A Read command should be command chained to
the Prepare command in order to transmit any
ensuing data to main storage. Any SYN characters
assembled by the 2703 are not transferred to main
storage.
The Prepare command should not normally be used
in switched-network operation, because no are performed to protect the system against hang-up. Write This command is used to transfer information from
main storage to the remote station. The length of
the information, in bytes, is determined by the
Write 01 Data Address
o 78
Count
32 3940 4748
31
63
count field of the CCW each time the Write command
is issued. A character of marks, a single "pad!! (all ones), and two SYN characters (except in trans­
parent mode) will be presented to the communications
line prior to the first character. If the Synchronous
Clock feature is installed, six SYN characters are
presented before the first character. All data or
control characters that are transm itted to the com­
munications line must ue originated in main storage
(except DLE and SYN when used as time-fill). The Write command will end as a result of any of the
following cond itions:
a. An ETX or ETB control character is detected
in the data stream (except in transparent
mode).
b. The 2703 detects common-carrier-equipment
malfunction.
c. Count field in Write CCW is decremented to
zero.
At the end of the Write command, a single pad
character is automatically sent after the last
character of block-check character (bcc) to ensure
that the data set will have time to transmit the last
character before data-set turnaround. PROGRAMMING NOTE: All Write commands will normally have
bit 33 set to one for command chaining to a Read command to
prevent loss of data. An exception to this re,!uirement is when
the Write command is used for sending EOT to a remote station.
58
Read
This command is used to transfer characters from
a BSC-equipped remote station to main storage.
For characters to be transferred to main storage,
Read
ccw 02, fl' f2' f3 I 02 I Data Address
o 78
Count
32 3940 4748
31
63
the lin(: must be in ch8T8.ctcr ph8.sC (i. e., must ha\'e
received two consecutive SYN characters). On a
read operation, the bcc and all SYN characters are
deleted in the 2703 prior to' transfer to main storage.
The Read command will end with Chan-
ne 1 End and Device End status (CSW bits 36 and 37)
when any ETB, ETX, NAK, DLE (and associated
ending follower characters), or ENQ characters are
detected. If an EOT is detected, the Read command
will end with Channe 1 End, Device End, and Unit
Exception status (CSW bits 36, 37, and 39). To be
recognized as valid control characters, the turn­
around characters EOT, NAK, ENQ, and DLE­
Stick must be followed by a valid pad character.
The Read command will end with Channel End,
Device End, and Unit Check status (CSW bits 36, 37,
and 38) should the following conditions occur:
Condition Sense bit set
Count decremented to zero 6
Halt I/O issued 6
Data check occurs 4
Overrun occurs 5
Common-carrier equipment 1
not operational NOTE: Sense-bit 6 is also set to one if data is lost because the
Read command was late in being executed.
If the time interval between the sequences of
signal SYN's followed by a non-SYN character is
greater than three seconds, of if the time interval
between thc acceptance of the Read command and the
receipt of eithe r SOH or STX or an End character (any
of those mentioned) is greater than three seconds,
the command will be ended with Channel End, Device
End, and Unit Check status (CSW bits 36, 37, and
38) and the Timeout sense bit (bit 7) set to one.

Read and Write Operations in Transparent Mode
Transparent mode is used during reading and writing
operations of text transmitted in !!bit-pattern structure" rather than "character-pattern structure" as in normal mode.
Transparent-Read Operation. The SF enters trans­
parent mode via the control-character sequence
DLE STX. SF exits from the transparent mode of
operation by the detection of a DLE followed by any
end control character (NAK, ETX, ETB, ENQ, EOT, or ITB). A DLE followed by a non-ending control
character results in the SF remaining in transparent
mode and in the setting of sense-bit 4 (Data Check)
to one. A DLE DLE sequence causes the SF to
remain in transparent mode with the first DLE
deleted from the data sent to main storage, as is
every first DLE in transparent mode.
Transparent-Write Operation. Transparent-write
operations require two Write commands to be
executed. The first Write command is used to trans­
mit the data stream and is ended normally when the
byte count equals zero. The command-chained
second Write causes the SF to leave transparent mode
when one of the following sequences is detected
during the second Write command execution--DLE
ETB, DLE ETX, or DLE ITB. The command ends
with Channel End and Device End status. If none
of these sequences occur, the byte count of the
second Write command (CCW), when decremented
to zero, causes the command to end and the SF
leaves transparent mode. Channel End and Device
End status are set.
The second Write command must be received by
the SF within three seconds of the ending of the first
Write command; otherwise a timeout occurs with
Channel End, Device End, and Unit Check status and
the Time sense bit (bit 7) set to one.
During transparent operations, the SF generates
a DLE character whenever it detects a transparent­
text-bit configuration that is the same bit config­
uration as a DLE character. This insertion of a
DLE prevents the receiving station from falsely
ending a block if the transparent-bit configuration is
the same as a transparent-ending sequence (e. g. ,
DLE E TX). The inserted DLE is deleted by the
receiving station prior to placing the received
message in the receiving station I s main storage.
To permit a bona fide ending sequence to be
transmitted, the transmitting SF must know When
to inhibit the insertion of the DLE character under
the conditions just cited. This is accomplished by
the use of the two Write commands as follows.
The byte count of the first Write command is
decremented to zero'upon the transmission of the
last byte (character) of the message. The second
Write is command chained to the first Write com­
mand, with the second Write command containing the
ending sequence. Thus the SF recognizes the ending
of the first Write command as the initiation of the
ending sequence provided by the second Write. Under this second Write command, the SF does not
generate a DLE upon detection of a main-storage­
provided ending sequence. See Figure 15. PROGRAMMING NOTES: For both read and write operations, a
DLE ITB sequence causes the SF to exit from transparent mode,
yet continue in text mode
The SF sends the block-check-character accumulation after
ETB, ETX, or ITB.
The DLE SYN sequence is used in transparent mode in the same
manner that the SYN is used in nontransparent mode. The
DLE SYN sequence is not recommended for use as time-fill.
In transparent operations, no VRC checking follows the initial
DLE STX se quence except on control characters.
The DLE ITB sequence is followed by the bee SYN SYN (double SYN generated by the equipment) sequence. If the data follow­
ing is to be in transparent mode, the DLE STX sequence must
follow the SYN characters. In this case, the DLE and STX characters are included in the bcc accumulation.
Search
This command is used when the synchronous-equipped 2703 is operating as a master station. The Search
command may be issued only after a Poll command.
Search ccw DE, f I, f 2, f 3 I DE I Data Address I D 78 31 Count
32 78 4748
Since the SF will normally be in character phase
when the Search command is issued, data will be
immediately transferred to main storage, starting
with the last index character 9 sent from main
storage under the preceding Poll command). The
last index character is followed by the data being
received from the communications line. I 63
All data received before going into text mode
(entry into text mode via recognition of the STX or SOH --including the first STX or SOH) is sent to main
59