Apple LC needs a monitor. Will CGA work?

Discussion in 'Apple Collectors' started by Radio_Dog, Sep 22, 2017.

  1. Radio_Dog macrumors newbie

    Radio_Dog

    Joined:
    Sep 22, 2017
    Location:
    Columbus Ohio
    #1
    Hello everyone! First post, and first day signed up for your lovely forums.

    The Story: Okay! So i inherited an old apple LC from a friends basement and this thing is so cool. But it's got a proprietary apple 15Pin Connector for video out. I'd like to run it, but i feel like using VGA is just cheating, no real love for that vintage feel of limited color palettes.

    What I DO have: Is a MAGNAVOX CM8762 074G vintage monitor from 1990.

    This monitor has two inputs. A CVBS connection, and a RGB TTL port with a special little adapter to run it into a DB9 port. So, I really want to use this old thing for my apple but information is sparse on the LC including workarounds. Everything points me to use a VGA converter, but i really like this monitor because it matches that vintage feel.

    So, can i build a converter to make the Macintosh LC use this CGA/ EGA connection? Most forms have told me that RGB TTL is a little different from standard RGB and it's been commonly referred to as CGA.


    Service Manual for this Monitor
    http://www.msxarchive.nl/pub/msx/mirrors/hanso/service_manuals/philipscm8833sm.pdf


    CGA pinout I found
    http://old.pinouts.ru/Video/CGA_pinout.shtml

    And to clarify this is a first generation Macintosh LC. Not a LCII or LC III Little computer still has a knockout for that dual floppy option, so I've done a little homework on it.

    Found this conversion for an apple 2 that may help too. But it's not specifically LC so, i've got no idea if this will even work for me.

    http://knut.one/A2E-CGA.htm

    Any help is appreciated guys. Also, If the conversion requires something weird I can solder it myself if the instructions are specific enough.

    Thanks for your time and I hope you can assist me in this conundrum

    ___ Radio ____
     

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  2. jerwin, Sep 22, 2017
    Last edited: Sep 22, 2017

    jerwin macrumors 68020

    Joined:
    Jun 13, 2015
    #2
    limited color palette? IIRC My brother had an LCII that supported 65,536 colors (at 512x384). He had the additional VRAM installed.

    see this page for more details on the LC's video output.

    http://lowendmac.com/2000/how-to-use-a-pc-vga-monitor-with-your-mac/

    The VGA standard dates to 1987; the mac LC was introduced in 1990.

    I recall back in 1986 (why, oh why do I remember these things?) that the Apple IIGS RGB monitor used an "analog" signal instead of the "digital" signal used by Apple IIe monitors, and that this would necessitate getting a new monitor to go with ones new motherboard. So from that perspective, TTL RGB was well and truly obsolete many years before the LC.

    (Don't get an Apple IIGS monitor, by the way. It's a 15 kHz display, and the mac's video signal is substantially higher frequency.)

    http://www.retrotechnology.com/herbs_stuff/macmons.txt
     
  3. Radio_Dog thread starter macrumors newbie

    Radio_Dog

    Joined:
    Sep 22, 2017
    Location:
    Columbus Ohio
    #3
    Well, it seems that based upon your post that it's not going to be that easy to do a conversion. I was hoping to use this mainly due to the look and feel of it. They do make a vga converter to Mac that's pretty cheap, it's just that I wanted to try this out due to how attractive it looks. Perhaps I'll look around in other forums. This doesn't seem impossible, just beyond my knowledge at the moment. Thanks for the advice though, if I buy another vga monitor I'll get the proper adapter.
     
  4. jerwin macrumors 68020

    Joined:
    Jun 13, 2015
    #4
  5. Anonymous Freak macrumors 603

    Anonymous Freak

    Joined:
    Dec 12, 2002
    Location:
    Cascadia
    #5
    tl;dr: No - CGA is a digital signal, not an analog signal as your Mac outputs.

    Long form:

    The video port on your LC sends variable-voltage signals for each of a red, blue, and green channel, which the monitor takes in and uses to change the intensity of the beam for the red, blue, and green video channels respectively. It is 100% analog the entire route from the plug to the tube. This is the same basic system as VGA. The Mac connector just has a different pinout, with slightly different "non-video-signal" pins.

    CGA (as well as EGA) is digital - red, blue, and green are each either on or off, with a separate line to indicate "intensity" (extra bright, on or off, not selectable per color channel, so you get dark cyan (blue+green) or light cyan (blue+green,) but you can't get dark blue + light green.) The monitor receives this digital signal and the circuitry within the monitor converts it for display on the tube. One oddity of this is that an official IBM CGA monitor's "dark yellow" color isn't actually dark yellow, it's brown. If you follow the logic of the adapter, the "dim" version of "green plus red" should be "dark yellow" as seen here (top, second-from-right.) But IBM didn't want dark yellow, so in the monitor it sees the "dim green plus red" signal and actually makes it "brown", as seen here. Many, but not all, clone CGA monitors also obey this "dark yellow is displayed as brown" behavior.

    The Apple II converter works because the listed Apple II video card contains a digital signal, meant for the extremely-short-lived AppleColor 100 monitor for the Apple IIe and Apple III, which worked nearly identically to a CGA monitor.

    There were converters to turn an analog (VGA) signal to a digital (CGA/EGA) signal, so you could use your old CGA or EGA monitor on a VGA computer, but they were ultra-rare. Far more common (yet still extremely rare today,) are the reverse - to take the digital CGA/EGA signal and output analog VGA. In theory, if you found one of the "hen's teeth rare" "send VGA signal to a CGA monitor" adapters, *PLUS* a Mac-to-VGA adapter, you could make it work. But that ain't gonna happen.

    * Note that EGA uses both a "low intensity" bit per color channel PLUS a "high intensity" bit per color channel, allowing up to three values per color channel - for a total of 64 possible colors. This allows "dark blue+medium green" for example, by having "low intensity blue" and "high intensity green" on at the same time, but *NOT* high intensity blue or low intensity green. It could further have "dark blue+bright green" by having low intensity blue, low intensity green, and high intensity green all on at the same time. EGA still uses a digital signal, though. It is on the monitor to interpret what precise voltages are applied to each color's beam for a given digital input.

    * Further note: The Apple IIgs RGB monitor, which uses the same physical plug as the Macintosh for video out, is a digital signal much the same as EGA. An Apple IIgs RGB monitor is *NOT* compatible with a Macintosh, or vice versa. (This is the reason I brought up how EGA does it.)
     
  6. jerwin macrumors 68020

    Joined:
    Jun 13, 2015
    #6
    What you need to be authentic is this:

    http://lowendmac.com/displays/mac-12-inch-rgb-display.html

    512 pixels by 384

    [​IMG]

    low resolution, limited to 60 Hz, but it does match the LC's case.

    wikipedia says:
    https://en.wikipedia.org/wiki/Macintosh_LC
    It's possible that this is no longer an issue.

    [​IMG]
     
  7. Radio_Dog thread starter macrumors newbie

    Radio_Dog

    Joined:
    Sep 22, 2017
    Location:
    Columbus Ohio
    #7

    Wow, that's an incredible explanation. And exactly what I needed to hear. Thanks for the technical breakdown and links. I was hoping for a intelligent argument if possible and you've definitely proven your point. Guess I'll have to look for a Vga monitor now. Thanks though, you've answered alot of questions I had.
     
  8. Radio_Dog thread starter macrumors newbie

    Radio_Dog

    Joined:
    Sep 22, 2017
    Location:
    Columbus Ohio
    #8
    Hey, I did a little searching around and found this.. https://www.amazon.com/Andoer-Portable-GBS-8100-Definition-Converter/dp/B00XKWX4F8

    Apparently there are arcade cabinets still using Cga monitors that need conversions pieces. It claims to go VGA-CGA but I'm skeptical if it will work. My monitor has a RGB-TTL port on the back that converts to a 9pin dsub connection. This lacks the 9 pin connector but it does have an rgbs output on those pins on the board. Think it might work? I'd also have to figure out what pins go where if I want to hook it up to my dsub connector.

    ANY help is appreciated
     
  9. Anonymous Freak macrumors 603

    Anonymous Freak

    Joined:
    Dec 12, 2002
    Location:
    Cascadia
    #9
    That adapter outputs a CGA-style NTSC signal, from what I can tell - it outputs to a "CGA television" rather than a digital CGA monitor. (Real CGA video cards had both the digital d-sub 9-pin connector as well as an RCA plug to composite video output - and through composite, it supported more colors, in similar fashion to the way an Apple II outputs color from what is really a black and white signal.) But that adapter's page is also badly written, so I'm not entirely positive *WHAT* it does.
     

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8 September 22, 2017