Data Communications
MULTIPOINT CONFIGURATION
Refer to Section 3 for
multipoint
configuration instructions.
MULTIPOINT PROGRAMMING INFORMATION
This
topic discusses
programming
information of
interest
to someone who
is
writing
a
data
communications
driver
or
controller
program
to communicate
with
this
type
terminal
in
an
HP
Multipoint
environment.
HP
Multipoint protocol is
similar
to IBM Bisynchronous communications
in
that
it
employs
control
characters
(embedded
in
the
data
stream)
to effect
an
orderly
transfer
of
data
between
the
host computer
program
and
the
various terminals.
Multipoint operation
requires
the
following:
• All communications follow a
strict
protocol.
•
Each
terminal
must
have
an
address
that
is
unique
within
its
communication line.
•
Data
is
transmitted
in
blocks.
• All
data
transfers
are
initiated
by
the
computer.
'. • All
terminals
on
the
same
communication line
must
use
the
same
transmission
format
(asynchronous
or
synchronous),
the
same
type
of
transmission
code (ASCII7/ASCIIB),
the
same
type
of
parity
(O's,
ODD, or EVEN),
and
the
same
baud
rate.
Polling and Selecting
All
data
transfers
are
initiated
by
the
computer
in
one oftwo ways, polling
or
selecting.
In
both
cases, device addresses
are
used to identify
the
desired
terminal
or
group
of
terminals.
TERMINAL
ADDRESSES.
Each
terminal
in
an
HP
Multipoint configuration
must
have
an
address
that
is
unique
on
the
communications line on which
it
is
connected (the
same
address
can, however, beusedon two differentlines).An address is
made
up
ofa
one-charactergroup ID
(GID)
and
a one-character device ID (DID) (figure 7-10).
7-23
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