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

means of a DIL switch. The higher speeds are used either for driving
printers or for direct computer-to-computer or computer-to-peripheral
connections. The normal maximum speed for transmitting along phone
lines is 1200 baud.
epending on how your computer has been set up, you may be able to
control the speed from the keyboard--a bit of firmware in the
computer will accept micro-instructions to flip transistor switches
controlling the wiring of the baud-rate generator. Alternatively,
the speeds may be set in pure software, the micro deciding at what
speed to feed information into the serial port.
n most popular micro implementations the RS232C cannot support
split-speed working (different speeds for receive and transmit). If
you set the port up for 1200 baud, it has to be 1200 receive and
transmit. This is a nuisance in Europe, where 75/1200 is in common
use both for viewdata systems and for some on-line services. The
usual way round is to have special terminal emulator software, which
requires the RS232C hardware to operate at 1200 /1200 and then slows
down (usually the micro's transmit path) to 75 baud in software by
means of a timing loop. An alternative method relies on a special
modem, which accepts data from the computer at 1200/1200 and then
performs the slowing-down to 75 baud in its own internal firmware.



Terminal emulators

We all need a quest in life. Sometimes I think mine is to search
for the perfect software package to make micros talk to the outside
world.
As in all such quests, the goal is occasionally approached but
never reached, if only because the process of the quest causes one to
redefine what one is looking for.
These items of software are sometimes called communications
packages, or asynchronous comms packages, and sometimes terminal
emulators, on the grounds that the software can make the micro appear
to be a variety of different computer terminals. Until recently, most
on-line computer services assumed that they were being examined
through 'dumb' terminals--simply a keyboard and a screen, with no
attendant processing or storage power (except perhaps a printer).
With the arrival of PCs all this is slowly changing, so that the
remote computer has to do no more than provide relatively raw data
and all the formatting and on-screen presentation is done by the
user's own computer. Terminal emulator software is a sort of
half-way house between 'dumb' terminals and PCs with considerable
local processing power.
Given the habit of manufacturers of mainframe and mini- computers
to make their products as incompatible with those of their
competitors as possible (to maximise their profits), many slight
variants on the 'dumb' computer terminal exist--hence the
availability of terminal emulators to provide, in one software