switched or private communications facilities are
used). If the transmission is to be via private line,
the command ends immediately, upon enabling the SF and the line, with Channel End and Device End
status (CSW bits 36 and 37).
If the transmission is to be via switched network,
the command ends with Channel End and Device End
status when the local data set is operationally
available.
This command permits the SF to be in condition to
answer dialed "calls" from remote station, if
required. No data is transferred in either direction
by this command.
Dial
This command permits automatic dialing of a remote
BSC-equipped station. If the Autocall feature (ACF)
is not installed, the Dial command will be rejected
the completion of the data communication with the
remote station, the dial operation is normally ter­
minated by the issuance of the Disable command.
If the Automatic Calling Unit is nonoperational,
sense-bit 1 (Intervention Required) is set to one and
the command is ended with Channel End, Device End,
and Unit Check status (CSW bits 36, 37, and 38).
Disable
This command is used to reset the line; i. e., it
deconditions the enabling of the line originally pro­
vided by the Enable command.
Disable CON 2F, f1, fa f3
[
2F Data Address
o 78 31 f" f" h Count J I 29 I 32 3940 4748 63
Data Address . """0------'78-------------------1
31 If operating on a switched network, the execution
Count
32 3940 4748
with a Command Reject (sense-bit 0 set to one) and
will also result in setting Unit Check status (CSW
63
bit 38). When Dial is executed, bytes are transferred
from main storage to dial equipment (provided by
common carrier) at a data rate determined by the
dial equipment.
The transferred bytes represent the dial digits
required for switching purposes. All dial codes are
regarded as decimal digits and are stored in main
storage, one dial digit per byte location. The SF does not check if the characters transmitted under a
Dial command are valid dial digits, as only the four
low-order bits of the dial-digit byte are transferred
to common-carrier equipment.
The Timeout sense bit (bit 7) is set to one if the
common-carrier equipment does not signal the ACF
of successful completion of the dial operation within
the recommended 40 seconds. This timeout duration
is initiated by the transmission of the last dial digit
to the common-carrier equipment. The command is
immediately ended with Channel End, Device End,
and Unit Check status (CSW bits 36, 37, and 38).
Successful completion of the Dial command results
in Channel End and Device End status (CSW bits 36
and 37).
The Dial command can be command chained (CCW
bit 32 set to one) to a Write or Read command. At
of the Disable command terminates an outstanding
call. If operating on a private line, this command
ends immediately with Channel End and Device End
status (CSW bits 36 and 37).
When disabled, the line will not answer any
incoming calls. Note that, following a Disable
command, a Set Mode command must be issued before
enabling or dialing. Prepare This command may be used in contention-type
communications systems to indicate when data is
arriving and to thus monitor the received data stream
Prepare CON 06, fit f
2
, f3 06 Data Address
o 78 31
Count
32 3940 4748 63
for SYN characters. The Prepare command will be
accepted only if the line has been previously enabled.
No data transfer to main storage occurs during the Prepare command execution.
The Prepare command ends when a sync pattern
has been detected and sets the Channel End and
Device End status (CSW bits 36 and 37).
57

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.