Interfacing a LCD with a legacy TTL video aircraft computer system - Page 1
Forum section: electronics / projects

author: unleashedko
author_rank: Newbie
timestamp: April 24, 2016, 12:12:08 am
content: As the question above states, I am in need of connecting an LCD with composite/VGA inputs to an old legacy monochrome TTL video aircraft system designed in the 80s. The video used to be displayed on a 5" CRT but the components that went to those monitors have reached their end-of-life and are no longer being produced. The system in a bit more detail is an aircraft computer system which outputs TTL signals (Hsync, Vsync, and Video). Intensity is unused.Hsync = 15.7KHz positive 5V pulse, Vsync = ~52Hz positive 5V pulseWhat would be the best way to get these signals over to the new LCD? I have been researching avenues to go on for both composite and VGA but have gotten stuck on both as of this posting.If going the composite route, it is possible to feed the signals through mixers to generate a composite signal that the LCD could use? I tired to find a composite encoder IC but have not found one that will work with a video signal, just RGB which isn't used in this application.I have also considered the VGA route, which seemed the "easiest" way to go, but I got stuck on trying to get the horizontal sync frequency up to the 31KHz needed. I was originally planning to just frequency multiply the sync signal but I feel that data stream would not be displayed correctly. I have seen some mentions of a "scan doubler" but I don't know much about them, how they work, or how to implement one in hardware. I'd rather not get into having to do a software solution as these aircraft are militaristic in nature and the simpler the better (meaning hardware that can be troubleshot and not custom firmware on an FPGA/CPLD/controller). This also means I cannot use a "converter box" that you see pop up in google.Google has given me a small amount of information but I need some help from display monitor gurus (I am most certainly not one...). Is there a solution to this dilemma? What route would you take? Other ideas that I would never think of? I could use any sort of direction or help regarding the above.

author: Delta
author_rank: Super Contributor
timestamp: April 24, 2016, 12:18:39 am
content: I can't help directly, but try reading up on retro computer gaming / arcade machine forums and sites. Those old machines output 15.7kHz video, and those dudes have various methods of interfacing them with modern displays. Best of luck!

author: KMoffett
author_rank: Regular Contributor
timestamp: April 24, 2016, 11:34:36 am
content: google: mda vga video converterKen

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2016, 12:01:44 pm
content: Do you know what the resolution of the display was, and was it greyscale or single bit per pixel? If it's fairly low resolution, you could reasonably get away with a microcontroller with DMA especially if it's single bit per pixel. If it's greyscale, you will limit the number of MCUs capable of doing this as they'll need a reasonably high speed ADC, and you'll need a DAC, and I don't know of any microcontrollers with sufficient DAC bandwidth for this on chip.That would be an interesting project though.Edit to add: the H and V are reasonably close to PAL, perhaps this could be done entirely in the analogue domain if your display will take composite PAL.To do black and white PAL composite from logic is as simple as a few resistors and open collector inverters for your sync signals.

author: unleashedko
author_rank: Newbie
timestamp: April 24, 2016, 01:38:17 pm
content: Does anyone know if the attached block diagram would work? Yes it was made in paint as I just through it together right now after waking uphttp://www.analog.com/media/en/technical-documentation/data-sheets/AD725.pdfThat is the datasheet to the IC mentioned in the paint model. I am unsure as to if feeding the single video signal into the three separate RGB channels would work correctly or not...Howardlong,The monitor would in fact take composite PAL. Could you give a bit more information on how you'd implement this with logic? I am currently not familiar with PAL so I'll have to go look up the specifications.

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2016, 02:27:09 pm
content: For black and white PAL is very similar to NTSC, but with different sync frequencies (particularly frame) and line counts.What is the video signal? Is it analogue or on/off? What are the levels?

author: unleashedko
author_rank: Newbie
timestamp: April 24, 2016, 02:44:34 pm
content: From what I could tell on my scope at work, the signal was "ON/OFF" with the peak voltage being around 2 volts.

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2016, 05:35:38 pm
content: What are the pulse widths of the hsync and vsync?

author: helius
author_rank: Super Contributor
timestamp: April 24, 2016, 05:58:18 pm
content: aerospace display technology typically uses specific standard formats like STANAG 3350.

author: unleashedko
author_rank: Newbie
timestamp: April 24, 2016, 08:17:11 pm
content: Howardlong,I'll try to go into work early tomorrow and take some screenshot captures from the scope and post them here. I am unsure of the pulse widths off the top of my head as I never thought they would be too necessary. In the meantime, do you think it would be worth it ordering an AD725 I linked earlier and tie the single video channel to the three RGB inputs? I saw someone mention doing something like that in another post I saw but have no idea what would happen when doing so.Helius,A special government standard may be used, but after a quick look at 3350, I don't think it could be that one as it states the use of component RGB which is not used here. I have requested more information from the end-user as the source signal but I have no idea when I may hear back from them so for now I have to work with what I can probe.

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2016, 08:55:55 pm
content: The standard for PAL is ~12us hsync and about a dozen lines worth for vsync, say about 750us. Once you add colour and interlacing things become rather more complicated.Here's something that I put together that covers the PAL voltage requirements given the other parameters you've given. It makes a few assumptions.O Syncs are TTL 5V active high, and are the right width for PAL;O 2Vp-p video in, negligible output impedance, can source about 30mA and sync about 10mA. If not, you'll have to buffer it;O Transistor is pretty much any small signal NPN.

author: unleashedko
author_rank: Newbie
timestamp: April 24, 2016, 10:09:27 pm
content: Howardlong,Thank you for the schematic. I'll get some screen captures in the morning at work and post them here to see if this would actually work or not. I need to compare the video signal with the horizontal sync as I didn't do this as of yet. This is one of those things you are given to do at work without actually having the time to be able to do it, so you have to find squeeze it in and yet make some results. Would you mind explaining how this circuit would work? Without being able to get too involved as I am doing other things at the moment, it looks like whenever one of the sync pulses rise it would forward bias the transistor and drop the video signal to ground? And the resistors are setting voltage levels I assume appropriate for PAL composite? I assume the "V2" is the system operating voltage? I do know that the monitor previously and will still operate off of 12VDC.

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2016, 10:36:18 pm
content: V1 is your 2vpp viseo inV2 is your 5vpp vsyncV3 is your 5vpp hsyncEither V2 or V3 going high clamp the output to ground using the transistor.Then there are some judiciously chosen resistors which, in conjunction with the 75 ohm load of the composite video represented by the z-a+jb box, present 0.3v for black and 1.0v for white given the 2vpp video signal as per the PAL spec.This assumes that the 2vpp signal is low impedance. If it's not, it'll need buffering, or if you know its impedance it might be possible fiddle the resistors a bit more if it's low enough.The circuit was drawn in a simulator which is why you get those strange circles and boxes which I set up to simulate your inputs and output.