A500 Keyboard - No VCC (+5V)

On an A500 or A500+ keyboard, if VCC (+5V) is missing, the _KBRESET signal is held LOW, so the machine fails to boot.

I will try to explain why this is the case.

Below is the keyboard connector CN13 which has the VCC (+5V) rail on pin 4, and the _KBRESET signal on pin 3.

On amigapcb.org using the board view for a rev 8A board, you can see the CN13 connector and where those pins are:

In the Mitsumi keyboard schematics at:

https://github.com/BleuLlama/AmigaSchematics/blob/master/A500_kyb_Mitsumi/A500_kyb_Mitsumi.pdf

RST_OUT is what is connected to _KBRESET on pin 3. This RST_OUT signal is the outpout of a 3 input NOR gate in an 74LS27 logic chip.

The truth table for a 3 input NOR gate is:

However, this is only when the logic chip has power and working correctly.

It seems that on an Amiga keyboard, when the 74LS27 logic chip does not get VCC (+5V), it keeps the logic Inputs and Output pins LOW. Hence why _KBRESET is always kept LOW and the A500 fails to boot.