Talk:IBM Monochrome Display Adapter

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Pin numbers

What pin numbers? Shown is the end of the connector without any numbering.— Preceding unsigned comment added by 216.188.252.240 (talk • contribs)

It would seem that when the above comment was added, this revision was current, but the issue complained of back then has long since been resolved. —ReadOnlyAccount (talk) 22:25, 1 December 2023 (UTC)[reply]

Pin 7 = Video?

What is the meaning of Pin 7 ('video')? I.E. What does this mean electrically? I understand the 'intensity' represents how bright the screen gets per pixel and ground is electrical ground and horizontal sync is used for making sure the pixels displayed line up the right way, but what is the 'meaning' of 'video' electrically?— Preceding unsigned comment added by 160.36.195.56 (talk • contribs)

I think this is the answer you're looking for: The video line carries a digital (on or off) signal indicating if the pixel was on or off. The intensity line made the pixel brighter than normal. There were three levels of pixel brightness - off, on, and on with intensity. The intensity signal just made the pixel brighter - if the video signal did not call for the monitor to paint the pixel on, the intensity signal had no significance.

In electronic parlance the name "video" on a signal line is often used to show that the line is carrying picture information - I've also seen laser printer circuit diagrams with the signal to the laser module named "video". 72.16.169.146 (talk) 19:42, 14 April 2008 (UTC)[reply]

All signals are using TTL levels. So 0 V on video pin means black, 5 V means gray or white, depending on pin 6 intensity.

    video   intensity   screen
    pin 7   pin 6 
    0 V     0 V         black
    0 V     5 V         black
    5 V     0 V         grey
    5 V     5 V         white

--Rôtkæppchen68 10:37, 7 November 2014 (UTC)[reply]

I think the original poster's description of the intensity pin betrays a misunderstanding – which is revealing, because maybe the article needs to be improved to explain exactly what the intensity pin was. First, think about CGA/EGA/VGA connectors. These have R, G, B pins for red, green and blue. Now think about building a video card that is strictly monochrome. For the sake of argument, let's say our hypothetical video card can only output black or white pixels, on or off. Ah, but there is a problem: The actual monochrome monitors on the market don't all use a white phosphor. Some use an amber phosphor, some use the famous green P39 long-persistence phosphor (to cut down on the flicker despite our card only managing to output at 50Hz). So you can't very well name that single output pin W for white. What do you name it? Maybe you'll make the decision to just name it "video". And that's what the MDA card's makers did – except there's a catch: They added an additional feature to the card that made the card no longer strictly monochromatic (but still monochrome in the wider sense): They added a second pin that could brighten the output, thus allowing for regular and more intense brightness, depending on whether that second pin was also turned on. They named that second pin "intensity". So it's important to understand the intensity pin is not the main video output pin, it's only an additional one.

MDA was also built such that if you only turned on the intensity pin and left the video pin off, the output remained close to black – maybe a little bit brighter, but it's not entirely clear if that was a bug or a feature from the start. (Remember also that MDA was not all-points-addressable, so in practice, changing the corresponding attribute bit in software affected not just single pixels but entire non-redefinable characters, hence the intensity pin would be repeatedly driven high for that character, on multiple affected scanlines.)

The intensity pin also existed on the CGA and later EGA cards, where they did pretty much the same thing, but for any given CGA or EGA-standard pixel and colour. The VGA connector however used its R, G, B pins in an analogue fashion – not just on or off, but set to some arbitrary level of brightness, depending on the capability of the VGA or VGA clone card. (I suppose you could argue the original VGA card only had 64 different levels per pin [for a palette of 262,144 colours], which was arguably still quantized and not strictly analogue, but they called it analog[sic]. Close enough for government purposes, I say.) This made an extra intensity pin superfluous. If you wanted to brighten a colour, you just drove the R, G, B pins higher. The intensity pin made sense with MDA/Hercules, and with CGA/EGA digital colour cards, but not with VGA's analog[sic] colour.

Whether or how this information should be added to this or some of the other articles mentioned I don't exactly know, but go ahead if you do. —ReadOnlyAccount (talk) 22:16, 1 December 2023 (UTC)[reply]

Fair use rationale for Image:Executive-Suite-DOS-1982.png

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BetacommandBot 03:52, 1 October 2007 (UTC)[reply]

Resolution

are you sure the pixel resolution of MDA is 720 x 350? When i take a screenshot of ibm pc emulators without scaling i get a 720 x 400 Pixel image out of it... 2001:9E8:6345:9700:35EF:ADC7:82E5:1948 (talk) 16:03, 1 November 2023 (UTC)[reply]

According to "Upgrading and Repairing PCs, 2nd Edition" by Scott Mueller(Que Books, 1992), table 10.1 on page 672, the monochrome display adapter has an effective resolution of 720 * 350, since it displays 80 columns of characters in 25 rows with a 9 by 14 character cell. A screen shot of an emulator is original research and not an acceptable reference for Wikipedia. --Wtshymanski (talk) 03:05, 5 November 2023 (UTC)[reply]

720x400px is the resolution of VGA card colour and monochrome 80x25-character text modes (character matrix 9x16px). A VGA card would use that in lieu of an actual MDA card's 80x25-character text mode output, which indeed was 720x350px (character matrix 9x14px). This means that when you used BIOS video mode 07h, results were not necessarily pixel-identical on all PCs. Close, but no CD. What you got depended on whether your PC sported an MDA/Hercules/EGA or VGA card. (Herc and EGA should've been pretty much pixel-identical with MDA in that mode, though perhaps some character ROMs had minor differences.) –ReadOnlyAccount (talk) 21:05, 1 December 2023 (UTC)[reply]

Disadvantage

This topic is in need of attention from an expert on the subject.

The section or sections that need attention may be noted in a message below.

I feel this recent addition to the article raises more questions than it answers. Does the cited assertion that the presence of an MDA card could halve the bus speed mean all MDA cards and clones did this, or only some? Did original IBM MDA cards do this? Did only some or all clones? Is 4MHz a rounded figure, or was even the original 5150/5160 PC/XT machines' bus speed also reduced, down from 4.77MHz? (I presume on those early machines running slower than 8MHz, bus speed was identical with CPU speed?) Was the speed limitation something that only negatively affected VGA card performance in dual-monitor systems given how VGA demanded more of the bus, or was there a speed impact that was noticeable absent a VGA card as well? What about CGA+MDA dual-monitor setups? Were there ever any 16-bit ISA MDA cards or clones available? I'm not against the inclusion of the cited concern in the article, but it's not very clear, and the section could benefit from expert attention. —ReadOnlyAccount (talk) 20:22, 1 December 2023 (UTC)[reply]

The I/O Channel Ready line is a handshake that allows older cards to pause the bus clock while they retrieve data. This mechanism allows the bus speed to be decoupled from the processor speed and for different cards to access the bus at different speeds. The performance impact would only be while the MDA is actively using the bus but this could gum things up enough to affect performance of a VGA card connected to the same bus. ~Kvng (talk) 15:34, 15 May 2024 (UTC)[reply]