Channel-Attachment Restrictions
The following restrictions pertain to the attachment
of any 2703 to the multiplexer channel.
1. The 2703 should have the first control-unit
position on the channel; in other words, it
should be the first to receive the channel­
scanning signals. When hvo 2703's appear on
the same channel, they will have sequential priority.
2. No shared subchannels will be allowed on the
multiplexer channel when more than 128 sub­
channels are required. In any case, if devices
using a shared subchannel are physically
attached to the channel, they must not be operat­
ed while the 270;) is in operatlOn.
3. The maximum line speed for any lines attached
to the 2703 is:
Start/ stop type lines--600 bps
Synchronous (BSC) type lines--2400 bps
4. The maximum number of lines attached to any
one 2703 is determined by the type of lines and
line mix. Figure 8 provides a complete listing
of maximum lines by the various possible line­
base configuratio'1s. OPERATION WITH THE MULTIPLEXER CHANNEL
The 2703 connects to and operates with the multi­
plexer channel via the I/O interface. This interface
consists of byte buses (In :;l1".d Out) and tag lines that
indicate the type of information on the byte buses
(e. g., command, address, data, and status),
channel-interlock controls, and interface-scanning
signals. The scanning Signals and interlocks estab­
lish priority among different 2703' s or other control
units attached to the multiplexer channel. When the 2703 requires data transfer on any of its communi­
cations lines (line 14, for example), the scanning
signal is intercepted by the 2703 and an interlock lead
is raised, indicating the interception of the scanning
signal to the multiplexer channel. The 2703 places
the address of the line requesting service on the
Input bus. When the 2703 receives acknowledgment
from the channel that the appropriate control word
lias been from storage, data transfer
between the 2703 and the channel begins. When transfer of a data byte (or bytes) is complete, the
interlock is dropped and the channel resumes scan­ ning the interface. Up to four data byieE can be
transferred serially by byte in one data-transfer
operation.
Selection of the next device (2703, card reader,
etc. ) is on a priority basis. However, the same 2703 is again selected if any line attached to this
unit requires service. and no higher-priority
machine on the channel interface is selected.
Usually the 2703 is attached to the multiplexer in
the position of highest priority.
The multiplexer channel initiates an operation to
a 2703 during the CPU execution of a Start I/O instruction. The specific 2703 operation desired is
defined in the channel-command word (CCW). Data
transfer in either direction across the I/O interface
is initiated by the I/O device after it is com manded . to start by the program.
1/ a Instructions
The System/360 operates with the 2703 through the follOWing I/O instructions: Start I/O, Halt I/O, and Test I/O. Start I/O A start I/O instruction executed by the CPU causes
initial command selection and the transfer of a
command byte to the 2703. Command chaining
within the multiplexer channel also causes selection
and transfer of a command to the 2703. However,
the 2703 will not signal Control Unit Busy status in
response to a command cycle resulting from comman(
chaining. This interlock is effected by presenting
unit status to the multiplexer channel only if the 2703 is free to accept a possible chained command.
During initial selection, the 2703 loads the line
address and the command byte into registers. The 2703 can make the following status responses to Start I/O: 1. If the command is acceptable to the 2703, an
all-zero status byte is sent to the channel.
2. If the command is not acceptable to the 2703, Unit Check is returned to the channel and the
reason for responding with Unit Check
(Command Reject or Bus-Out Check) is set in
the sense byte stored in MCW-2. 3. If the 2703 is busy, it signals Control-Unit
Busy to the multiplexer channel. Control­
Unit Busy is defined for the 2703 as the busy,
status -modifie r, and control-unit-end bits
being ON in the status byte transferred to the
channel. NOTE: This condition occurs only in cases where the channel
traffic is exceptionally high.
Halt I/O Once the 2703 has responded to initial command
selection, the channel can signal Halt I/O. When the 2703 detects a Halt I/O, it loads the line address into
a register, the same as for Start I/O. The addresse
l MCW is commanded to halt. When the current com- I

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