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.
BSC Special Features
The BSC permits wide selectivity in the features
employed, thus permitting equipment options designed
to the user's communications network and his opera­
ting modes. Synchronous Terminal Controls (STC) and Synch­ ronous Bases lA, 1B, and 2A provide for information
transfer between core storage and the communications
facility. The Synchronous Base performs character
assembly on data being read into core storage and
character disassembly on information passing in the
opposite direction. Synchronous Terminal Controls
A Synchronous Terminal Control (STC) is required
for attaching synchronous terminals (or stations) to
the 2703. Synchronous Terminal Controls are
optionally available for communicating in EBCDIC, USASCII, or SBT. Each of the three available STC's contains circuitry for transmission-error-detection
operations. The method used in error detection
varies depending on the transmission code used. The STC controls all lines (independent of the nurrb e1' of Synchronous Bases, number of lines, and line speed
employed) of a specific code type. Up to three STC's can be installed in each 2703. Mixes of Synchronous Terminal Controls with start/stop terminal controls
(e. g., Telegraph Terminal Controls or IBM Terminal
Controls) are permissible provided only two STC' s
are installed per 2703. No more than two types of Synchronous Terminal Controls may be associated
with one Synchronous Base. (See Figure 3.)
The three transmission codes available are:
EBCDIC--Extended Binary-Coded-Decimal Inter­
change Code. This eight-bit code allows trans­
mission of 256 different bit patterns. Ten (option­
ally eleven with ITB) of these bit patterns represent
data-link characters assigned as line-control
characters. In aCIdition, the following characters
are currently assigned in EBCDIC:
52 alphabetic characters (upper and lower case) 10 numeric characters
22 end-to-end characters
33 special graphics (including space)
EBCDIC is code-compatible with the internal code
used in System/360, thus permitting maximized
utilization of communications facilities and of
channel-to-control-unit data paths to the 2703, CPU and main storage. Information is stored in main
storage directly as received without need for trans­
lation. For the thr8e transmission codes, bit­
position 7 of the byte in main storage is always trans­
mitted onto the communications line as the first bit.
The first bit received from the transmission facility
always goes to bit-position 7 of the byte in main
storage. USASCII-United States of America Standard Code for
Information Interchange. * The USASCII code consists
of seven data bits plus an odd-parity check bit in the
eighth bit position. USASCII provides up to 128 bit
patterns, all of which have assigned characters as
follows: 10 data -link characte rs
52 alphabetic characters (upper and lower case) 10 numeric characters
23 end-to-end characters
33 special characters (including space and delete) USASCII characters are received by the 2703 directly as received from the communications
facility. Figure 12 gives examples of code trans­
lations of a received character to EBCDIC or ASCII-8. SBT--Six-Bit Transcode. The SBT provides for the
transmission of 64 bit patterns assigned the following
characte r repre sentations: 10 data-link characters [includes ITB (US--Unit SeparatoT) ]
26 alphabetic characters (upper case) 10 numeric characters
12 special characters (includL3 space)
6 end-to-end characters SB T is essentially a card-oriented transmission
code for information entry from remote card
machines not requiring the extended code of EBCDIC. SBT is not available for 4800 bps operation.
Transmission-Code Transparency
Each of the three transmission codes may be used
in transparent-text mode. Transparency permits the
unrestricted use of all bit patterns, within each
transmission-code type, to be transmitted and
received as strictly a binary-bit stream, using a
special procedure for control-character recognition. Within the several transmission codes,
transparent-text mode is useful in transmitting
messages as:
Fixed-point data
Floating-point data
Packed-decimal digits
Logical information
*This code is compatible with the UnitE'd States of America Standard Code for Information Interchange (USASCII). However, this does
not imply full compatibility with non-IBM synchronous USASCII devices currently being marketed.
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