24 Aug
2010
24 Aug
'10
1:24 p.m.
Hello all,
I would like to play^H^H^H^H use some of my vintage portables the way
they were intended. For example, the Radio Shack Model 100 and the HP
110 Portable have 300bps internal modems. I have several other modems
thru which these machines could talk to a *NIX but I do not want to pay
for multiple phone lines in my home. How can I simulate a POTS
connection between these modems? I don't need a dial tone or ringing,
etc., as I can just tell one machine to connect and the other to answer.
Way back I heard that a few modems would work if the tip and ring wires
of the modems were crossed and connected, but most need something more
than that. Is there some simple circuit I could build? Surely I'm not
the first to want to avoid phone bills for such play^H^H^H^H testing.
What you need is a 'telephone line simulator' There are many versions
from the very simple (one phone going off-hook causes
the device to send
ringing voltage to the other device, then when that goes
off-hook they
are linked), via the more complicated proper dialing andf ringing tones),
up to the 'real' ones which allow you to delibrately degrade the
connection (introduce losses, noise, etc) for testing modems and the like.
I seem to rememebr last time I did a search for this on google, I found a
fairly simple circuit in the second category. There was a simpler one
publisehd in the US magazine Popular Electornics (Poptronix???) and
reprinted in 'Telephone Projects for the Evil Genius', but that book
seems to have so many errors (including extra/missing connections on
schematics!) that I am not sure I recomend trying to follow it.
If you don't need ringing -- that is you can maually set both devices
off-hook, then very often jsut linking the tips and rings of the 2
devbices will work. While telephones need a DC power supply from the
phone line (commonly called 'battery' over here since the telephone
exchanges ran off large lead-acid batteries), Modems don't generally draw
any power from the phone line, so you don't need this DC supply. If for
some reason you do need it, then try connected tips and rings as before
and connect a 24 V DC supply in series with a 1k resistor (or so...)
between tip and ring.
-tony