DLE Character
The DLE control character, plus a defined follower
character, initiates a control sequence. Thus, for
example, the DLE STX sequence places the SF in
transparent mode. While in transparent mode, the SF inserts a DLE character into the data stream
whenever a DLE is received from main storage.
Time-fills consist of DLE SYN sequences. The
DLE SYN sequence is generated by the equipment;
thus the programmer cannot use DLE SYN for time­
fill while in transparent mode.
The first DLE, after a DLE STX in any sequence,
is not included in the bcc accumulation, nor is a
DLE SYN sequence included.
Any transmitted DLE followed by an end control
character must be contained in a second Write,
CCW which is command chained to the first Write,
CCW.
DLE-Stick Sequence
DLE-Stick will be ignored as a control character in
transmit mode.
Pad Character
The pad character is an all-ones character. To
ensure that the last meaningful bits in a transmission
are sent properly by the data set, one pad character
will be sent following every turnaround character
(or the bcc(s) that follow ETB or ETX). NOTE: By definition, the bit configuration of a valid pad is
1111 1111; however, the 2703 checks only the first four bits
tha t arrive (xxxx 1111).
ENQ Character
The ENQ control character is not recognized as a
control character if detected under a Write com­
mand. If ENQ is detected under a Poll command,
it turns the SF from transmit mode to receive mode.
The ENQ does not signal End to the program under
a Poll command.
Receive Operations Using Data-Link Characters
For binary-synchronous communications, receive
operations are those occurring when data is
received from a remote station. The SF is
considered in receive mode whenever it is not in
transmit mode. SYN Character
All SYN characters are deleted from the received
data stream by the SF before the received data is
transferred to main Any SYN characters
detected by the SF in the received data stream are
not included in the block-check-character accumu­
lation. SOH and STX Characters
Either of these two control characters initiate the
bcc accumulations. The ir initial detection sets the SF in text mode. The initial SOH or STX control
characters are not included in the bcc accumulation.
Subsequent SOH and STX characters in a data block
are treated as noncontrol characters (i. e., data
characters), and are included in the bcc accumu-
lation. Exit from text mode is accomplished when
the SF signals End to the program.
ETB and ETX Characters
These control characters signify that the check
character is following. The SF remains in receive
mode. On a Read command, the ETX or ETB always
signals End to the channel. When the SF is in error
index byte mode (see Set Mode command), an ETX
or ETB causes an Error Index byte (EIB) to be
inserted immediately after ETX or ETB in the data
being transferred to main storage. The bcc accumu­
lation includes the ETB and ETX characters.
ENQ and NAK Characters
Either of these control characters, if followed by a
valid pad character (all ones), ends the command
with Channel End and Device End status. It does not
change the SF from receive mode. No bcc compari­
son is performed after detection of these characters.
The SF hunts for a new sync pattern. ENQ and NAK
are included in the bcc accumulation. NOTE: If the pad character is not valid, the 2703 will maintain
character phase, place the invalid pad in main storage as
data, and continue to receive.
DLE Character
This control character, followed by a defined
follower character, initiates a control sequence when
the SF is operating in transparent mode.
49

To initiate transparent mode, the SF monitors the
incoming data for the DLE STX sequence. While in
transparent mode, a DLE DLE sequence is inter­
preted as a normal data character with the first DLE
being deleted and the second DLE is sent to main
storage. The DLE SYN sequence is detected as
time-fill with the DLE and SYN deleted from the
data stream going to main storage. Neither the
DLE nor SYN is included in the bcc accumulation.
During a receive operation in transparent mode,
a single DLE character, followed by any character
except DLE, SYN, ETB, ETX, ITB, or ENQ, sets
the Data Check bit in the sense byte. Receiving
continues.
A DLE, followed by end control characters ITB,
ETB, ETX, or ENQ (if the ENQ is followed by a
valid pad), causes the SF to leave transparent mode.
The end character is handled the same as in normal
transmission. The DLE character preceding the
end character and the first DLE in a DLE DLE
sequence are not included in the bcc accumulation.
All DLE sequences must be contiguous characters
on the transmission line. NOTE: For DIE sequences as pertaining to ACK 0, ACK 1,
WACK and RVI (for the several transmission codes), see the
publication, General Information--Binary Synchronous Com­
munica tions, Form A27 -3004. EOT Character
The EOT with its required trailing pad character
will cause the SF to remain in receive mode. No
bcc comparison is performed on the received data.
Channel End, Device End, and Unit Exception status
is signaled to the CSW.
ITB Character
When the SF detects the ITB character, the charac­
ter(s) following will be the block-check character(s).
The SF remains in text mode and the accumulation
of a new bcc is restarted with the next character
(except SYN) following the last bcc. The bcc accu­
mulation is reset immediately after the intermediate­
block-check operation has been performed. The ITB
49.1