[GreenKeys] ASR33 modem

[Chris Elmquist]

Hey Nico...

Yes... you can also connect to the terminal block but when you do, you
will still need to unplug the modem from the molex connector since it
would be in parallel with the connections on the terminal block if it
is plugged in.

Many computer connections to an ASR-33 run the loop with 24VDC...  which
is derived from +/- 12V supplies also used to power the RS232 transmitter.

There is typically an opto-isolated interface from TTL to the loop and
the computer supplies the current on both the keyboard and the print
loops, which are kept seperate to provide a (full) duplex connection to
the computer.  I have examples from Altair, SWTPC, Heath H8, KIM-1, etc.
All machines c. 1974 to 1980 or so.

Chris

On Thursday (08/05/2010 at 08:50AM +0200), Nico de Jong wrote:
> ----- Original Message ----- 
> From: "B M" <iamvirtual at gmail.com>
> > On my ASR-33, an Anderson-Jacobson acoustic coupled modem (ADT-233) is
> > installed.  Does anyone have information on this modem?  I have
> > googled around, but have only come across the patent for the device,
> > but no further information.
> >
> > I am looking to connect the TTY up to my ancient PDP-11 computer, but
> > I need to be sure the modem is sufficiently disconnected for the
> > direct connection.
> >
> When you look at the back of the ASR33, you will see 8 white connectors. 
> Below these connectors you find a terminal block with 9 screws. The two 
> leftmost are for the AC. Number 3 is Send+, number 4 Send-, number 6 RCV- 
> and number 7 RCV+
> Connect 4 to 6.
> 2 and 7 are brought out on a jackstik. This is to be connected to a current 
> source of 20 mA, e.g. 160VDC and a resistor of 8K (10W).
> 
> Your now have a serial connection, 110 bps, 8 bits, 1 stop bit.
> 
> I know nothing about PDP, so what's written below is for a PC:
> 
> http://www.baudot.net/tty-connect/hardware.htm
> 
> This box comes between a PC COM port and the jackstick.
> 
> Nico
> 
> 
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-- 
Chris Elmquist