6530 Programmer's Guide

Block Mode Operation
3-72
6530 Programmer’s Guide
Outbound
Compression
Outbound compression provides compression of repetitive characters
sent from the 6530 to the Tandem host. In order for an application to
determine if a particular terminal supports outbound compression, the
value associated with the READ CONFIG (Esc ?) letter f (66h) must be
checked. A value of 1 or 2 indicates support for outbound compression.
A value of 0 indicates no support.
Outbound compression operates only in block mode and is always reset
on entry to block mode (SETMODE 8,1). This is to protect host
applications that have not implemented support for outbound
compression. The application must set outbound compression on after
entering block mode by using SET CONFIG (Esc v) to set the value for
the letter f to 2. For example:
Esc v f 2 CR
1b 76 66 20 32 0d (hex values)
If outbound compression is set on, an outbound sequence of 4 or more
repeated characters is replaced by a Limited Data Compression (LDC)
sequence or an Extended Data Compression (EDC) sequence.
Start Limited Data Compression (DC4)
The control character DC4 (14H) specifies the start of a 2-byte character
sequence that defines Limited Data Compression (LDC) characteristics.
A 1-byte parameter that follows the DC4 control character specifies
whether the character to repeat is an ASCII zero (30h) or an ASCII blank
(20), and indicates the number of times the character is repeated. The
format for the sequence is as follows:
DC4 p1
where:
DC4 indicates the start of the LDC sequence.
p1 is a byte that specifies the number of times a character is
repeated. A count ranging from 0 - 31 or from 0 - 63 is represented
by one of the following formats: