when the modem (data set) does not provide the
synchronizing signals. Character phase (character
synchronism) is established, after bit synchronism,
by the recognition of a "sync pattern" (two consecu­
tive SYN characters). Once character phase is
established between two stations involved in a given
transm iss ion, it is maintained until the trans mission
is terminated. Synchronis m is aborted if no sync
patterns or terminating or turnaround control char­
acters are detected within a pre-established timeout
period.
Transmit Operations Using Data-Link Characters
During hi m r::-synchrnnnus ('om munic8tions, tr8 n8-
mit operations are those occurring when data is
transferred from main storage to a remote station
via the SF and com munications facilities. The SF is said to be in transmit mode when trans mitting
information in this direction. See the publication, General Information--
Binary Synchronous Communications, Form A24- 3004. for a conceptual discussion of the total binary­
synchronous operations, including line-control
repertoire, description f)f control procedures,
control-character sequencing, and transmission­
code structures.
The following discussion of control characters
relates to the various applicable control characters
as implemented by the SF. SYN Character SYN characters are generated at the beginning of
the Write command and are inserted into the data
stream as time-fill whenever a character is not
available in time during a write transmission. The SYN character is generated into the data stream in order to maintain character synchronization. SYN characters are not included in the block-check­
character (bcc) accumulation (see "Transmission­
Code Checking" later in this section of this manual).
The programmer can insert a SYN character
whenever there is need for time-fill in the program;
howe\'er, the SYN characters will be deleted by the
receiving equipment involved in the data transmis­
sion.
The SF does not transfer the time-fill (SYN) to
recei ving storage.
For synchronization purposes, two cOl"'secuti ve SYN characters are inserted into the data stream cver:y second \vhile tl1e SF is in text mode. In trans-
parent mode, a DLE SYT\ sequence is inserted every
second (or more often, depending upon the speed and
whether business machine clocking is used). SOH and STX Characters
The SF monitors for the presence of these two
control characters. Upon detection of either SOH or STX. the SF enters into text mode and initiates
block-check-character accumulation. After the SF has entered into text mode, any subsequent SOH or STX characters detected are treated as non-control
characters (i. e., data characters).
In any particular data block, the SOH or STX that initiated bcc accumulation is not included in the
bcc accumulation; however, any ensuing STX or SOH characters in the data block are included in the bcc
accumulation.
During transmission, exit from text mode is
accomplished by the SF detecting either an ETX or
ETB control character or an Interface Stop, or by
the Halt I/o instruction. Since the SF requests multiple bytes from main
storage, bytes following the ETX and ETB may be
transferred into the 2703 prior to the ending of the
command. These additional bytes are not trans­
ferred to the line.
ETB and ETX Characters
These control characters cause the SF to exit from
text mode and also cause the accumulated block­
check character and one pad character to be trans­
mitted. After the transm ission of the bcc and pad
characters, the SF goes into receive mode (i. e. , SF stops transmitting) and hunts for a sync pattern.
ETB and ETX characters are included'in the bcc
accumulation. EOT and NAK Characters
The se two control characters are ignored as control
characters when the SF is operating in transmit
mode. Both are included in the bcc accumulation.
ITB Character
This control character always causes the block­
check character to be sent following the transmission
of the ITB character. The SF continues to transmit.
Two SYN characters are generated by the SF and
transmitted immediately after the bcc character.
The SF begins accumulation of a new bcc after the
ITB bcc sequence has been transmitted, and ITB is
included in the bcc accumulation. The direction of
transmission is not reversed following the ITB, as
it is following the ETX.

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