+5v Current Limit From The Keyboard Connector?

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+5v Current Limit From The Keyboard Connector?

I'm trying to design a pcb using atmega32u4 as an adapter for a PS2 keyboard and I want to know what is the maximum current that can be drawn from the +5v on the keyboard connector on the Apple 1? I know that ps2 keyboards can draw 30ma-100ma but some can draw as much as 300ma.

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I think the connector itself can take up to an amp, but ...

I think the connector itself can take up to an amp, but other factors are involved ...

1. Insulation displacement vs. a soldered connection (soldered is best)

2. Length of the ribbon cable & wire gauge from the connector.

3.  Available power supply current. (this is most important)  I would be really cautious how much load you put on the Apple 1's LM-323K regulator.  The computer itself uses over half it's capacity and the device gets hot enough on it's own.  For this reason, I would use a clean external +5V source and tie the 5V grounds common if you plan on using a mini procesor, unless you can keep it under 250ma or so.

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There are already a couple of

There are already a couple of PS/2 adapter boards that plug into the //e keyboard port and draw their power from there.  I think at least one of them uses a similar microcontroller so you can probably get away with it.

 

Edit -- oops, didn't notice the question was about the Apple 1.  For that application I would probably consider adding your own 5V external supply or tapping into somewhere else if possible.

 

 

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You should design a PS/2 interface to the 300 milliamp spec and also include whatever draw your adapter might also need.  I had issues with my Atmel AVR based PS/2 adapter sufferring from brownout issues when powering up.  I had to tweak the brownout settings on the AVR to make it work reliably.  I think the biggest problem wasn't the DIP connector, but the voltage drop in the ribbon cable that connected the motherboard to the adapter during power up.

 

regards,

Mike Willegal

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Thank you guys!

Thank you guys!

 

On my build im actually just using a hand-built arduino atmega328p which works just fine with any ps2 keyboards i have at home even with an old IBM AT keyboard. 

 

I might just go with an external power source for this project.

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