dialing operation is aborted and the command ends
with Channel End, Device End, and Unit Exception
status. In the event the data set fails to go on-
hook, the Unit Check status bit, as well as the
Equipment Check sense bit, is also set.
Bytes are transferred from the line to the channel
each time the data-word buffer is filled with char-
acters, or at the end of a message.
controls provide for deletion of control characters
(upshift, downshift, idle, and delete) from the in-
The Read cemmand is terminated
at the end of a message by the setting of Channel End,
Device End, and Unit Exception status. These status
bits are set when the 2702/03 receives a
the communications line while in text mode. When
the 2702/03 is in control mode, either a
(only for IBM Line Sets) causes the Channel End,
Device End, and Unit Exception status bits to be set.
Otherwise, the command is ended normally with
Channel End and Device End.
For other ending-
status conditions, refer to "Status-Byte. "
Read (2703 BSC Only)
The Read command is used to transfer characters
from a BSC-equipped remote station to the
For characters to be transferred to the chan-
nel, the line must be in character phase
have received a SYN character). On a read opera-
tion, the bcc (block check character) and all SYN
characters are deleted in the 2703 prior to transfer
to main storage.
The Read command will end normally with Chan-
nel End and Device End status when any ETB, ETX,
ACK, NAK, DLE (and associated ending follower
characters), or ENQ characters are detected.
EDT is detected, the Read command will end with
Channel End, Device End, and Unit Exception status.
For other ending-status conditions, refer to "Status
On acceptance of the Inhibit command, the 2702/03
performs normal read operations except that idle-
line timeouts are inhibited (except during Timeout
Sequence, which is performed just before the com-
This command is normally used in a contention-type
communications system to notify the processor when
data is arriving. It is also useful to check for the
Break signal, to indicate when a Break or elongated
Space signal ends (the line returns to Mark). When
a valid start bit is detected by a line instructed to
Prepare, a character is strobed off.
time the line is at Mark (which indicates the line has
returned to the normal idle condition or that a char-
acter has been received), the command is terminated
with Channel End and Device End status. The char-
acter assembled is not transferred to the multi-
the line was at Space at stop-bit
time, the 28-second line timeout is started. If the
line returns to Mark before the timeout is complete,
Prepare is terminated with Channel End and Device
End status. If the line does not return to Mark before
completion of this timeout, then the Prepare com-
mand is terminated (indicating an open-line condi-
tion) with Channel End, Device End, and Unit Check
status, and the Timeout sense bit is set.
No data transfer occurs under Prepare, and no
characters received during its execution are trans-
ferred to the processor (they cannot be recovered).
However, line-control characters received under
this command continue to be recognized (as they are
under Read) and they perform their normal function.
NOTE: To avoid a hang-up condition when operating with the
2741 Break feature or with any telegraph device, a Halt I/O
command should be issued immediately following a Prepare
The Halt I/O command is essentially ignored while
the line is still at Space.
However, when the line is at Mark,
the Prepare command is ended immediately.
Prepare (2703 BSC Only)
This command may be used in contention-type com-
munications systems to indicate when data is arriv-
ing and to thus monitor the received data stream for
SYN characters. The Prepare command will be
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 Channel End and Device
A Read command should be command chained to
the Prepare command in order to transmit any ensu-
ing data to main storage. Any SYN characters
assembled by the 2703 are not transferred to main
Normally the Prepare command should not be
used in switched-network operation, because no
timeouts are performed to protect the system against
This command is provided to allow the processor to
ignore terminal-to-terminal messages on telegraph