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Data Communications
MULTIPOINT COMMUNICATIONS
The terminal is capable of operating in a polled multipoint
environment. This means that one or more terminals can
share the same communication line.The terminal can be
used in networks using asynchronous or synchronous
communications. Operation is similar to IBM Bisynchron-
ous communications. 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 code (ASCII/EBCDIC) and parity.
•
A minimum of 8K of display memory.
MULTIPOINT PROTOCOL. The terminal uses a mul-
tipoint protocol that is similar to IBM Bisync. The protocol
is made up of sequences of one or two control characters.
Table 5-10 contains a list of the control characters used
along with a short description.
BREAK Key Operations. The
IIlI
key allows the user
to tell the CPU or application program that he wants to
abort the current operation. (Long text transfers from the
CPU can be stopped by holding down
IIlI .)
When the
terminal is in Text-In mode and thelma key is held down,
an RVI (DLE
<)
is sent to the CPU instead of an ACKO or
ACK1 after the current text block is received. The CPU
software must then respond to the RVI in an appropriate
manner.
If the terminal is in the Text-Out or Control mode and the
III
key is pressed, the terminal will clear all data in the
data comm output buffers (the data is lost) and then it will
send \
GIDDID
q,
lie
in response to the next poll from the
CPU. The CPU software must then respond in an appro-
priate manner.
PA and PF Key Functions. Multipoint operation al-
lows you to enter an escape sequence to select operation
comparable to the CLEAR, PA, and PF keys on the IBM
3270 terminal. The escape sequence can be entered from
the keyboard or a tape unit or datacomm. The PA and PF
functions allow you to send a single character to the com-
puter or preface the data with a special character. Then
depending on how the computer is programmed, it can use
this character to branch to various data handling routines.
The escape sequence to define the character to be sent is as
follows:
~
&
9
I I I
{F
or
A}
where:
### is the octal code of the character to be trans-
mitted. It can be made up of 0 to 3 octal digits. This
5-36
character must have an octal code in the range 040
to 176. Note that the DELETE character (octal 177)
cannot be used. Ifno character is defined, the default
value returned will be an octal 047 (27 hex) if Ex-
tended Text Mode is selected.
The softkeys (. . - . .) can be loaded with the escape
sequence. Refer to the description of "softkeys" elsewhere
in this manual for procedures. Note that the softkeys are
cleared by a full reset.
P A Operation. If the last character of the escape
sequenc~
is an A, it will cause the single character indi-
cated by the octal code to be sent to the computer the next
time the terminal is polled. This is done by creating a new
text block in the output buffer.
Example:
Et:
Ir
9
122 A
(Note: R
=
122 octal)
This would cause the following text block to be sent to the
computer:
<\><GID><DID><R><Iie><BCC><PAD>
PF Operation. If the last character in the escape
sequence is an F, it will cause the defined character to-
gether with the data currently displayed on the terminal
screen to be sent to the computer the next time the termi-
nal is polled.
Example:
Et:
Ir
9
120 F
(Note: P
=
120 octal)'
This would cause the following text block to be sent to the
computer:
<~)<GID)<DID)<P)<~creendata)<~)<BCC)<PAD)
Note that if the screen data exceeded the terminal block
size the transmission would use the normal multiblock
format.
When in Extended Text mode the PF escape sequence will
cause the character coded in the sequence to be sent as the
AID character. (Refer to Extended Text Feature.)
Example:
Et:
Ir
9
120 F
(Note: P
=
120 octal)
This would result in the following text block in Extended
text mode:
<\><GID><DID><P><CCA><CCA><~creen
text>
(1ie>(BCC>(PAD>
Typical Applications of the P A and PF Func-
tions. Use of the PA and PF functions allow the com-
puter to use a general poll to find out more than just which
terminals have data ready to send. If the PA and PF
functions are used, the character returned from each ter-
minal can be used to determine whether the terminal has
data, no data, a large amount of data, or high priority
data. For example the terminal's programmable function
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