"Hugo Cornwall "The Hacker's handbook"" - читать интересную книгу автора

Pin 3 characters arriving at the computer

The remaining connections are for such purposes as feeding power
to an external device, switching the external advice on or off,
exchanging status and timing signals, monitoring the state of the
line, and so forth. Some computers and their associated firmware
require one or other of these status signals to go 'high' or 'low' in
particular circumstances, or the program hangs. Check your
documentation if you have trouble.
Some RS232C implementations on microcomputers or add-on boards are
there simply to support printers with serial interfaces, but they can
often be modified to talk into modems. The critical two lines are
those serving Pins 2 and 3.
A computer serving a modem needs a cable in which Pin 2 on the
computer is linked to Pin 2 on the modem.
A computer serving a printer, etc, needs a cable in which Pin 3 on
the: computer is linked to Pin 2 on the printer and Pin 3 on the
printer is linked to Pin 2 on the computer.
If two computers are linked together directly, without a modem,
then Pin 2 on computer A must be linked to Pin 3 on computer B and
Pin 3 on computer B linked to Pin 2 on computer A: this arrangement
is sometimes called a 'null modem' or a 'null modem cable'.
There are historic explanations for these arrangements, depending
on who you think is sending and who is receiving--forget about them,
they are confusing. The above three cases are all you need to know
about in practice.
One difficulty that frequently arises with newer or portable
computers is that some manufacturers have abandoned the traditional
25-way D-connector, largely on the grounds of bulk, cost and
redundancy. Some European computer and peripheral companies favour
connectors based on the DIN series (invented in Germany), while
others use D-connectors with fewer pin-outs.
There is no standardisation. Even if you see two physically
similar connectors on two devices, regard them with suspicion. In
each case, you must determine the equivalents of:

Characters leaving computer (Pin 2)
Characters arriving at computer (Pin 3)
Signal ground (Pin 7)

ou can usually set the speed of the port from the computer's
operating system and/or from Basic. There is no standard way of doing
this; you must check your handbook and manuals. Most RS232C ports can
handle the following speeds:

75, 110, 300, 600, 1200, 2400, 4800, 9600

and sometimes 50 and 19200 baud as well. These speeds are selectable
in hardware by appropriate wiring of a chip called a baud-rate
generator. Many modern computers let you select speed in hardware by