I have an old system that I want to connect two monitors with. I have only one VGA port, a parallel port and a 9-pin serial port on the system. One of the monitor only has a VGA port while the other one has a VGA as well as a DVI port. From this question, I know that I cannot use the 9-pin serial connector. How can I connect two monitor to this system, can I use a "VGA to parallel adapter" (?) to connect the two monitors?
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17The parallel port electronics would be far too slow to be able to drive a VGA signal at any speed that would allow a good display.– Mokubai ♦Commented Feb 26, 2023 at 15:53
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8Parallel to VGA adapters do not exist. Maybe there could be one having 1 frame per minute or so.– zomegaCommented Feb 26, 2023 at 15:55
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5What kind of internal ports does the system have? If it's a desktop, does it have any spare PCI/ISA/AGP slots?– grawityCommented Feb 26, 2023 at 16:31
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3VGA is analog while the parallel port is digital. The "adapter" would have to be effectively a parallel-port-connected GPU, and I don't think those exist. If you just want two copies of the same output image e.g. for presentation purposes, there are "VGA signal splitters" for that.– telcoMCommented Feb 27, 2023 at 7:22
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6The answers also make me wonder - how old a system is this?– Journeyman Geek ♦Commented Feb 27, 2023 at 11:17
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6 Answers
Unfortunately, no. You would not be able to connect a monitor to a parallel port. They were designed for slow devices, like printers, scanners, etc. Not to mention it would require special device drivers. The easiest method of connecting another monitor to your computer would be to add a graphics card that supports two video outputs or put in a second video card. Alternatively, you can use a DVI or VGA to USB adapter. Those absolutely work. However, if the port is USB 2, you could use it for regular desktop use (email, word processing, spreadsheets, etc), but not video playback or gaming, as it would be too slow and choppy. USB 3 is fine, but it the machine is old, its doubtful you have it. If you only have USB 1 or 1.1, then I wouldnt try it.
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2@Holger we only know it's got VGA and parallel, we don't know anything else. Loads of machines had VGA, parallel, and USB1 with no HDMI/DVI so it's reasonably likely to have useless USB– Chris HCommented Feb 27, 2023 at 16:04
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3@ChrisH The OP did state: "I have only one VGA port, a parallel port and a 9-pin serial port on the system"– MrWhiteCommented Feb 27, 2023 at 17:25
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6@MrWhite, but does "only" apply to the whole list, or does it just mean that there's only one VGA port? I read it as the latter, partly because we all know the list isn't exhaustive (there's bound to be some form of keyboard port, and almost certainly either a 2nd serial port or more likely a PS/2 mouse port)– Chris HCommented Feb 27, 2023 at 18:23
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1I used a USB 2.0 to DVI adapter for a couple years and was surprised at how well it performed. No gaming sure, but web browsing, even video, was okay. YMMV– notlocCommented Feb 27, 2023 at 20:27
Parallel? No not without a lot of skill and patience and a probably-still suboptimal result. Parallel is just too slow for graphics.
A serial console on the other hand, can present text characters in a far more useful way. You could hide an older rasperry pi around the back of your monitor and run a USB/serial cable from that to your main front PC. Then set the Pi to boot linux, run a serial terminal like minicom on boot, and get your main machine to run a getty on its own serial port.
Pi's have HDMI, and you can get a HDMI-DVI cable to make the monitor connection easy.
You can also put a Hercules or MDA card in the box with your VGA card, and have a second display. You might choose to have the second montior as a logging console in linux, or a second terminal, or even run its own X server. Useful links at https://www.seasip.info/VintagePC/dualhead.html
A third option is to consider what you want to do - some software supports multihead output using a remote client accessed via a network. Classic examples include Doom 1.1 with its -left and -right parameters,
from https://www.techeblog.com/doom-1-1-multi-monitor-setup/
or the famous MS Flight Simulator from 2002, which is over 20 years ago.
Downside - all these depend on some additional hardware from what you describe, and most require more computers, power, and space while adding complexity.
Some very-few video cards of the "old" era may support multiple monitor natively. I had three displays on one linux host in around 2005, using one AGP and one PCI NVidia card which worked well. Finding suitable video cards may be your shortest path to success, but the OS will play a big part too.
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1You can also run a VGA monitor of a Raspberry Pi with an HDMI-VGA adaptor cable. It's a good way of using up old VGA-only monitors for equipment that needs its own console, but only occasionally. I had student setups that were mainly logging, for example. These active adaptors were cheaper than HDMI-DVI passive cables!– Chris HCommented Feb 27, 2023 at 16:08
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6Awesome gaming setups! (Now let's pack it for the LAN party ;-).) Commented Feb 28, 2023 at 2:37
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@Peter-ReinstateMonica that the last one was from a FILM CAMERA shows just how long ago it all was. (neither are my photos btw) 21 years... there are mods on SE who are younger than that photo.– CriggieCommented Feb 28, 2023 at 2:44
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2Certainly, neither Doom nor Flight Simulator sent the video signal over the network. They just sent some player data and the PCs then rendered the view accordingly. Not sure if that's a fair comparison to what OP wants. Commented Mar 1, 2023 at 12:41
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@ThomasWeller you are correct there - OP hasn't told us what the end goal is other than a second monitor on an "old" system.– CriggieCommented Mar 1, 2023 at 18:44
I started to wonder if this might actually be possible, if one were to bit-bang the parallel port signals and synthesise VGA signalling. However, a VGA dot clock is 25.175MHz and a parallel port in ECP mode can transfer 2.5MBPS at best so on the fact of it that's not an option, and even implementing something more complicated, there just isn't enough throughput.
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2A special driver and custom hardware, though you'd be limited to lol-frames-per-second (I'd say maybe 6-8?). You're not going to run games on it, but maybe you could run terminal programs or PowerPoint slide shows.– phyrfoxCommented Feb 27, 2023 at 1:30
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2@phyrfox: There would not be any frames per second rate limit from driving the VGA this way. Rather there would be a very very low horizontal resolution limit of around 64 pixels or so. Otherwise it should work. Commented Feb 27, 2023 at 1:49
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1I was thinking more of a custom video card with 2MB of "VRAM"(double buffered) that collects a full 640x480 every 8th of a second or so while keeping the signal for the VGA, but if we're going full custom, anything in between would be possible.– phyrfoxCommented Feb 27, 2023 at 2:22
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3@phyrfox: At that point you'd just get a cheap Raspberry Pi and connect its GPIO pins to the parallel port. That's more than enough processing power to decompress a simple RLE scheme. The PC driver part will be way harder.– MSaltersCommented Feb 27, 2023 at 12:31
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@R..GitHubSTOPHELPINGICE Otherwise it should work. - While numerically 25 "mega" vs 2.5 "mega" seems like a factor of 10, it's a 8-bit parallel port, so the result is 25 MHz vs 312.5 kHz, a factor of 80. As VGA line frequency is something 31.5 kHz, and ~10% of a scanline is hsync, for playing safe one could output a hsync signal, an empty pixel, 7 actual pixels, and an empty pixel. And even then it's not obvious that the monitor will accept it. Commented Feb 27, 2023 at 13:34
I have an old system that I want to connect two monitors with. I have only one VGA port
Then install a second video card. PCs have no limit to the number of VGA cards as long as you can find interrupt channels for them. Driving them is tricky, but Windows can handle that.
It needs the throughput available on the ISA or PCI bus in order to render graphics worth a darn.
I get where you might think that. I just got a new Mac and it has USB-C ports on it. Apparently I can connect additional monitors to the USB-C ports. As well as the scanner, printer, keyboard, anything can go into those USB-C ports. Darn, you hardly need any other kind of port!
Old computers can't do that. Any given port did One Thing. Or to be more precise spoke One Protocol, you could certainly connect an HP Laserjet III via parallel OR serial OR ethernet.
The parallel port would not have the throughput to do anything useful with a monitor graphics-wise. I suppose you could buy/build a single-board computer that would emulate a printer and drive a VGA monitor. Be easier to get a thing that emulates a VT100 terminal using a VGA monitor, but that would work better on the serial port. You would be able to do VT100 tier graphics on that, but that's pretty lame compared to a VGA card.
answered Feb 28, 2023 at 1:06
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@user253751 There's no designed-in limit - nobody at a manufacturer said "Let's deliberately architect the system to lock in only 2 video cards max". But as a practical thing you run out of resources before you get too many. Back in the DOS days, driver and BIOS problems were on you :) Commented Mar 1, 2023 at 19:15
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@user253751 Some have jumpers or DIP switches to let you move them. Commented Mar 2, 2023 at 19:57
Assuming you have an ISA bus system you can have two video cards, one color card (eg: VGA or EGA or CGA) and one monochrome card (eg MDPA or HGC, or monochrome EGA or VGA)
Or you could buy a "VGA splitter", and put two VGA monitors on the one VGA output.
There are two kinds of VGA splitter: the simplest, which just displayed the same content on two monitors, and the kind at one time used by anybody who wanted a large display, which splits the VGA signal into 2 sections, or 4 or 6 or 16 or whatever, so that you can get a wall of diplay as big as you want.
