The most challenging part when working with dot matrix screens is the interface circuit. Compared to a backlight circuit, which is relatively simple, the screen interface can be quite complex. A typical backlight board usually only requires three lines: power, ground, and a backlight enable signal. Most of the time, these signals are high-level, making it straightforward to handle.
LVDS (Low Voltage Differential Signaling) is a type of interface that evolved from TTL. For example, a single 6-bit TTL interface would require at least 20 lines. This includes power lines, three primary color signals (R0–R5, G0–G5, B0–B5), horizontal and vertical sync signals (HS, VS), data enable (DE), and clock (CLK). The large number of lines makes it prone to interference, which is why it's not commonly used anymore. Most modern LCD screens now use LVDS interfaces instead.
LVDS operates at a low voltage, typically around 1V, and is often a single-row connection. It commonly uses 20-pin or 30-pin connectors. The data is transmitted in pairs, ensuring better signal integrity and reduced noise.
Common configurations include single 6-bit (D6L, SI6L), single 8-bit (D8L, SI8L), double 6-bit (S6L, D06L), and double 8-bit (S8L, D08L). These terms refer to the number of data bits per pixel and how they're transmitted.
In terms of power supply, most screens operate at 3.3V, 5V, or 12V. Typically, screens smaller than 15 inches use 3.3V, 17–24 inches use 5V, and 26 inches or larger often use 12V. However, there are exceptions, so it’s important to check the specifications before connecting. Attempting to power a 12V screen with a lower voltage can cause damage, so caution is advised.
To determine whether a screen uses a single 6-bit, single 8-bit, double 6-bit, or double 8-bit configuration, you can look at the number of twisted pairs in the cable. A single 6-bit interface usually has 4 pairs, while a single 8-bit has 5. Double 6-bit uses 8 pairs, and double 8-bit uses 10. Each configuration has a specific arrangement of data and clock lines.
Most stock screens are available in 8-bit versions because it's easier to adapt them to 6-bit configurations by reducing the number of data lines. However, converting a 6-bit screen to 8-bit is more complicated. If you don’t have the right screen cable, it's better to modify the existing one rather than trying to force a mismatch.
When changing screen cables, the recommended order is to go from double 8-bit to double 6-bit, then to single 8-bit, and finally to single 6-bit. Always pay attention to pin positions, as incorrect connections can damage the circuit board. Most boards have markings to guide you.
If the original screen cable is a flat flexible cable (FFC), you can also identify the interface by examining the logic board. Power and ground connections are usually straightforward, often using fuses or large copper areas. Data and clock lines are typically adjacent and connected through small resistors, usually 100 ohms. By counting the resistors connected to the screen interface, you can estimate the number of data bits.
Understanding the screen interface goes beyond just knowing its definition. It's also important to understand how the data lines, ground, and power are connected. Additionally, knowing the screen resolution and size is crucial, though size may not always matter.
Once the screen cable is properly connected, the motherboard should be able to recognize the resolution and display the image. However, even if the screen lights up, the software might not be fully compatible. Factors like character display, button functions, and overall performance need to be considered. If issues persist, you may need to adjust or replace the software.
This information comes from my own experience, and I hope it helps others who are working on similar projects. If you notice any mistakes, feel free to correct me. I'm always open to learning and improving.
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