[GreenKeys] Model 28 Keyboards

Mon Jul 5 12:36:43 EDT 2004

Hi

Here's a couple of parts that could be used for a simple interface to 
the low voltage contact keyboards:

1) Simple setup with RS-232 chip

Run a Maxim MAX232A 2 RX / 2 TX RS-232 chip. They run about $1.85 and 
the four chip caps would probably run $0.50

The chip has two transmitter channels and two receiver channels. Set 
one transmit channel high and the other low to give + and - 10 volts to 
bias the keyboard(s).

Run the output(s) of the keyboards into the RS-232 receivers on the 
chip.

Harris / Intersil also sells the same chip for a bit less money.

2) Go totally nuts setup

Run a Maxim MAX865 5V to +/- 10V converter. Converter is $1.30 and the 
four chip caps probably would run $1.00.

If you already have +/- 12 then forget about the Maxim chip.

Add a National LM3302 quad comparator, this adds $0.28.

The 3302 is an open drain part so you can wired or the outputs 
together.  In order for this to work the output convention would be V+ 
= mark at the comparator output.

Next parts would be something like a 2N3904 and a 1N4148 to convert to 
open collector, ground referenced output. Figure on a three 10K ohm 
resistors. One to pull up the transistor base to +5. One to pull the 
base down to the wired or junction on the comparators. One to pull up 
the collector of the 3904 and give you a CMOS output. The 1N4148 goes 
reverse paralell between the base and emitter to protect the 
transistor.

That's a few more parts than the RS-232 version but it would run 4 
keyboards instead of only two.

In both cases a few capacitors and resistors to improve RF immunity 
would be a good idea if you run a transmitter. They probably are not 
needed for reception since dry keying is very low RFI ...

Don't know if all of that is very clear or not. It's pretty cheap stuff 
though and the keyboard contacts will run forever with dry keying (and 
no ants ...).

If you dry switch the keyboard and solid state (or tube) switch the 
magnets the RFI from the TTY is a lot less than with a wet switched 
inductive loaded loop. The arc on the mechanical contacts is a *very* 
good RF generator.

RFI is the main reason I like to go this way. It's more or less the 
same thing that drove the Tempest certified designs as well. Having the 
machine mess up the receiver is not all that uncommon if your antenna 
is in a less than perfect location. Of course all of you with eighty 
foot boom log periodics up 250 foot towers need not worry about any of 
this ....

One nice thing about BPL coming to your neighborhood is that it will 
mask all the other sources of RFI so this sort of thing may not matter 
.....

	Take Care!

		Bob Camp
		KB8TQ

On Jul 5, 2004, at 12:09 AM, Bob Camp wrote:

> 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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>
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