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

character is always acted upon; it is also included in
the bcc accumulation.
Each time an ITB is detected, the SF checks the
bcc accumulation and passes on to main storage an
Error Index byte following the ITB (or ETB or ETX)
character provided the SF is in error index byte
mode. If the Error Index byte mode is not set, the
Error Index byte will not be passed on to main stor­
age. The Error Index byte (EIB) reflects the condi­
tion of the last block of data recei ved (a non-zero
content indicates a transmission error).
The EIB informs the program of data-check or
overrun conditions detected while the block of data wati ut;;ing rt;;ceived. These conditions set the
following bits in the Em:
Bit Position
4
5
Condition
Data Check
Overrun
The Em character is stored in the byte location
immediately following the ITB (or ETB or ETX)
character of the data block involved in the read
operation.
The ITB character is used to break up a long
record into shorter blocks; each block (except the
last) is ended with an ITB character. The direction
of transmission is not reversed following the ITB,
as it is following the ETX.
The record will end normally upon detection of
an ETB or ETX character. When the SF is not in
text or transparent mode, DLE-Stick (followed by a
valid pad character) will end a read-type command
with Channel End and Device End status. Some of
the defined DLE-Stick sequences are: ACK 0, ACK 1, WACK, and RVI. The DLE-Stick sequence
will be treated like the NAK character.
Both characters must be of proper parity when
parity checking is performed. All DLE sequences
must be contiguous characters. If text mode is set,
the DLE-Control sequence will be treated as data
and included in the bcc accumuladon. SYNCHRONOUS OPERATIONS The Synchronous Base (SB) provides the circuitry
for two-way, nonsimultaneous (half-duplex), serial,
synchronous data communication over based or
switched transmission facilities having voice-grade
qualities. Information transmission (consisting
of data bytes, logical information, line-control
characters, error-checking characters, etc.)
consists of binary streams, serial by bit and by 50 character, between two BSC-adapted stations. The
stations operating over the data link may be BSC­ adapted computers, terminal, remote I/O devices,
control units, or other equipment. For example,
two System/360 computers can typically operate
over a communications path as shown in Figure 11.
The Synchronous Attachment is a prerequisite to
attaching any ESC capability to the 2703. Two
Synchronous Bases can be installed in a 2703. The
attachment of the Synchronous Attachment feature
in a 2703 requires the installation of the Base
Expansion feature.
Operationally, the SF is fully controlled by the
channel program in conjunction with the data-link­
control ::;ignals it receives from the remote station
via the communications network and the signals it
receives via the multiplexer channel attached to the
main-storage CPU. The SF performs the following functions: • Provides the required buffering between the 2703 and the attached BSC-adapted remote
station. • Checks the of received data between
the 2703 and the remote station. • Scans the received data -link-control characters
and control-character sequences and initiates
certain actions. • Initiates data transfer to main storage on read­
type commands and to the remote station on
write-type commands. • Automatically generates time-fill and check
characters as r.equired. • Generates several timeouts--of fixed duration-­
to prevent system "hang-up" and unwanted
looping.
The SF informs the processor if its status (thus
reflecting the status of attached stations) via the
status and sense bytes. The program interrogates
each received response to determine if any further
action is required. For example, if:
--the remote station signals EOT (end of trans­
mission).
--an attempt to transmit or recei'vc a data block
fails.
--erroneous or invalid characters are trans­
ferred in either direction between main storage
and the 2703; or erroneous or invalid characters
are received from the remote station.
--an error in sequencing of certain commands
occurs, or if invalid commands are attempted
to be executed.
--timeout conditions occur.