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Specific terminal emulation - Some software has pre-formatted sets of
characteristics to mimic popular commercial 'dumb' terminals. For
example, with a ROM costing under г60 fitted to a BBC micro, you can
obtain almost all of the features of DEC's VT100 terminal, which
until recently was regarded as something of an industry-standard and
costing just under г1000.
Other popular terminals are the VT52 and some Tektronix models, the
latter for graphics display. ANSI have produced a 'standard'
specification.

Baudot characters - The Baudot code, or International Telegraphic
Code No 2, is the 5-bit code used in telex and telegraphy -- and in
many wire-based news services. A few terminal emulators include it as
an option, and it is useful if you are attempting to hack such
services. Most software intended for use on radio link-ups (see
Chapter 10) operates primarily in Baudot, with ASCII as an option.

Viewdata emulation - This gives you the full, or almost full,
graphics and text characters of UK-standard viewdata. Viewdata tv
sets and adapters use a special character-generator chip and a few,
mostly British-manufactured, micros use that chip also-- the Acorn
Atom was one example. The BBC has a teletext mode which adopts the
same display. But for most micros, viewdata emulation is a matter of
using hi-res graphics to mimic the qualities of the real thing, or to
strip out most of the graphics. Viewdata works on a screen 40
characters by 24 rows, and as some popular home micros have 'native'
displays smaller than that, some considerable fiddling is necessary
to get them to handle viewdata at all.
In some emulators, the option is referred to as Prestel or
Micronet--they are all the same thing. Micronet-type software usually
has additional facilities for fetching down telesoftware programs
(see Chapter 10).
Viewdata emulators must attend not only to the graphics
presentation, but also to split-speed operation: the usual speeds are
1200 receive from host, 75 transmit to host. USA users of such
services may get them via a packet-switched network, in which case
they will receive it either at 1200/1200 full duplex or at 300/300.
Integrated terminal emulators offering both 'ordinary'
asynchronous emulation and viewdata emulation are rare: I have to use
completely different and non-compatible bits of software on my own
home set-up.

Modems


Every account of what a modem is and does begins with the classic
explanation of the derivation of the term: let this be no exception.
Modem is a contraction of modulator-demodulator.
A modem taking instructions from a computer (pin 2 on RS232C)
converts the binary 0's and 1's into specific single tones, according