word (MDW-l, 2) in the common-controls-and­
storage section. When four bytes are accumulated,
they are transferred to the multiplexer channel via
the riO-interface controls.
Terminal Controls
This section performs functions associated with a
particular type of terminal equipment. The terminal­
controls section acts as the extension of common con­
trols, performing such unique duties as: determin­
ing character bit length, character-sequence recog­
nition, shift-character recognition, pad-character
control, and initiating LRC checking. A terminal
control is assigned by line set, and all lines con­
nected to a specific line set must be of the same ter­
minal type. The addressing circuit that selects the
line also selects the associated delay-line storage
(base control word--delay-line type storage), MCW,
and terminal control.
Line Bases
The line-base section performs bit sampling while
accepting bits directly from the line adapters during
a receive operation. It buffers and time-multiplexes
the data before sending the bits (characters, for syn­
chronous operations) to the common-controls-and­
storage section. The reverse procedure takes place
during a transmit operation. A base-control word in
the delay line of the line base is used for bit buffer­
ing (character buffering, for synchronous operations)
and bit sample control. The line base is limited to a
certain number of lines, depending on the type of base.
Line Sets The line sets interface the communications facilities
(lines) to the 2703 and provide bit buffering for the
Transmit Data line. At least one line set must be
selected for 2703 operation. Each line set consists
of eight line adapters (four, for synchronous), with
each adapter servicing one communications line.
IBM 2711 Adapter Unit This is a free-standing unit that provides housing and
power for up to 32 IBM Line Adapters (Shared-Line,
Leased-Line, and Limited-Distance Type 2--eight
miles). These line adapters interface with the 2703 via the Data Line Set special feature, with one
line set required for each eight lines. The 2711 may
be attached on a per-line basis to more than one 2703. Any combination of line adapters up to 32 can be
accommodated by the 2711.
IBM 2712 Remote Multiplexer
The 2712 consists of a free-standing unit at the re­
mote communications point and several special fea­
tures available for adding to the 2703. These 2703 features allow up to 10 lines oJ:"'rating at 134.5 bps
or 14 lines operating at 74.2 bps to be bit-multiplexed
onto one voice-grade line. Refer to "2712 Modell
and Model 2 Adapter Features" under "Start/Stop Special Features" for further details.
9

SYSTEM/360--I/0 OPERATION Input/output operations for the IBM 2703 are initiated
and controlled by information with two types of
formats: instructions and commands. Instructions
are decoded by the CPU and are part of the CPU program. Commands are decoded and executed
by the channels and I/O devices, and. initiate I/O operations, such as'reading and writing. One or
more commands arranged for sequential execution
form a channel program. Both instructions and
commands are fetched from main storage and are
common to all types of I/O devices, although the
modifier bits in the command code may specify
device-dependent conditions for the execution of a
data-transfer operation at the device.
The CPU program initiates 2703 I/O operations
with the Start I/O instructi<>n. This instruction
identifies the channel and the device, and causes the
channel to fetch the channel-address word (CAW)
from a fixed location in main storage. The format
for the CAW is:
Command Address
o 3 4 78 31
The CAW contains the protection key and designates
the location in main storage from which the channel
subsequently fetches the first channel-command
word (CCW). The CCW specifies the command to be
executed and the storage area, if any, to be used.
The format for the CCW is:
o
Command
Code
78
Data Address Flags I 000 32 36 37 3940 47 48
31
Count
63
If the subchannel associated with the addressed I/O device is not busy, the channel attempts to
select the dey ice by sending the address of the
device to all control units attached to the channel.
The control unit that recognizes the address connects
itself logically to the channel and responds to its
selection by returning its address. The channel
subsequently sends the command-code part of the
CCW oYer the interface, and the device responds
with a status byte indicating whether it can execute
the command.
DETAILED DESCRIPTION OF OPERATIONS NOTE: The 2703 does not operate in burst mode; neither can
the 2703 operate concurrently on the same channel with any
other device operating in burst mode.
At this time, the execution of Start I/O is term­
inated. The results of the attempt to initiate the
execution of the command are indicated by setting
the condition code in the program -status word (PSW) ,
and, under certain conditions, by storing pertinent
information in the channel-status word (CSW).
If the operation is initiated at the device and its
execution involves transfer of data, the subchannel is
set up to respond to service requests from the device
and assumes further control of the operation. In the
case of operations that do not require any data to be
transferred to or from the device, the device may
signal the end of the operation immediately on
receipt of the command code.
An I/O operation may involve transfer of data to
one storage area, designated by a single CCW, or to
a number of noncontiguous storage areas. In the
latter case, a list of CCW's is used for execution of
the I/O operation, each CCW designating a contiguous
storage area, and the CCW's are said to be cGupled by data chaining. Data chaining is specified by a
flag in the CCW and causes the channel to fetch
another CCW upon the exhaustion or filling of the
storage area designated by the current CCW. The
storage area designated by a CCW fetched on data
chaining pertains to the I/O operation already in
progress at the 2703, and the 2703 is not notified
when a new CCW is fetched. Provision is made in
the CCW format for the programmer to specify that,
when the CCW is decoded, the channel request an I/O interruption as soon as possible, thereby
notifying the CPU program that chaining has pro­
gressed to a particular CCW in the channel program.
Termination of the I/O operation normally is
indicated by two conditions: Channel End and Device
End. The Channel End condition indicates that the I/O device has received or provided all information
associated with the operation and no longer needs
channel facilities. The Device End signal indicates
that the I/O dev ice has terminated execution of thE operation. For the 2703, the Device End conditioL
always occurs concurrently with the Channel End
condition.
The conditions signaling the termination of an 110 operation can be brought to the attention of the
program by I/O interruptions. These conditions
cause storing the CSW, which contains additional
information concerning execution of the operation.
At the time the Channel End/Device End condition
is generated, the channel identifies the program the
11