User`s manual
3.6.1
Originating
and Answering
Calls
The
Control
Status
Register
(CSR)
arid
the
Line
Stat-
us
Register
(lSR)
are
provided
to
enable
the
PDP-II
program
to
originate
and
answer
calls
to/from
remote
modems.
Initially,
the
local
modem
is
enabled
and
the
operating
mode
(interrupt
or
non-interrupt)
is set,
as
described
in
Paragraph
3.5.1.
An
inter-
change
then
takes
place
between
the
PDP-II
pro-
gram
and
the
MCU
to
originate
a call,
as
follows:
1.
PD
P-II
program
sends
Data
Terminal
Ready
(LSR
01)
to
cause
enabled
modem
to
hold
the
line
once
the
call is
established.
2.
PD
P-II
program
dials
remote
number
via
DN
11
Automatic
Dialing
Unit,
or
an
operator
manually
initiates a call
to
the
remote
modem.
When
the
call
has
been
established,
the
DN
11
will
hold
the
line
via
the
Call
Request
line
and
Data
Termi-
nal
Ready.
In
the
manual
dialing case,
the
operator
switches
to
"Data
Mode"
and
Data
Terminal
Ready
holds
the
call.
3.
PD
P-II
program
waits
on
Data
Set
Ready
(DSR)
transition
from
the
enabled
modem
(CSR
12).
If
the
MCU
is
oper-
ating
in
the
non-interrupt
mode
with
only
one
line
enabled
(as reflected by
the
con-
tents
of
CSR
00-03)
LSR
04
may
be
read-
ily used
to
monitor
the
DSR
line.
4.
When
DSR
is detected,
the
PDP-II
pro-
gram
sends
a
Request
to
Send
(LSR
02)
to
set
the
data
mode
for
transmission.
5.
PDP-II
program
waits
on
Carrier
On
(CO)
and
Clear
to
Send
(CS)
transitions
(CSR
14
and
13)
from
the
enabled
modem.
6.
When
CO
and
CS
are
detected,
the
PDP-
11
program
starts
the
DVII
Data
Han-
dling
Section
and
initiates
data
transfer.
Answering
a call
consists
of
the
PDP-II
program
detecting
the
Ring
transition
from
the
enabled
mod-
em
(CSR
15),
then
I.
PD
P-II
program
sends
Data
Terminal
Ready
(lSR
01)
to
cause
enabled
modem
to
answer
the
call.
3-42
2.
PD
P-II
program
waits
on
Data
Set
Ready
(DSR)
transition
(CSR
12)
and
the
Carrier
On
(CO)
transition
(CSR
14)
from
the
enabled
modem.
3.
When
CO
is
detected,
the
PDP-II
pro-
gram
starts
the
DVII
Data
Handling
Sec-
tion
and
initiates
data
transfers.
3.6.2 Resynchronization During Reception
If
line
synchronization
initially fails
or
is lost,
the
PDP-II
program
can
command
resynchronization
during
reception
by setting
bit
01
of
Line
State
sec-
ondary
register
to
one.
The
DVII
then
1.
defines a
"Resync
Flag
Expected
interval"
(Line
Progress
secondary
register
bit
07
set
to
one),
during
which
any
receiver
characters
for this line
already
buffered
in
the
D V
11
are
discarded
2.
clears
the
Resync
Command
bit
(Line
State
01)
and
Receiver Active
(Line
State
00),
and
3.
searches
for
the
synchronization
character.
When
the
synchronization
character
is
found,
the
DVII
sets
the
Receiver
Active
bit
to
one
to
enable
receipt
and
storage
of
subsequent
characters
on
the
resynchronized
line.
The
program
should
not
request
resynchronization
again
until
at
least
one
character
has
been
received since
the
previous
resynchroniza-
tion
request.
3.6.3
Termination
of
Transmission
and
Reception
The
DV
11
terminates
transmission
on
a line
when-
ever
both
principal
and
alternate
byte
counts
have
reached
zero,
or
a
non-existent
memory
or
memory
parity
error
condition
is
encountered.
The
DVII
sets
Transmitter
GO
(Line
State
secondary
register
bit
02)
to
zero
to
terminate
transmission.
The
PDP-II
pro-
'gram
may
set
Transmitter
GO
to
zero
to
abort
transmission.
The
PDP-II
program
shuts
down
reception
on
a line
by
clearing
Receiver
Enable
(LCR
13)
and
setting
li,ne
State
secondary
register
bit
01
(Receiver
Res-
ynchronize)
to
one.
The
DVII
then
I.
clears
the
Resync
Command
bit
(Line
State
01)
and
the
Receiver Active
bit
(Line
State
00),
and
2.
discards
any
receiver
characters
already
accumulated
for
the
line.