[GreenKeys] Model 28 Keyboards

[Bob Camp]

Hi

What I was thinking about was using a very normal RS-232 transceiver 
chip to hook up the keyboard. Pick any one of the five volt supply / 
internal voltage multiplier parts. from Maxim or Intersil.  Use the 
outputs to drive a nice + and - voltage to bias up the contacts. Then 
use a RS-232 receiver to demodulate the output.  It should work just 
fine to give you a TTL level from a normal 28 or gold  plated 28 
keyboard. No dual supply required. Low current / moderate voltage 
switching just like the Teletype docs call out.

If you have a dual supply available, say from a TU then bias up the 
contacts with +/- 12 or +/- 15 volts. I would still consider using a 
RS-232 receiver.

The only problem with using the RS-232 receiver is that the distortion 
will be slightly higher than if you use a true zero point switch. 
Either way it still will have *way* less distortion than a dry keyed 
single supply setup. The dry keyed setup will always have less 
distortion than it's inductive loaded wet keyed cousin. Of course none 
of that counts ants and grime on the contacts as a source of 
distortion.

	Take Care!
		
		Bob Camp
		KB8TQ


On Jul 4, 2004, at 6:23 PM, AI2Q wrote:

> Thanks for this information Bob.
>
> It looks like I'll have to add an OR or NOR gate to my logic circuit 
> that
> I'm deriving to drive my Model 28 and process the keyboard. I was 
> going to
> use an opto-coupler xstr for driving the selector magnets (and will 
> retain
> that) and put the keyboard in series for local print. Now I will 
> return to
> the drawing board and likely use an OR gate to derive local copy while
> generating logic for FSKing my transmitter.
>
> Vy 73, AI2Q, Alex in Kennebunk, Maine
> http://users.adelphia.net/~alexmm/ai2q.htm
>
>  .-.-.
>
>
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net
> [mailto:greenkeys-bounces at mailman.qth.net]On Behalf Of Bob Camp
> Sent: Sunday, July 04, 2004 11:16 AM
> To: Greenkeys maillist
> Subject: [GreenKeys] Model 28 Keyboards
>
>
> Hi
>
> Well they made me pick up a little here and I found correction T-1 to
> Navships 93788 Vol 1. It has the keyboard stuff in it. To more or less
> quote:
>
> Page 1-30 Keyboard LAK45ARE, LAK45BRW, LK51ARN : These keyboards have
> gold plated contacts designed for 3 volt 70 *micro* amp non inductive
> signaling. Maximum breaking power is 120 mili watts at 25 to 70 VDC.
> An RFI filter is *not* included with these
> contacts..........................
>
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