One method for connecting two computers
together to transfer files is by a direct
modem to modem connection. Some
reasons to do so: the two computers may
not have diskette compatibility; one
computer may have a modem, but no free
serial port; a serial port may be free, but
(especially on some modern PC's with Asian
made motherboards) the back panel
connector may be wired incorrectly; getting
two different computers to communicate
correctly via serial port connection (i.e., "null-
modem") may be difficult; getting files other
than text files to transfer via rs-232 may be
tricky. A direct modem connection has the
advantage that standard terminal programs
are designed primarily to work with a
modem. Another possibility is that of
printing via a fax machine; some PC printers
are multi-function: fax/scanner/copier/
printer. This is another way an old Mac
might access such devices. I have seen this
mentioned, but haven't been able to try it
yet (I don't yet have a compatible Mac fax
program). It might also be possible to use a
fax machine as a scanner this way; connect
one computer modem to the fax machine.
One necessity for a reliable connection of
this type is a "line simulator", a device to
provide simulation of a telephone line,
supplying the line current that modems are
designed to work with. For our purposes, a
very simple home-made circuit will do the
job (you should be able to find most, if not
all, of the parts at a Radio Shack store). This
connects between two telephone modular
jacks, and the two modems plug into the
jacks: Note that the resistor value depends on the
actual voltage used. For 24V about 1K Ohms
max will give at most 24mA (12V @ 500
Ohms, 9V @ 380 Ohms); the resistance of
the modem circuit will reduce this slightly
(you may need to reduce the resistor value,
but if it works with the values mentioned,
leave it at that; I'm using 380 Ohms with a
voltage input of 14V). The telephone
company guarantees about 20mA minimum
in an actual phone line, and we want to be
about the same minimum. Note also that the
battery shown can be replaced by a "wall
wart" power supply; most of these are un-
regulated (my "9V @ 130mA" plug in DC
supply gives about 14V on this circuit when
connected to a telepone for testing) and
consequently will need an electrolytic
capacitor of about 2200 uF across the
power supply + and - terminals to reduce
the "ripple" voltage (i.e., AC "noise"); be sure
to match the polarity of the Electrolytic to the
polarity of the power supply.
A more elegant solution when using an
unregulated power supply is to add a 12
volt regulator such as a 78L12 (or a 7812 if
you can't find the low power "L" version) as
shown here:
In the circuits above, the 10uF is a Tantalum,
but you can substitute electrolytic. The 0.1uF
is ceramic, and the 0.47uF should be an
audio grade bypass; I used a polypropelene
capacitor here. Make sure the capacitors
together to transfer files is by a direct
modem to modem connection. Some
reasons to do so: the two computers may
not have diskette compatibility; one
computer may have a modem, but no free
serial port; a serial port may be free, but
(especially on some modern PC's with Asian
made motherboards) the back panel
connector may be wired incorrectly; getting
two different computers to communicate
correctly via serial port connection (i.e., "null-
modem") may be difficult; getting files other
than text files to transfer via rs-232 may be
tricky. A direct modem connection has the
advantage that standard terminal programs
are designed primarily to work with a
modem. Another possibility is that of
printing via a fax machine; some PC printers
are multi-function: fax/scanner/copier/
printer. This is another way an old Mac
might access such devices. I have seen this
mentioned, but haven't been able to try it
yet (I don't yet have a compatible Mac fax
program). It might also be possible to use a
fax machine as a scanner this way; connect
one computer modem to the fax machine.
One necessity for a reliable connection of
this type is a "line simulator", a device to
provide simulation of a telephone line,
supplying the line current that modems are
designed to work with. For our purposes, a
very simple home-made circuit will do the
job (you should be able to find most, if not
all, of the parts at a Radio Shack store). This
connects between two telephone modular
jacks, and the two modems plug into the
jacks: Note that the resistor value depends on the
actual voltage used. For 24V about 1K Ohms
max will give at most 24mA (12V @ 500
Ohms, 9V @ 380 Ohms); the resistance of
the modem circuit will reduce this slightly
(you may need to reduce the resistor value,
but if it works with the values mentioned,
leave it at that; I'm using 380 Ohms with a
voltage input of 14V). The telephone
company guarantees about 20mA minimum
in an actual phone line, and we want to be
about the same minimum. Note also that the
battery shown can be replaced by a "wall
wart" power supply; most of these are un-
regulated (my "9V @ 130mA" plug in DC
supply gives about 14V on this circuit when
connected to a telepone for testing) and
consequently will need an electrolytic
capacitor of about 2200 uF across the
power supply + and - terminals to reduce
the "ripple" voltage (i.e., AC "noise"); be sure
to match the polarity of the Electrolytic to the
polarity of the power supply.
A more elegant solution when using an
unregulated power supply is to add a 12
volt regulator such as a 78L12 (or a 7812 if
you can't find the low power "L" version) as
shown here:
In the circuits above, the 10uF is a Tantalum,
but you can substitute electrolytic. The 0.1uF
is ceramic, and the 0.47uF should be an
audio grade bypass; I used a polypropelene
capacitor here. Make sure the capacitors
have
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