24 Aug
2010
24 Aug
'10
10:51 a.m.
On 2010 Aug 24, at 8:51 AM, Jim Scheef wrote:
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.
If both modems in a given connection are older ('80s), lower-speed
(300-1200/2400) it will likely work by simply connecting them together
and commanding one off-hook in answer mode (ATA ?), then the other
off-hook in originate mode (ATD ? ATH1 ?). I have successfully done
this with particular modems up to 14.4Kbps but might be forgetting the
commands.
The older, lower-speed modems are likely to be transformer-coupled onto
the line and will inject audio energy onto the line without need for DC
line current. Some higher-speed modems do this as well and will work,
other higher-speed modems use opto-couplers and need the DC line
current to modulate.
You shouldn't need to worry about 'crossing' tip and ring. Technically
it is more correct to connect tip-tip and ring-ring, although it is not
likely to matter one way or the other. At the electrical level it is
not a null-modem-type situation where one has to swap xmtr-rcvr
circuits.