MDA / VGA dual setup in a 5150?

Today I tried to add a SVGA card to a 5150 (while keeping the original IBM MDA) with no success. When powering on the bios emits a beep error (1 long 2 short) and the IBM 5151 doesn't show the cursor, only strange things. The SVGA initializes well its monitor. On the motherboard, the DIP switches are configured to set the MDA as primary display.

Is this combination possible?

What version of the PC is it? The early models with 64K RAM on the motherboard and ROM BIOS dated 1981 do not support a VGA card. You need a later model with 256K on the motherboard and the ROM dated 1982, which supports ROM extensions in adapter cards (VGA, hard drive controller, etc.). You can also retrofit the 1982 ROM chips into an older model (IBM sold upgrade kits to do so).

It is a 64-256kb motherboard with the last BIOS. The VGA card works correctly if the MDA is not present and the DIP switches are configured for no video.

AFAIK the only dual card combination that was supported was MDA with CGA.

In any case, if you have an EGA or VGA card installed (no matter if it is primary or secondary), you should set the motherboard switches such that the VGA card is used. If you want the VGA as a secondary display, you may have to alter some settings on the VGA card itself. If the VGA card don't have such settings (very few VGA cards - if any - had such settings), then you can't use the card for that combination. However, almost all EGA cards have jumpers for such settings.

per said:

In any case, if you have an EGA or VGA card installed (no matter if it is primary or secondary), you should set the motherboard switches such that the VGA card is used.

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I have read that the MDA must be set as primary to let the PC bios initialize it: if the motherboard switches are set to use the VGA as primary display, the MDA card won't be initialized and the 5151 can be damaged. Anyone knows if this is true?

nestor said:

I have read that the MDA must be set as primary to let the PC bios initialize it: if the motherboard switches are set to use the VGA as primary display, the MDA card won't be initialized and the 5151 can be damaged. Anyone knows if this is true?

Click to expand...

That is if the system BIOS is being used. The system BIOS can do 3 things depending on the switches: 1. Initialize the MDA, 2. Initialize the CGA in either 80-col or 40-col text mode, 3. Skip the whole initialization and call the BIOS Expansion at absolute address D0000 (if present). It can only do one of them, and that's what you are choosing when you alter the switches on the motherboard.

If your VGA/EGA card has settings for selecting primary or secondary displays, the code in the BIOS Expansion present on the VGA/EGA card will initialize the MDA adapter instead of the System BIOS. This way, you should set the motherboard switches as VGA/EGA and set the switches on the VGA/EGA to use MDA as the primary/secondary display.

None of my VGA cards have jumpers to configure it at master or slave... so, will be dangerous for the 5151 if I put one of these VGAs and configure the motherboard switches to set it as primary?

nestor said:

None of my VGA cards have jumpers to configure it at master or slave... so, will be dangerous for the 5151 if I put one of these VGAs and configure the motherboard switches to set it as primary?

Click to expand...

I've heard that the 5151 can be damaged if it is powered on while not fed with any video signal, but in practice, I found that mine just shuts down its HV circuitry in the absence of a synch signal, with no ill effects.

Today I set the VGA as primary card with the MDA plugged, but without the 5151 connected. The system booted fine, without error beeps.

- Under DOS, if I type 'mode mono' the text don't appear on the VGA monitor (I think it should be displayed in the MDA card, but I didn't connect the 5151 to it).
- Issuing 'mode co80' makes the VGA monitor recover the "focus", displaying again the text.

To explain this behaviour, I assume that the VGA is initialising the MDA card on boot, and it is safe for the 5151 to be plugged on the MDA card... or am i mistaken?

I don't want to blow this beautiful 5151, it is in perfect condition with no shadows burnt in the phosphor.

Try plugging in the 5151 after you issue the MODE MONO command, and see what you get. If you get a valid text display, then it should be safe to keep it plugged in all the time. When you then issue MODE CO80 again, it may even keep showing whatever was on the screen at the time you made the switch, because the text will remain in the MDA's memory buffer until you or a program accesses it again.

Some programs, such as AutoCAD, even supported simultaneous dual-monitor use, with the MDA monitor used for input and output of text commands and the color monitor (up to Super VGA) for displaying the graphics.