Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

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Sherlock's picture
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Joined: Dec 20 2014
Posts: 135

I have recently switched my Mimeo to MK4027P-4 RAM chips instead of the standard MK4096 chips, mainly because I could get them in gold ceramic, and I like the look. They work great, and I've put them through tests and they seem to be holding up well. There have been warnings about increased power consumption of using other RAM chips, so I'm looking into reducing the power consumption where I can, especially after seeing some of speedyG's current/heat calculations in recent posts.

One area where may be a possibility is in Mike W's PS/2-Serial adapter board, which on my setup is currently multiplexed with the Apple II keyboard via a Corey's keyboard multiplexor design. What I would like to do is add a mechanical switch to turn the PS/2-serial board on or off, since I don't expect to need it most of the time.

In my envisioned setup, I would simply separate the 5V line from the PS/2-serial board and route this through a switch. All other connections (ground, data signals, etc) would be always connected.

The microcontroller in the PS/2-serial board reportedly puts its I/O pins into the high-impedance state when there is no power to the chip, so it should have no effect on the circuit when powered off, and thus should not affect the keyboard multiplexor or the Mimeo.

Does anyone see any issues with a setup like this?

Additionally, I would think I would be able to plug/unplug a serial cable to the PS/2-serial board while it's in the powered-off state, since the serial port on the board is connected to the microcontroller, which is powered off. I don't think this would qualify as hot-plugging, since the board has no power.

So here's that scenario:

1. Mimeo is powered on, and the PS/2-serial board is connected to the keyboard-multiplexor which is also connected to the Apple II keyboard
2. PS/2-serial board is powered off
3. plug in serial cable to PS/2-serial board
4. turn on PS/2-serial board
5. download a program from a host serial connection
6. turn off PS/2-serial board
7. unplug serial cable

Any thoughts or issues with this?

Thanks.

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Joined: Jun 5 2008
Posts: 382
Re: Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

That adapter should draw less than 20 milliamps current, which is the same as adding a single typical LED to a design. If you are not connecting a keyboard, which can draw up to 200 or 300 hundred milliamps (I can't remember the exact limit in the PS/2 spec), I wouldn't bother with a power switch.

Otherwise your idea will probably work OK.

regards,
Mike Willegal

Sherlock's picture
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Joined: Dec 20 2014
Posts: 135
Re: Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

Mike WIllegal wrote:

That adapter should draw less than 20 milliamps current, which is the same as adding a single typical LED to a design. If you are not connecting a keyboard, which can draw up to 200 or 300 hundred milliamps (I can't remember the exact limit in the PS/2 spec), I wouldn't bother with a power switch.

Otherwise your idea will probably work OK.

regards,
Mike Willegal

Wow, only 20 mA? That's tiny. Cool.

Thank you for this info and confirming that the above sequence should be okay. It's enhancing my basic electrical understanding!

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Re: Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

I would leave the grounds connected and just switch the power.

regards,
Mike Willegal

Sherlock's picture
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Re: Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

Mike WIllegal wrote:

I would leave the grounds connected and just switch the power.

regards,
Mike Willegal

OK, thanks!

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Posts: 382
Re: Thoughts on a power switch for PS/2 Keyboard-Serial Adapter

One more thought- early versions of my PS2 adapter automatically assert reset after power up. Later versions make it a jumper option.

Regards,
Mike Willegal