HP 2641A Reference Manual page 74

The J07 switch is used to select the code to be used to
represent data. The codes available are as follows:
J07
o
(closed) = ASCII
1 (open)
=
EBCDIC
Block Check Character (BCC). The Block Check
Character is used to verify the accuracy of transmitted
data. Switch J06 allows you to select the type of test used.
The terminal will then automatically perform the proper
test and generate the same type of check character sent to
the computer. The types of check character available are
as follows:
Data Communications
apostrophe (') 47 octal ('047'). If you use the PA or PF
functions refer to their descriptions elsewhere in this
section.
CCA - Current Cursor Address. This is a two character
address and will always be SP,SP ('040040'). This is the
cursor home position (0,0).
Note that if you have configured the terminal for text
mode compatibility and are not operating with such a
system, the first three characters in the first text block
received by the terminal will be ignored. Also, the three
leading characters (AID, CCA, CCA) will be enibedded in
the transmitted text block.
J06
o
(closed)
1 (open)
Longitudinal redundancy check
(LRC)
Cyclic Redundancy Check (CRC
16)
Buffer Size. You must set the amount of terminal mem-
ory allocated for use as input and output communication
buffers. When the terminal is inputting data it uses this
space for a single input buffer. When the terminal is out-
putting data the buffer space is divided into two or more
output buffers. The basic terminal configuration uses a
500 byte input buffer or two 250 byte output buffers.
EXTENDED TEXT FEATURE. The Extended Text
Feature is selected by setting the J05 switch on the mul-
tipoint interface to the 1 (open) position.
J05
When the terminal is selected, any data waiting in the
output buffers is lost. The output buffers are then used as
a single input buffer to hold data sent from the computer
until the terminal can process the characters.
3 leading characters
/ \
\
~
WCC TEXT
~
The Extended Text feature can be used to generate and
delete three special characters used with an IBM 3270
terminal. After the computer has selected the terminal to
receive data, the first text block will have the following
form:
1 (open)
o
(closed)
Extended features
No effect
The terminal will respond to select sequences with a
WACK when there are fewer than 250 bytes available in
the input buffer. The terminal will respond with an ACK
as soon as 250 bytes of buffer space become available. Note
that if too large a block is sent to the terminal following
the ACK it may result in a buffer overflow and an EaT
will be returned.
It is often desirable to increase the size of the communica-
tion buffers to optimize use of the computer. The size of the
terminal's input buffer can be set on the communication
interface. The input buffer size can range from 500 to 4000
bytes.
Note that the characters follow STX and precede the text
block. Since these characters are not used by the terminal,
they would normally be accepted as a part of the text
block. Selecting the Extended Text feature will cause the
terminal to discard these three characters before process-
ing the text.
When the first block of text is sent to the computer in
response to a POLL sequence, the computer expects to see
the following:
If there is no RAM memory installed in the address range
48K-52K, buffer space is allocated from the terminal's
display memory (see figure 7-10). This means that the
larger the buffer size, the smaller the amount available to
display memory. (A minimum of 8K of display memory is
recommended.)
Input buffer size is set by switches J17 and J16 as follows:
J17 J16 Input Buffer Size
\ GID DID AID CCA CCA TEXT
~
\ /
3 leading characters
The leading characters that are sent by the terminal are
as follows:
AID - attention J.D. This character will normally be an
o
o
1
1
o
=
500 bytes
1 = 1000 bytes
o
=
2000 bytes
1
=
4000 bytes
where 1
=
open
o
=
closed
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