[GreenKeys] GreenKeys Digest, Vol 68, Issue 15

[John Nagle]

> 
> Message: 3
> Date: Fri, 18 Sep 2009 17:49:31 -0600
> From: B M <iamvirtual at gmail.com>
> Subject: [GreenKeys] ASR33
> To: greenkeys at mailman.qth.net
> Message-ID:
> 	<2645f9870909181649g114a5e0bg21d0fa72b9f1f99a at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> Is the ASR33 capable of generating voltage/current spikes via the 20mA
> connection?  I realize the connection is passive, but what exactly is
> the 20mA connection driving? Should I be concerned about connecting
> the ASR33 directly to my old PDP-11's?  I am more concerned about
> ensuring the PDP-11 will be ok and not as concerned if something bad
> happens to the ASR33 since the TTY is much easier to fix/replace :-)
> 
> Is there any tests I could perform before hooking the TTY to my computer?
> 
> Many thanks for any information.
> 
> --barrym

    Unlike the older Teletype machines, which were purely electromechanical,
the ASR33 has a drive transistor driving the selector magnet.  Here's
a schematic of an ASR33:

http://www.pdp8online.com/pdp8cgi/query_docs/tifftopdf.pl/CHD/model33_line_set.pdf

See page 3, and note transistor Q2.  The input is driving the transistor, not
the selector magnet itself.

Provided you have a PDP-11 serial card wired for 20mA output, you shouldn't
have any problems.  That's what PDP-11 serial cards were intended to drive.

    For older Model 12/14/15 machines, I've designed a basic
RS-232 to current loop board.  See

http://www.animats.com/nagle/aetheric/connecting.html

Those things do, in fact, generate 400V spikes from the selector
magnet, which have to be snubbed and isolated.

But you shouldn't need that to connect an ASR-33 to a PDP-11.

				John Nagle