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I have an old computer from around 1995. It's from a computer brand that I've never heard of called Samanna Compu-Solutions. I am unsure of the model. The problem is, I only have one PS/2 port to work with. Since the computer is from 1995, there are no USB ports. The PS/2 port is labeled as a mouse port, and I've tried a keyboard on there to no avail. My question is: why isn't there a keyboard port? Without a keyboard, there's no way to get past the BIOS.

I'm also getting a CMOS checksum error. The computer is using the Award Modular BIOS v4.51PG. It has 64MB of RAM, and a Pentium MMX 233MHz CPU.

Keyboard error or no keyboard present
CMOS checksum error - Defaults loaded

Press F1 to continue, DEL to enter SETUP

Without a keyboard, I can't press F1 to continue, or press DEL to enter the setup.

I have tried:

  • Clearing the CMOS/removing the motherboard battery
  • Unplugging the keyboard and plugging it back in (while the computer is off, I've heard PS/2 is not hot swappable)

Basically, what I'm trying to solve is how I can use a keyboard and mouse at the same time. Any help would be appreciated.

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    Pictures might help to answer.
    – UncleBod
    Commented Aug 25, 2020 at 14:48
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    The CMOS checksum error just means your CMOS battery has run out and all the settings have been lost.
    – Mark
    Commented Aug 26, 2020 at 0:59
  • @Mark the thing is I've already replaced the CMOS battery twice. Could be a dead socket, though. Commented Aug 26, 2020 at 15:36
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    @TheComputerMan as long as you don’t enter the BIOS setup (once you’ve got a keyboard connected), the error will remain — the checksum won’t be fixed until you save the CMOS settings from the setup. Booting loads defaults when there’s a checksum error, but it doesn’t save them. Commented Aug 26, 2020 at 16:21
  • Something to remember, is that a mouse was an optional component of most PC systems until Windows 95 came out. Users would have been used to using purely a keyboard and character based UI (and probably a monochrome monitor) Your motherboard basically adds a mouse port to an existing motherboard design, and the user can continue to use their existing 5PinDin style keyboard. Even older designs would have used an RS232 style mouse port.
    – Neil
    Commented Dec 9, 2022 at 15:59

4 Answers 4

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Given your computer’s vintage, I’m going to guess that the reason there’s a single PS/2 port, intended for use with a mouse, is that its keyboard port is a 5-pin DIN connector as used in the IBM PC AT and its descendants. This was quite a common setup in the mid- to late nineties — most socket 7 motherboards were AT boards and included these two ports (I still have a Pentium computer with this configuration), and some post-socket-7 boards continued the trend. The 5-pin DIN connector disappeared with the switch to ATX, but that ended up taking quite a long time.

Here’s a photo illustrating the two ports, from Phil Storrs PC Hardware book:

A 5-pin DIN connector next to a PS/2 connector

This isn’t strictly AT-compliant, a more common configuration was to have the PS/2 port on a bracket or another cut-out in the back of the case.

If your computer does have such a DIN port, you can use a PS/2-to-DIN adapter to connect a PS/2 keyboard to it.

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    I find it somewhat humorous that the keyboard and mouse use different pins on the same connector, and that while some computers wired the pins in parallel to allow the ports to be used interchangeably (as was the case with Apple Desktop Bus used on the Macintosh), and while ADB would allow a user to plug in two or more keyboards or pointing devices (and have them all work), the PS/2 interface wouldn't work at all if two keyboards or two mice were plugged in.
    – supercat
    Commented Aug 25, 2020 at 21:28
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    @supercat, as far as I've understood, PS/2 uses the same pins for both keyboard and mouse functions with the other two unused, so you couldn't in fact make the ports interchangeable (but you could make a splitter that allowing plugging both a keyboard and a mouse in one port that was specially made for that, I think I had laptop with such a port once.)
    – ilkkachu
    Commented Aug 26, 2020 at 4:30
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    Working as a PC techie in 1997-2000, I saw Pentium II (slot1) boards in AT format, having this 5-pin DIN keyboard connector.
    – Jonathan
    Commented Aug 26, 2020 at 8:48
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    @ilkkachu I have two systems like that; they work with either a keyboard or a mouse connected directly to the port, or both with a splitter. The port is coloured green and purple (half-and-half). Commented Aug 26, 2020 at 18:21
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    @ilkkachu: I just checked, the ASRock FM2A88X EXTREME4+ here accepts a mouse, or a keyboard, or both. In the latter case it doesn't matter whether I connect the keyboard to the adapter plug labelled K and the mouse to the M one, or the reverse. So it definitely detects what to do (at least during boot up). The port is coloured purple for the upper half and green for the lower half. (And yes, apart from the old KVM switch, I use the PS/2 port to connect a mid-1990s mechanical Cherry keyboard which originally even had the AT plug.)
    – ecm
    Commented Aug 26, 2020 at 19:19
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The IBM PC and its successors the XT and AT used a 5-pin din connector for the keyboard. On the XT and AT this was the only external connector placed directly on the motherboard (the original PC also had a cassette connector), other connectors were on add-in cards.

IBM's next system the PS/2 introduced in 1987 brought us the mini din keyboard and mouse ports but unlike previous IBM systems, the PS/2 was not widely cloned. Many PC clones continued to use derivatives of the AT form factor and hence continued to use the AT keyboard connector. Other PC clones used vendor-specific form factors and some of these did adopt the PS/2 keyboard connector. The PS/2 connector also became the norm on laptops with many supporting a splitter to allow a single connector to support both a keyboard and a mouse.

Systems using the AT form factor varied in how they handled the mouse. Some did not have a PS/2 mouse socket at all, requiring the user to use a serial mouse. Some had a PS/2 mouse port on a flylead to an expansion slot backplate or a mounting hole on the case. Some apparently used a variant of the form factor with a PS/2 mouse port on the motherboard though I have never seen this personally.

In 1995 intel introduced the ATX standard. This replaced the dedicated location for a keyboard connector with a flexible IO panel. For the first decade or so of ATX's life*, this IO panel nearly always incorporated a double-stacked mini din connector for the keyboard and mouse.

Intel pushed ATX heavily with the Pentium 2 and nearly all Pentium 2 systems used ATX, hence the PS/2 keyboard connector. Systems using AMD and Cyrix processors hung on for longer but ultimately the death of socket 7 around 2000 was also pretty much the death of the AT/Baby AT form factors and the 5-pin keyboard connector.

The AT and PS/2 keyboard interfaces were electically the same, only the connector was different. During the period that both standards were in use, adapters were readilly available and when I do a quick google for "ps/2 At keyboard adapter" it seems they are still available today.

* Eventually as USB became dominent, many motherboards dropped the PS/2 connectors or replaced them with a single combined keyboard and mouse connector, but that came much later.

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Fun fact: the PS/2 standard had a specific mouse and keyboard slot. I know because I once had the namesake IBM PS/2 computer that first used it.

I had to move the computer one day and I put everything back together but the computer refused to POST. Since it was a new machine, I couldn't believe it had broken so easily. I tried reseating things, but to no avail. I finally broke down and called tech support. The fix took all of 20 seconds: I had accidentally plugged the keyboard into the mouse port and vice versa. The tech himself admitted it was a bad design on IBM's part.

This was long before color coding would help sort things out (green for mice and purple for keyboards).

If you have a modern motherboard, you might have a slot like this

Combo PS/2 slot

This type of slot can accept either a keyboard or mouse. Otherwise, you'll need to look carefully on the stamped metal. There should be a label telling you what that specific PS/2 port is for.

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you already have an answer, but just some other options:

  • i think there was an adapter that allowed to plug a ps/2 device into a com-port

  • you could add an additional card that gives you another port

  • maybe the port would be available on the motherboard and just not wired to the back of the computer, so you can buy another port and attach it

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    "i think there was an adapter that allowed to plug a ps/2 device into a com-port" not exactly, there were adapters with a serial port plug and a PS/2 socket, but they only worked with dual-mode mice (which afact was most PS/2 ball mice but not PS/2 optical mice), not with keyboards. Commented Aug 26, 2020 at 2:00
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    "you could add an additional card that gives you another port" again no, connecting keyboards through expansion cards didn't become feasible until the USB era as the standard keyboard controller can only exist once in a given system. While you could put a PCI usb controller card in the machine and plug a USB keyboard into it said keyboard would only work when running a modern OS, it would not work in the BIOS setup. Commented Aug 26, 2020 at 2:04
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    "maybe the port would be available on the motherboard and just not wired to the back of the computer, so you can buy another port and attach it" again no, no manufacturer would sell a machine without a keyboard port and the keyboard port was directly on the motherboard right from the first PC. Commented Aug 26, 2020 at 2:06
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    Still won't work in the BIOS though. Commented Aug 26, 2020 at 3:34
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    @manassehkatz-Moving2Codidact: Many did, but I think serial port-based ones were probably about as common, and some devices used proprietary cards, perhaps to let the PC's CPU do more of the decoding.
    – supercat
    Commented Aug 26, 2020 at 6:31

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