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Keyboard Loopback

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Booting without a Keyboard

When setting up a server or any other sort of headless machine, booting without an attached keyboard can be somewhat problematic. However, most new off-the-shelf consumer computer hardware has an included BIOS option that explicitly allows you to boot without keyboard. I have found this to be the case on almost all systems capable of using a USB keyboard. Older systems may lack this feature, but nevertheless the issue is also addressable from the BIOS. These older machines [should] have a bios option regarding error halts during POST; to boot successfully without a keyboard, one is simply required to select that errors "halt boot on all but keyboard." (thats the phrase most often used, or some variation of it). Failing that - for even older and/or non-standard equipment, a very simple hardware level workaround is available.

During boot, most [most*] systems detect that a keyboard is present and active by monitoring the current levels in the keyboard port. In order to circumvent this check, merely wire a 10K ohm resistor between ground (pin 3) and +5 volts (pin 4) on any standard PS/2 style keyboard connector. If the keyboard is an older PC/AT keyboard - the large 5 pin connector version - you may simply use the same wiring scheme with an AT-to-PS/2 adapter. Alternatively, connect the resistor between ground (pin 4) and +5V (pin 5).

With truly ancient parallel port (DB25) and serial port (DB9) keyboards, the above approach may also help you to find success. Your mileage may (will) vary.

I highly suggest using an extension cable from the motherboard port so that you may detach this setup. Otherwise, inserting a resistor directly into the motherboard port will work too.

Note that the pinout of each connector is somewhat counterintuitive:

PC/AT:
n
1    3
4    5
2

PS/2:
n
5    6
3    4
1 2

(perspective of looking into the connector on the keyboard, not the PC. The character "n" denotes the notch in the shield around the connector.)

* by most, I imply "not all." Some systems insist upon initializing the keyboard circuit, setting the caps/num lock, etc. Therefore, this keyboard loopback trick with a resistor will not work on every system; as with everything, your mileage may vary. However, in general, if it does NOT work, one of the other methods usually will.

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