of data in addition to images. Their bandwidth is usually insufficient for smooth transmission of high-definition video at high frame rates. This is because the amount of information that needs to be transmitted in a unit of time to generate a smooth FHD or 4K image is very large.
For example, a 10-inch display with a resolution of 1280 x 800 pixels and 8-bit color depth, refreshed at 60 Hz, requires a transmission of about 250 million bits per second. This is a data transfer of 2 Gbps. Meanwhile, universal communication interfaces typically reach speeds on the order of kbps or Mbps.
For this reason, interfaces such as SPI, I2C, RS232 or UART tend to be used in simple, low-resolution displays where the amount of information to be transmitted is not large. They are also used in communication with other components, such as memories or I/O circuits. Their advantage is their simplicity and universality of use.
- SPI (Serial Peripheral Interface) is a serial interface based on master-slave architecture. It uses the MOSI and MISO data lines, as well as the clock line and the SS device selection line. Communication is synchronous. SPI has a relatively high transmission rate (as high as 37.5 Mbps), but limited resistance to interference at higher frequencies. Used in small displays with resolutions up to approx. 320×240 pixels.
- I 2 C (Inter-Integrated Circuit) is a two-line serial communication interface between integrated circuits. It uses the SDA data line and the SCL clock line. Communication is done in multi-master mode (multiple masters can initiate transmission). I2Cis characterized by slow transmission and susceptibility to interference at longer distances. Used, for example, in simple OLEDs or touch screens, where the amount of data transferred is small.
- RS232 and UART are serial interfaces of simple design, using two data lines each. Used in devices with low-resolution displays due to bandwidth limitations. Their advantages are reliability and simplicity of implementation.
Internal interfaces for image transmission
The task of interfaces dedicated specifically to image transmission is to ensure fast, smooth transmission of the large amount of data necessary for high-frequency screen refresh. Among the most popular today are RGB, LVDS, MIPI, Vx1 and eDP.
These are differential interfaces (except for RGB), i.e. using symmetrical signal transmission over pairs of wires. This gives high resistance to interference, which translates into the ability to transmit signals over long distances without loss of quality. These interfaces allow transfer rates in the hundreds of MHz or even GHz, which is essential for high-resolution screens with refresh rates above 60 Hz. They are used in both large industrial displays and consumer devices.
- RGB (Red, Green, Blue), in which the name comes from the colors sent in parallel to the screen: red, green and blue is based on parallel data transmission using multiple wires, with each data line transmitting one-bit RGB color intensity information for one pixel. Therefore, the 24-bit variants use 24 wires for each pixel. In theory, the interface could be fast, but due to the lack of differential, the transmission speed is limited to 1.2 Gbit/s, and even worse, the need for more wires in the configuration increases susceptibility to electromagnetic interference. RGB usually works with small diagonal screens – less than 7 inches, as the small image resolution becomes a limitation.
- LVDS (Low Voltage Differential Signaling) is currently the most common interface for image transfer in industrial LCD-TFT displays. Its dominant position is due to its versatility of application and proven, stable performance over the years. Data is transmitted in series, differentially (symmetrically) over pairs of wires, and the conversion occurs at the receiver. This allows high-speed transmission with immunity to interference even over long distances. The maximum data transfer rate for LVDS in a point-to-point configuration (with one transmitter and one receiver) is approx. 3.125 Gbps.
- MIPI DSI (Mobile Industry Processor Interface – Display Serial Interface) is a state-of-the-art interface designed for mobile devices where low power consumption is crucial. It is also increasingly appearing in industrial applications (e.g., in portable measuring equipment). Like LVDS, it uses differential signal pairs. It is distinguished by two-way communication and very low power consumption. This makes it suitable for battery-powered mobile devices. The maximum data transfer rate for MIPI is approx. 10 Gbps.
- Vx1 (V-by-One) is a very high-speed interface for high-resolution displays, such as 4K. Like LVDS and MIPI, it is based on differential data transmission over pairs of wires. It does not require a separate pair for the clock signal. This interface is even more resistant to electromagnetic disturbances than LVDS. Used in large UHD (3840×2160) TVs and monitors.
- The eDP (Embedded DisplayPort) is an interface increasingly common in modern industrial displays. In terms of data transmission, it resembles LVDS and is considered its successor. The maximum data transfer rate for eDP is approx. 20 Gbps. Suitable for applications with 4K and higher matrices. The eDP interface is also relatively immune to electromagnetic interference.
Comparison of interface parameters
To better illustrate the differences in the capabilities of the various interfaces, below is a table of maximum data rates:
|Interface||Maximum data transmission speed|
|DVI-D dual link||25 Gbps|
As you can see, interfaces specifically designed for image transfer allow transfers an order of magnitude faster than universal communication buses. This translates into the ability to work smoothly with matrices with high resolution, color depth and fast refresh rates.
New technologies are gradually displacing older solutions. For example, MIPI and eDP are slowly replacing LVDS, which has been popular for years, in new designs. Vx1 is used in top monitors and TVs. There is a clear trend toward increasing transfer speeds to meet increasing demands for image quality. Now let’s look at the interfaces at the end of the table, i.e. those with the highest data throughputs.
External interfaces are used for communication between different devices, such as a computer and a monitor. Among the most popular standards today are:
- USB-C enables high-speed video and audio transmission. An additional advantage is the ability to transmit considerable power, up to 100W. This makes it possible to power the monitor directly from the computer with a single USB-C cable. This is by far one of the most forward-looking interface solutions – currently transfer speeds reach 80 Gbps.
- HDMI (High Definition Multimedia Interface) is a commonly used interface for transmitting digital video and audio. There are different variants of HDMI connectors – standard, mini and micro. The maximum data transfer rate for the latest HDMI 2.1 is as high as 48 Gbps. Supports resolutions up to 10K. It is backward compatible with older versions.
- DVI (Digital Visual Interface) is a digital visual interface, similar in signal to HDMI, but without built-in audio capability. It comes in several varieties, including. With optional analog VGA interface. The maximum data transfer rate for DVI-D dual link is 25 Gbps.
Expanding the capabilities of displays using AD Board
In addition to standard interfaces, many display manufacturers offer the ability to expand their functionality using additional PCBs called AD Boards. These boards mount directly on the display and allow for additional interfaces such as HDMI, VGA, DVI or audio. Some manufacturers, such as. Litemax, offer ready-to-use display kits with pre-installed AD Boards. Their big advantage is simple integration and immediate readiness for operation.
It is also possible to mount the AD Board on standard displays. This allows you to extend the functionality of the display according to the specific needs of your project. Our engineering team is ready to modify and expand virtually any commercially available display using AD Board.
We hope that the information presented in the article will be helpful in selecting appropriate interfaces for specific applications in modern electronic projects. Feel free to contact us if you have any questions related to the topic of interfaces or to discuss custom solutions.
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