Transmit and Receive Operations The 2703 can accommodate various transmission
codes not exceeding an eight-level, eleven-unit
structure (eight data bits and three start/stop bits).
For start/stop type transmission, the start and stop
bits are generated for transmission onto the com­
munications line. On receive, the start and stop
bits are removed from the code structure before
each byte is transferred to the multiplexer channel.
For synchronous operations, sync idles are inserted
and removed on a similar basis. All transmission
codes of less than eight bits are placed in processor
storage in the low-order bit positions, with the pro­
per number of high-order zeros inserted.
In transmit or receive operations, each attached
communications line is scanned to determine whether
it requires data from the processor for transmission,
or whether any line has data to send to the processor.
During receive, the 2703 samples signals on the line
to derive bits, and it later assembles these bits into
characters that are transferred to the channel as
data bytes. To store these bits before assembly, a
delay-line storage is used. To assemble a byte,
magnetic-core storage is used for start/stop and the
delay-line storage is used for BSC. A maximum of
four bytes (eight bytes for BSC) can be aSf,embled a
and stored before being transferred to tUb multiplex­
er channel; a parity bit is generated by the 2703 be­
fore transfer of the byte to the channel.
For data transmission from the 2703, groups of
up to four bytes are transferred from processor
storage (main storage) and stored in the 2703 mag­
netic core storage before transmission over the
communications line. The one-byte characters are
then transferred serially by bit for start/stop and
serially by char acter for BSC to the delay-line stor­
age. When transmission of the last character is
started, a request for four additional bytes is sent
to the multiplexer channel.
All character and bit control, character decoding,
data handling, and matching to common-carrier
equipment is accomplished by the functional sections
within the 2703 (see Figure 3). These sections are
described under "General Description of Operations." GENERAL DESCRIPTION OF OPERATIONS Here, in general terms, is described the operation
of the main internal units or functional sections of
the 2703. These sections (Figure 3) are the 1/0- interface controls, common controls and storage,
terminal controls, line bases, and line sets. A gen­
eral description of the IBM 2711 Line Adapter Unit
and IBM 2712 Remote Multiplexer is also presented
here. 1/0- Interface Controls
The I/O-interface-controls section connects the 2703 to the System/360 multiplexer channel. It recog­
nizes signal sequences from the channel and returns
the required sequences in response. The response
is variable, depending on the status of the other func­
tional sections. Command buffering and multiple­
byte buffering for data is provided from main storage
and from the multiplexer channel. If the channel
wishes to transmit or receive through the 2703 con­
trol.unit, it initiates this operation by issuing a Start I/O to the desired communications line. The
data transfer then occurs in short bursts of up to four
characters or bytes. Each byte consists of eight data
bits plus an odd-parity bit.
The operation of each communications line is con­
trolled through a CCW (channel-command word)
stored in the processor core storage. See "System/ 360--1/0 Operation" under "Detailed Description of Operations." The I/O operation is initiated for a
given communications line by issuing the Start I/O instruction with the subchannel address correspond­
ing to the desired communications line specified in
the instruction format. The 2703 will terminate both
input and output operations after data transfer occurs,
unless some unusual condition exists.
Common Controls and Storage This section accepts commands and data from the
multiplexer channel (via the I/O-interface-controls
section). It performs all the functions that are com­
mon to all communications lines attached to the 2703. A magnetic-core storage is provided to store control
information and to buffer commands and data. Each
communications line has storage reserved for its
exclusive use, for its main control words (MCW-l
and MCW-2), and for its main data words {MDW-l
and MDW-2)--see Figure 4. The storage locations
are accessed in a prescribed sequence or priority.
The control words buffered in 2703 storage con­
tain fields that define such areas as: assembly/dis­
assembly, character address (within the data word),
character shift (upper or lower case), mode of oper­
ation (text or control), command, longitudinal redu
dancy check (LRC) accumulation, sense information,
and unit status condition.
The common-controls-and-storage section re­
ceives data in groups of up to four bytes and trans­
fers them bit-by-bit through the line base to the
communications line. When the line presents bits to
the 2703 line base, the line-base section receives the
data bit-by-bit and stores this data in the control
word. The data is stored in byte form in the main
control word (MCW-l) and is stored in the main data
See Note 113 To Terminals
via Data Sets or Line
r-----, I Line Set Ill-I See Note 112 Line
Base I Line I r-----, I Terminal I See Note #1 To Terminals
via Data Sets or Line ----, Base --L __ --1 Common
Controls and
Adapters See Note 114 D -----r - Line I I Base ---L __ --1 t:l2W: 8 'I-Termina! Controls-Max. of 6 per 2703. *2 Line Bases*-Max. of 3 per 2703. '3 Line Sets**-Per Line Base a",J Per 2703. Table 1 under Line Sets ',n Start/Stop Special Features gives detailed information.
#4-Lines per Line Set-Max. of 8***per sis line set.
Max. of 4 per Sync. line set.
S/S-Start/Stop type operations
Sync. -Synchronous type operations
*-Check Figure 8 for various line base configurations.
**-These maximums are influenced by the Feature linitations Per 2703, described
in the Special Features Summary.
***-Except where this is increased to 16 lines by an expander feature.
Refer to the 2712 Modell and Model 2 Adapter Features for details of how the 2712
expander features increase the number of available lines.
Figure 3. 2703-Internal Organization P 0 2 3 4 5 6 7-Bits
To Multiplexer
Channel Connection , I
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