Interfacing a monochrome 7" CRT with HSYNC, VSYNC and 0.5-3.0V video - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: December 12, 2022, 01:02:04 am
content: Hello,I salvaged this neat little CRT module from a marine radar head unit (working pull) that I'd like to connect to a VGA/CGA output for some retro goodness. It seems fairly straightforward:QuoteI tried hooking it up to a 12 V power supply and gave it HSYNC and VSYNC from a DP>VGA converter, set to 320x240 with a 60 Hz refresh (480i). I used the green output for VIDEO, but got nothing. Not unsurprising since the 0.7 V of a VGA output is close to (or even below) the black level of this monitor (it expects 0.5-3.0 V). It's too nice a display unit for me to break it with further crude experiments, so I thought I'd ask for help - does anyone have any suggestions how I can make this thing go?

author: Benta
author_rank: Super Contributor
timestamp: December 12, 2022, 01:24:35 am
content: Nice lil thing!I suspect it might be laid out for TTL levels for the syncs and a primitive R-2R ladder driven from TTL as well for the luma.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 12, 2022, 01:49:09 am
content: Quote from: Lomax on December 12, 2022, 01:02:04 amYou can't feed VGA to a NTSC/RS-170 display.

author: james_s
author_rank: Super Contributor
timestamp: December 12, 2022, 03:44:06 am
content: VGA has a 31KHz horizontal, your display is 15KHz, you'll need something that produces CGA compatible video. I've driven similar monitors from FPGA retro computers. Be careful feeding it incorrect signals, many monitors will blow the horizontal stage if you drive them at the wrong frequency.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 12, 2022, 10:48:10 am
content: Quote from: Benta on December 12, 2022, 01:24:35 amThank you! I have scoured the service manual, but although it seems to promise a video signal timing diagram, figure 4-6 shows a block diagram for the radar function instead (see attached pics), and a video signal timing diagram is nowhere to be found. I think someone made a mistake. Also, the video signal is not carried by a coax cable as claimed in the service manual, but by a simple single core wire in a 4-pin PCB connector (G, V, H, D) - see second and last photos in original post. The "V" and "H" have bars, suggesting they are inverted.Quote from: Alex Eisenhut on December 12, 2022, 01:49:09 amQuote from: james_s on December 12, 2022, 03:44:06 amMy apologies - I was referring to the connector only, not the video standard; I used a custom modeline and Xrandr to force 480i. The VGA connector sits on a DP > VGA converter, which is seen as HDMI-0 for some reason.Code:xrandr --newmode "480i" 24.021 1204 1258 1372 1525 460 472 480 525 -hsync -vsync Interlacexrandr --addmode HDMI-0 "480i"xrandr --output HDMI-0 --mode 480iThat said, I'm not sure if this generated the correct timings, and in any case the signal levels are incorrect - which brings me full circle to my initial post...

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2022, 02:09:35 am
content: You'll want to verify the timings with a scope. Also some monitors are really picky about sync polarity so try all four permutations of positive and negative H and V sync if it is not specified. For the video you can use a variety of techniques to turn it into a TTL compatible signal.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 13, 2022, 03:59:02 pm
content: Quote from: Lomax on December 12, 2022, 01:02:04 amOh OK, in that case can you elaborate on what you mean by "got nothing"? Do you hear a nice staticky crackling sound on the CRT when you power up? Like, is the thing alive? Even with no video input, if it's working, you can probably pick up the flyback whine or wave a scope probe around in the air near the CRT and pick up the free-running frequency, just to see signs of life.That is, if it is supposed to power up and just wait for video and not do smart things like power down if there's no valid video.

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2022, 06:29:42 pm
content: I really doubt it has any kind of auto-power-off, it's meant to be integrated into a piece of equipment.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 09:51:52 am
content: It's definitely working - not only do I see/hear signs of life, I removed it myself from a working radar head unit. I was able to confirm that it was nice and bright with no signs of burn-in, and looked quite impressive when on; sharp and high resolution for its diminutive size. That, and the fact that the video interface seemed quite simple, plus that it runs off a single 12 V supply, is why I decided to keep it and see if I could build something cool with it.But even if (and TBH I'm not even sure this is possible; the Internet provides conflicting information) I'm able to coax a graphics card into providing 480i@60 timings, I still need to convert the VGA* voltage levels to TTL... I imagine this would involve a trio of transistors, at least one of which needs to be fairly high bandwidth (~15-20 MHz?). I think there's a 5 V rail provided on the PCB that I could tap, but I'm very inexperienced when it comes to the dark art of RF design and was wondering if anyone had any suggestions.*) Again, I am referring to the electrical interface, not the video standard.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 10:41:42 am
content: Another thought: what about dividing the horizontal frequency in half? FromWikipedia:Quote

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 14, 2022, 03:22:03 pm
content: You can't. Not easily. You really have to supply 15KHz RS-170/NTSC timings.

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2022, 05:45:45 pm
content: Do you have any interest in FPGAs? Grant Searle's "Multicomp" will run on a cheap one, they used to only be around $14, and that can be configured to drive all sorts of different monitors. It's what I used to run a very similar one I got from an old CNC control.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 08:02:11 pm
content: Just had a look in the user manual for a motherboard I have been saving for exactly this kind of project, and to my amazement I see this in the BIOS configuration section:QuoteIt seems the S3 Savage4 BIOS on this board has been customised to support CGA output, and more! I had heard of some VGA eraand latervideo cards having been made specifically with backwards compatibility in mind (mine's from 2004), but I had no idea the very board I was hoping to use came with this functionality built in. RTFM!Surely this means I'll be able to dosomethingto get this working?

author: Benta
author_rank: Super Contributor
timestamp: December 14, 2022, 08:33:12 pm
content: Seems to me you need some kind of basic NTSC video source to test the little monitor.Check your attic/basement for VHS/DVD players or older digital video cameras. They all have CVBS output.Second, build a little circuit using the LM1881 to generate VS/HS (it's an easy IC).Now you have a basic set of signals for your CRT. Getting the volage levels and inverted/noninverted is the next simple step.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 08:50:31 pm
content: I have considered several options:Buy a second-hand pattern generator (cheap as chips nowadays)Bit-bang the output on a Raspberry Pi, akin to GertVGA 666Modify one of the many examples available for generating VGA on an Atmel MCUNone of these get me any further than verifying that the monitor works though, and I already know this. I want to be able to run DOS and Linux textmode applications, and ideally Windows 3.11 & 98, for which I would like to use my Arcom Olympus PIII board - I think that would be a marriage made in heaven. Build a custom case for it, adding an Epson SD-800 dual 5 1/4" & 3 1/2" floppy drive, an optical drive,an industrial mini keyboardand a few other bits & pieces...Think steampunk Mac SE/30

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2022, 09:23:38 pm
content: Windows 9x is going to be unusable on 320x240, if there's even a driver that supports that. What you need is something like a 8088/286/386 with CGA, a more modern board may work for that but even so you'll be limited to DOS or Windows 3.0 or lower. It's a great display for a retro 8 bit PC though, I run CP/M on my Multicomp board with a similar monitor.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 10:19:45 pm
content: I was hoping I could use it at 640x480 interlaced, with 8 levels of brightness. But you're right, that still wouldn't be great, even if I could make it happen. Perhaps I'll just save this little monitor until I can come up with a project to use it for. Maybe one day it will be possible to generate the appropriate signals using the GertVGA 666 method from a Pi, and then I can do something with that.I do have a cute 9" SVGA colour CRT which may be a better fit for the steampunk Mac SE/30 project - it even does XGA interlaced quite well. But it's a whole other level in terms of bulk, weight, and power. An order of magnitude more difficult to fit into a custom case. Some pics attached.

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2022, 10:48:57 pm
content: That 9" SVGA monitor looks great, I can think of all sorts of projects for that.I don't think you'll get 640x480 out of the CGA monochrome monitor but it still has many uses. There were a huge number of vintage computers that used that resolution. Also arcade games, it would make a good test monitor for those. I knew several guys that would modify the monitors to make the video input analog instead of TTL and use them as test monitors.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 14, 2022, 10:57:16 pm
content: Quote from: james_s on December 14, 2022, 10:48:57 pmI bought it many years ago thinking I'd replace the monochrome CRT in an actual Macintosh case (as well as the Mac mainboard), but I'm not keen on that idea any more. I'm a lot better at scratch building stuff these days - and I have better tools for it.Quote from: james_s on December 14, 2022, 10:48:57 pmWhy not? The radar head unit seemed to be able to do it just fine. Looked super crisp and impressive to me - which is why I decided to hang on to it!

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2022, 11:02:16 pm
content: Quote from: Lomax on December 14, 2022, 10:57:16 pmOh I missed that the spec did mention interlaced, well you might get that to work, most of those monitors were run non-interlaced, it wouldn't surprise me if the radar was 320x240 or thereabouts, CRT displays tend to look a lot better than the resolution would suggest compared to LCD. If you get the H and V timing correct it should work.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 15, 2022, 01:24:30 am
content: Quote from: james_s on December 14, 2022, 11:02:16 pmDefinitely 640x480 interlaced: it even had an adjustment to align the fields - see attached crop from service manual.Edit: Added another crop from the service manual where it clearly states 640x480 is the resolution.

author: aeg
author_rank: Regular Contributor
timestamp: January 01, 2023, 09:30:13 am
content: Keep the current hsync and vsync inputs and just put 3 volts DC into the video input. If it doesn't light up, fix your sync or whatever else is wrong. If it does light up, put the 0.7V video input back in and try cranking up the brightness pot.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2023, 12:27:29 pm
content: Look for the ICs in the video section and check the datasheet for information about tweaking the input signal.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 01, 2023, 01:44:25 pm
content: Quote from: aeg on January 01, 2023, 09:30:13 amThe video input might be ac coupled. If so, that 3 volts dc test will not work.