mand at the addressed MCW is terminated, the
proper status information is returned to the channel.
If the current command is Write, the character
being transmitted and the character buffered, if
present, are sent before Channel End and Device End
status are presented to the channel. A maximum
three-character delay (up to 500 ms) can occur
between the time the Halt I/O is accepted and the
presentation of Channel End and Device End status to
the channel.
Test I/O The 2703 responds unconditionally to an all-zero
command byte during initial command selection
with the status-modifier bit of the status byte. Any
existing interrupt conditions in the 2703 are not
cleared; the multiplexer channel remains unchanged. PROGRAMMING CONSIDERA TIONS From a programming standpoint, the 2703 appears
as a number of individual communications-control
devices. When an operation or sequence of opera­
tions is to be performed, the programmer prepares
a list of one or more channel-command words
(CCW's) in main storage. (Refer to "System/360-­ I/O Operation" earlier in this manual for the format
of the CCW.) The channel-command word specifies:
1. The command (operation) to be performed
(Write, Dial, Read, etc.).
2. The number of bytes contained in the record.
3. The address in main storage where data is to
be placed when receiving, or the address of
the first byte to be transmitted when sending.
4. Command flags to control possible modifica­
tion in command execution. The flags are:
chain data, chain command, suppress length,
skip, and program-controlled interruption.
When the CCW's have been formed, the pro­
grammer specifies the channel and line address of
the communications line. The execution of a
Start I/O instruction causes the command, count,
data-address, and control information to be stored
in a specified subchannel within the multiplexer
channel. The channel then selectci the 2703 and
relays the command and line address to it; the 2703 accepts the command if valid. The channel then
indicates successful or unsuccessful execution of
the Start I/O instruction to the program. Once a command has been accepted by the multi­
plexer channel and the 2703, the CPU program is
unaware of the of the operation until the
message has been received or transmitted, or until
the multiplexer channel requires interruption either
to perform functions such as dynamic storage allo-
cation or because an unusual condition is detected
during execution. Since the multiplexer channel
contains all the necessary information pertaining to
the current operation, data transfer between main
storage and the 2703 can be overlapped with CPU processing. The extent of the overlap varies,
depending on the processor model (30, 40, 50, 65,
or 75) of System/360.
If the 2703 is reset, either by a general system
reset, by a power-on reset, or individually at the
CE panel, the communications line must be enabled
by issuing the Enable command where necessary.
This command must be issued before transmitting
in all cases except for a link using a half-duplex data
set or an IBM Line Adapter.
The following 2703 functions require special
programming considerations:
1. The 2703 signals Control Unit Busy in response
to initial selection when the interface registers
are in use with a previous command cycle or
when the 2703 is executing a machine reset
resulting from a system reset or power-on
reset. The 2703 responds to the interface
signals, Address Out and Select Out, and to
a valid address on Bus Out with the interface
tag, Status In, and to the busy, status-modifier,
and control-unit-end bitQ on Bus In.
2. On an end-of-transmission sequence, when a © is sent under the Write command to a
private line using a common-carrier data set
with an IBM Terminal Control Type I or II, at
least two additional consecutive @ 's must be
sent by the program. This prevents receiving
a false start bit, generated by the remote data
set, when the terminal turns off its Request to
Send lead.
Automatic Wraparound
The automatic-wraparound (autowrap) capability is a
standard feature of the 2703 and can be utilized by
the program to determine the source of error for a
given 2703 line. The wraparound function is initiated
by issuing the Wrap command to the 2703 line address
on which trouble is suspected.
The 2703 wraps the output of the line to which the
Wrap command is issued to the input of the line v'ith the lowest address within this 2703. Wraparound is
accomplished within the line adapter to avoid line­
termination mismatches, since the line with the
lowest line address is not necessarily the same type
of line as the line being tested. The execution of
Wrap is always one way; that is, the transmit
operation is performed on the line in question and the
receive operation on the lowest line address. A Read
command must be issued to the low line address be-
15

lure autowrap is started in order to permit the data by the Wrap command to be received. program must ensure that normal operations 'l low line have been completed before issuing 'Yrap to any communications line on the 2703. The i'-l'dlIl must not issue Wrap to more than one
, .tt l( line at a time. The program can '" :'--b':-character comparison of re- , .; I d rlata with transmitted data and/or monitor the " J'ation for recognizable control characters.
The low line is not reserved as a test line, but is
used during the autowrap process. (However, it
should be recognized that the low line itself can ne\'cr be issued a Wrap command.) Since the low :: '\'.?y::tparolJnd opcratiC'ns, the auto-
wrap operation selects the proper terminal control
so that this low line operates as if it were the same
kind of communications line as the line being tested.
In this \vay, a complete check is made of the common
controls and storage as well as the terminal control
of the line in question. Receive operations are
checked because a common terminal control performs
the work. NOTE: Output data can be blocked from going onto the
communications line uy issuing the Disable command to
the 2703 line just before the Wrap is issued.
2703 Common Controls The common-controls-and-storage section of the 2703 contains the storage controls that are
common to all lines and line bases (see Figure 3). Common controls stores the I/O commands and data
bytes, assembles the line-base data bits into charac­
ters, and disassembles characters in to line-base
data bits.
Four 36-bit words are reserved for each possible
line address. These words are:
Main Control Word 1 (MCW-l) Main Control Word 2 (MCW-2) Main Data Word 1 (MDW-l) Main Data Word 2 (MDW-2)--used exclusively
for synchronous operation
The main control words contain the control infor­
mation necessary for the 2703 to know exactly where
it is when sequencing through its operations. They
also store status and sense information. The main data worns each provide four character buffers for
data transfer to and from the channel. These control and data \vords (Figure 4) allO\v t!1e 2703 to
multiplex the operation of up to 176 transmission
lines \\'Hh one transmission control unit.
The field assignments within the control words differ slightly between start/stop and S}11Chronous ty1)e operations as indicated by Figure 4. These
differences are described under MCV/-l and MCW-2. 16
The field assignments for the data words are identi­
cal for both types of operation.
Main Control Word 1 MCW-l stores the operating controls for its
associated line and the data character that is being
assembled or disassembled.
Assemble/Disassemble Field. The assemble/
disassemble (A/D) field stores the data byte being
transmitted or received. Data in the A/D field can
be shifted left one bit position at a time. On a receive operation (for start/stop), the bits in the A/D field are shifted left one position, and the
line-base data bit is inserted into bit-position 7. On a transmit operation (for start/stop), the
high-order bit of the A/D field character is sent to
the line base, and the A/D bits are shifted left one
position.
On a timeout operation, the A/D field is used as
a count A timeout operation counts time be­
tween characters. For example: The 2703 is
receiving text from a 1050 terminal and the 1050 operator fails to send EOB (end of block) following
the last character; a timeout operation allows the
2703 to end the receive operation 28 seconds after
receiving the last character.
For synchronous type operations, bits 4 and 5
within the A/D field are used for data-check and
overrun indication when receiving in intermediate­
block (ITB) mode. These conditions are set in the
error-indicator byte (EIB) following an ITB, ETB
(end of transmission block), or ETX (end of text)
generated by the 2703 while executing the Read
command in ITB mode. Data check can also be set
while the 2703 is monitoring the line in the absence of
a command, so that once a command is accepted the
data-check bit is set in the sense byte within MCW-2. During a transmit operation the A/D field acts as a
character buffer, buffering the next character to be
transferred to the Synchronous Line Base for trans­
mission.
Character Address. In a Read command this ad­
dress refers to the position in the MDW to which the
A/D field will be transferred when a character has
been assembled. Similarly, in a Write operation,
this address refers to the next character to be taken
from the MDW-l, or MDW-2, and placed in the
A/D field.
Bit Count. This counter increments as the AID field obtains more bits from the start/stop line base,
or sends bits to the start/stop line base. When this
count reaches a specific number set by the terminal