[GreenKeys] TTL and Model 28 Keyboards

[AI2Q]

Hhmmm...good food for thought Bob. I've used Maxim RS-232 chips before, and
they work very nicely for a variety of applications.

Anyway, I've got two opto-couplers in "series" on my logic board (under
development), and decided to use the Model 28's internal supply and a 60-mA
loop, as shown in the Model 28 manuals and diagrams.

On my board, one opto is used to derive TTL from the Model 28, for keying
the rig (an Omni-V), and the other opto-isolator drives a HV xstr for keying
the loop from TTL derived in a switched-cap TU that I have here. I'm picking
off a logic signal from the tail-end of the filter/discriminator portion of
old Kantronics Universal terminal Unit (UTU). It's an 8039-based TU with
firmware in EPROM for copying Baudot, ASCII, CW (!), and AMTOR. Using it I
hope to copy to the glass tube (occasionally) as well as the Model 28 (the
prime mover).

I'd be happy to send anyone JPEG schematics of my board if interested.

Vy 73, AI2Q, Alex in Kennebunk, Maine
http://users.adelphia.net/~alexmm/ai2q.htm

 .-.-.

-----Original Message-----
From: Bob Camp [mailto:ham at cq.nu]
Sent: Monday, July 05, 2004 12:10 AM
To: ai2q at adelphia.net
Cc: 'Greenkeys maillist'
Subject: Re: [GreenKeys] Model 28 Keyboards


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