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There are many benefits to integrating a small display into your embedded system. In this project article, Stuart takes a 3.5″ LCD video monitor and builds a circuit that provides a text-based terminal that can plug into the display. This basically emulates an old-style alphanumeric computer terminal, but using modern low-power LCD technology.

I picked up a $15 LCD video monitor from Amazon. It has two standard, composite video inputs. The screen is only 3.5″, so you would never use it to replace a 12″ or 14″ monitor. It is intended for use as a display for backup cameras in vehicles.

All that said, a display like this has some other uses. For example, it could be a status display for an embedded system. Of course, you could always carry around a tablet, or even display things on your phone. But what if you want the display self-contained? Or you don’t want to add Wi-Fi to your design? What if it’s a piece of equipment that is located where phones and computers aren’t allowed?

I thought it would be interesting to build a circuit to provide a text-based terminal that can plug into the display. The effect would be similar to the old alphanumeric computer terminals used in the 1970s and 1980s, but with a much smaller, lighter, and less power-hungry screen. To make this work, I had to generate an output that would drive the composite video input of the monitor.

The composite video waveform dates back to the days of monochrome televisions, when they were built with vacuum tubes. Because the signal was sent over the air, everything needed to display a complete picture had to be embedded in the transmitted waveform. The picture on the screen was painted as a series of lines from left to right and from top to bottom. There was no digital communication. It was all done with voltages and resistor-capacitor timing.

The composite waveform is shown inFigure 1. Since the video terminal circuit that I built is only for painting characters—not a television picture—all the video information is either white or black. In contrast, on a television broadcast the video portion would vary between white and black to reproduce the grayscale image of the transmitted picture. The composite waveform, when sent over a cable, is normally a 1V P-P (peak-to-peak) signal. When broadcast over the air, the amplitude varies with signal strength. Circuits in a television receiver normalize it, so the signal can be used.

In displaying text on a monitor using composite video, each character needs multiple lines on the display. In the 1970s and 1980s, typical characters were either 5×7 or 7×9—that’s 5 dots wide by 7 dots high, or 7 dots wide by 9 dots high. To display a complete character, 7 or 9 scan rows would be needed, with at least one blank row of dots between each character row to separate the characters. The monitor would therefore display around 20 or 24 character rows.

Similarly, with each character being 5 or 7 dots wide, clocking the dots at a reasonable rate to make the characters wide enough to read, results in a row of typically 70 to 80 characters. Filters in a television to remove the audio subcarrier mean a television can’t display that many dots on the screen, but a monitor with composite video input can.

If the first line of the display contains the characters ABC, then the first three locations in the RAM will contain the ASCII codes for those characters. As the scan moves across the screen, the first scan line will address the top line of character A, then B, then C, and then the rest of the line will be blank. The second scan line will address the second line of character A, then B, then C, and so on. Each character row is displayed, one scan line at a time, all the way to the bottom of the screen.

Later versions of the composite waveform inserted information into the vertical blanking interval for things like channel information and closed captioning. Although composite video has been superseded by higher resolution VGA, DVI and HDMI in computers, the ability to run a full video signal down a single coaxial cable still has advantages in some situations. And, now, of course, the television broadcast itself is a digital HDTV transmission.

The composite video standard defines a standard way of getting a video image to the monitor, but a signal containing intelligible information still has to be generated. Early video terminals were constructed with TTL (transistor-transistor logic). The logic had to walk through the display memory, retrieving stored ASCII characters in sequence.

The output of the display memory would go to a ROM (read-only memory) that contained the codes for converting the ASCII from the RAM into dot patterns for each scan line of the character. A shift register produced white/black outputs that produced dots on the screen. Additional logic was needed to generate the horizontal and vertical signals when needed, and still more logic was needed to handle incoming serial data, and write it into memory. The Lear-Siegler ADM-3A was one such terminal. You can find a maintenance manual for it with schematics on the Internet. That terminal had over 100 ICs.

Using internal RAM has an additional advantage—no complicated RAM synchronization is needed. Early video cards needed a mechanism to ensure that the computer microprocessor did not write new characters into memory during the live scan time. If they did, the display would be covered with “snow”—white dots that randomly appear on the screen. If the writing occurs during the live scan time, it interferes with reading the memory to display the characters. One way this was fixed is by writing only during the horizontal or vertical blanking periods, when the screen is blanked anyway.

Early terminal designs used a shift register to shift out the dots. The TM4C design uses one of the synchronous serial ports to output the dots. Internal timers generate the synchronization signals, and the entire process is driven by the horizontal sync signal. The TM4C provides a nearly self-contained video terminal with few external parts needed (not counting, of course, the electronics in the LCD video terminal, itself).

Due to the tiny size of the monitor, a DIP switch selects either a 75-character x 20-row output, or a 35-character × 10-row output. The 75×20 output is for display on larger terminals. You can use it on the tiny display, but it’s impractical.

Synchronous serial interface number 3 is used to generate video dots and blanking. The output of SSI3, on PD3, is pulled to ground, because it needs to be at blanking level when not active, and the TM4C normally floats the output when inactive. SSI3 is configured to transmit 7-bit words, since the character matrix is 7 dots × 9 dots. The SSI shift register is clocked at 10MHz in 75-character-per-line mode, and at 5MHz in 35-character-per-line mode. This is the dot rate, and allows the 52.5µs live scan time to display 525 dots. Using a 7×9 character format, that’s 75 characters per line. With a 5MHz clock, it’s 37 characters per line. I set it to 35 in the firmware.

U5 is a 3.3V to RS232 converter that connects serial port 1 (UART1), on PB0/PB1, to the host system. UART1 is fixed at 9600 baud in the firmware, though that can obviously be changed. Header J3 is a 10-pin header that connects to a D-series connector with a short cable. Transistor Q1 mixes the dot and sync signals to produce the composite video output. To keep the dots crisp, Q1 is biased so that it never goes fully into saturation or cutoff.

There are some special considerations in the TM4C firmware to make working video. The “character generator ROM,” which is actually an array in flash memory, is configured for 7×9 dots. But the actual display memory is sized for 10 dots per character, to make a blank scan line under each character row for vertical separation. This extra scan line isn’t included in the character matrix. It’s just an extra line that is initialized to all zeros (blank). So, the start of the first character row is at location 0 in the display memory array, and the next one is at location 750 (75 characters per row × 10 scan lines per character). To blink the cursor, the blank line under the character where the cursor is located is switched once per second between white and black.

Although the horizontal characters per line can be changed simply by changing the dot clock and forcing the DMA to send only 262 dots (35 characters) per scan line, the vertical spacing isn’t so easy. In 35-character-per-line mode, if the same number of character rows were used as in the 75-character-per-row mode, the characters would be vertically “squashed,” and only the upper half of the display would be used. So, in 35-character-per-line mode, each line of dots is sent twice to expand the vertical height of each row. The full frame is still 262 scan lines, but each character row now takes 20 scan lines instead of 10.

The software scrolls the screen when a new character needs to be inserted and the screen is at the end. This is accomplished by moving an index that indicates where the top row starts. At initialization, location 0 in the display RAM area is the start of the first line. When the bottom of the screen is reached, the screen is scrolled up by making location 750 (the start of the second character row) the top of the screen, and the row starting at location 0—now the last row on the screen—is blanked. The circuit board and the monitor, displaying a few lines of test text, are shown in Figure 6. See the sidebar “ Composite Video Display Calculations” below as a guide if you wish to try out alternate clock frequencies or different character generator dimensions.

When color television was introduced, the monochrome video waveform was modified to accommodate the addition of color. The waveform had to be compatible with existing monochrome televisions, so the changes had to be invisible to existing monochrome receivers. Color was added by sending a color sync burst during the horizontal synchronization back porch, which synchronized the chrominance phase with that of the transmitter. The color information is phase-encoded with a 3.58MHz signal on the video. Circuits inside the television extract the 3.58MHz color signal to display the color information with the luminance (white/gray/black) information. The video circuit here does not include color. That would take additional hardware and a significantly faster processor.

As currently implemented, the terminal is fairly simple. Characters are received via the serial port and written to the display at the current cursor position. When the end of the display is reached, the screen scrolls up one line, and new characters are inserted at the bottom. The addition of direct cursor addressing would allow the host to send characters that tell the terminal to place the cursor at a specific line and column position.

Another improvement would be to add underline or inverse video for the characters. The high bit of the incoming byte can be used to indicate when a character is to be displayed in this enhanced mode. The original ASCII character set needed only 7 bits, so the highest bit can be used for special features. Some word processors of that era used these features. when available.

Although this entire project was based on displaying ASCII characters, there is no reason you can’t display graphics information on the screen. The resolution isn’t very high, and, as mentioned, adding color would require different hardware. But unlike the character-based terminals of the 1980s, you aren’t locked into displaying characters only.

Since there is no hardware character generator between the RAM and the dot output, you can display low-resolution graphics just by filling the memory with the graphics image. In fact, the first test I did on the system when I connected it to the LCD was to display a line in the middle of every other character position in each character row. The result was a display that had just a series of horizontal dashed lines. I did this because it was easy to do and didn’t require any of the character processing routines to be functional. If you look at the source code, you will see that the simple code to do this is still there, just commented out. The source code for this project can be downloaded from Circuit Cellar’s article code and files webpage.

This project started out to re-create something resembling the terminals that we used on in the 1980s. Unless you are interested in that sort of project, the original intent for this circuit wouldn’t be of use. But the simplicity of the circuit provides an easy way to build a text-based or even graphics-based monochrome display that will drive any monitor with a composite video input. It provides an inexpensive way to produce a display for diagnostics, monitoring and general visual displays, especially given the low cost of the monitor used for the prototype.

The calculations for the composite video display are given here, should you wish to tinker with alternate clock frequencies or different character generator dimensions:

The LCD"s (mostly TFT"s but some CSTN for the cheaper ones) used in digital photo frames and incar entertainment systems are quite simple to drive.... digitally. The require a horizontal sync, and vertical sync to tell the display drivers when to start a new frame, and when to start a new line. Also a single "clock", which on a transiston reads digital inputs which control the brightness of each sub pixel (Red, green, and blue for each "pixel"). These are usually 5/8-bit numbers.

It IS complicated, I have managed it (sort of) with a couple of CPLD"s, and some parts robbed form a PSone LCD display. And, to be fair, to buy one pre-made IS a lot cheaper, and easier. But most of the above can be done with a simple IC, containing all the necessary buffers and ADC"s. It won"t be the cheapest chip ($9?) but will do the job when coujpled with a small microcontroller to program its registers for settings.

That said, it doesn"t have to be expensive! A PSone LCD isn"t a TFT, it is in fact a CSTN (colour STN LCD) and so its contrast isn"t great, nor is its quality... but composite video was deisgned for PAL/NTSC anyway, which isn"t the best resoluiton in the world. It has built in speakers, and can happily take in standard composite video/audio, and display it without trouble. These go on ebay for anywhere from $10 to $25 US.

As someone mentioned "in car displays" I have several of these, also from ebay. There were "ripped" frmo a car so they aren"t nicely housed, but all that was needed was soldering a phono plug connection to the PCB, and it displayed PAL OR NSTC. Also has a nice I2C interface to change aspect ratio, refreshrate, contrast, and brightness (all digitally).

Usually both the above displays are not really "manufactured/designed" by the manufacturer. In fact, generally the circuitry used to drive the display from composite video is part of the display itself (all on the same PCB on the back of the LCD, which it is mounted on). So you CAN buy displays that take in analogue and display it, as OEM, not consumer products.

While looking for lcd monitors in bulk, Lcd monitors are more affordable and can be used for a variety of purposes. Browse through smartphones wholesale and more suppliers on Alibaba.com to find the ideal lcd monitors for bulk and other wholesale purposes.

A LCD monitor with aposite inputs can be used for both purposes and at the same time. For customers looking for a more portable LCD monitor in bulk, they will be able to adjust the gamma, contrast, sharpness, and color rendering.

A LCD monitor with aposite input is standard for all-in-one devices. They also be available for a variety of custom media options, such as wall-mounted monitors, and for the use of LEDs for a variety of custom media options.

The RMM1024 is a 10.1”, 2-screen, 4RU rack-mountable LCD monitor with 1920x1200 resolutions. The RMM1024 provides analog composite, 3G/HD/SD-SDI and HDMI video along with stereo analog audio input/output.

Additional features include precise color replication, and analytic functions supporting 16 channel audio meter display, full feature waveform and vector scope making the RMM1024 an obvious choice for camera control, ingest, transmission, flight pack, OB Van and EFP applications.

CRTs can be abundant if you have good places in your local community to look, but they are getting trickier to track down each passing year. And while nothing’s stopping you from using a modern LCD for retro gaming, you may want to track down a square-shaped LCD (mostly 4:3 aspect ratio) so your classic content seems a bit more “at home” — avoiding those black bars.

So whether you’re looking for a cheap 4:3 LCD to use with your MiSTER FPGA setup, have a classic personal computer, or just want something for watching “full screen” video content or a emulation box to run on, I’m hoping this guide is helpful in your shopping.

Granted LCDs, especially older models, aren’t ideal for retro gaming compared to CRTs. LCDs often down’t have as deep of black colors and there is input lag involved among some other issues.

However, LCDs are easier to move around and store and use less electricity. And even though there are many die-hard CRT fans out there, there are also many long-time retro fans that have been using the recommended monitors below for a while and been happy with their performance. I’ve even quoted and handful of enthusiasts to give you a good perspective on these recommendations. And don’t forget, eventually LCDs might be much easier to find and purchase remotely then the CRTs counterparts.

Early LCD monitors may show a lack of technical maturity, however, near some of the later ones that we are focusing on have some decent IPS technology with reasonable color and response times.

In the end, I thought this would be a fun, lighthearted look at some options for those that are interested. If you have any commentary on the matter, please share your thoughts and suggestions in the comments section below. Just be respectful — not looking for CRT vs LCD wars