following the last index character. Additional
characters are requested when required. The
old index character is destroyed.
When the CSW receives the Status Modifier signal,
the Poll command should then chain to either a Read
or a Search command (depending on whether operating
as point-to-point or multipoint). The first character
received during the Read or Search command execu­
tion is the last index character sent out from main
storage in the previous Poll. After the index
character has been transferred to main storage, the
SF immediately sends to main storage the data
received via the communications line. If the Time­
out sense bit had been set during the Poll command
execution, the SF ends the command chained to Read
or Search with Channel End, Device End, and Unit Check status and with the Timeout sense bit set to
one.
EFFECT OF HALT I/O INSTRUCTION AND ZERO BYTE COUNT All the commands implemented by the Synchronous
Base employ a command structure as follows:
o
Command Code 7 8
Data Address
31 Count 32 33 3435 36 37 3940 4748 63
A complete discussion of each of the fields within
a channel-command wt>rd (CCW) is included in the
publication, IBM System/360, Principles of Operation, Form A22-6821.
When the byte count in the current CCW is
decremented to zero, and data chaining is not speci­
fied (bit 32 set to zero), a condition known as
interface stop occurs in the event additional bytes
are requested. This condition signals the 2703 that
no additional bytes are to be transferred between
main storage and the 2703. Various actions take place, depending on the
command being executed, as follows:
Command
Set Mode
62
Action
Command ends when stop is
signaled. If no error or unusual
condition occurred during execu-
Dial
Read
Write
tion, the ending status is Channel
End and Dc\' ice End status (CSW bits 36 and 37).
Stop indicates the last dialing
digit has been transferred.
After the last dialing digit has
been transferred to common­
carrier dial equipment, the Auto­
call featurc will wait for a signal
from the data set indicating that
the connection to the remote
station has been established.
Command ends with Channel End
and Device End status (CS\V bits
36 and 37). If the call cannot be answered
at the remote station, the Dial
command ends with Channel End,
Device End, and Unit Check
status (CSW bits 36, 37, and 38)
and the Timeout sense bit (bit 7)
set to one.
The Interface Stop signal causes
the command to immediately
cease transferring data to the
cham1el. The Unit Check status (CSW bit 38) and the Lost Data
bit (sense-bit 6) are set to one.
Since any data that has been read
but not transferred to main
storage will not be transferred,
part of the received data will
always be lost as a result of an
Inte rface Stop signal. Stop signal indicates the last
character has been transferred
under the current command. The SF sends any characters it has
buffered to the line and ends with
Channel End and Device End status
(CSW bits 36 and 37). The SF goes into receive mode after
transmitting the last character.
If the SF was operating in trans­
parent mode, it will continue in transparent synchronism by
sending transparent sync idle s
until another Write command is
issued. This new Write is used
to transmit the end-of­
transparent-block (DLE ETB) or
end-of-transparent-text (DLE
ETX) sequence.

Search Poll
Enable,
Disable,
Adprep,
Prepare Sense Halt I/O Instruction
The Interface Stop causes the
command to cease transferring
data to the channel and stop
monitoring for valid addreRses. Unit Check status (CSW bit 38)
and Lost Data (sense-bit 6) are
set to one. Part of the received
data will always be lost when the
Interface Stop occurs.
The stop signal indicates that
the last character (EaT) has
been transferred from main
storage to the SF under the cur­
rent Poll command. The SF sends any buffered characters
and ends the command with
Channel End and Device End
status (CSW bits 36 and 37). The SF waits for the reply to the
poll operation before ending the
command. Note that the program
must ensure that the byte count
is exactly as large as the poll
list so that the last character
transferred by the Poll com­
mand is index character for the
last poll in the list.
No data-byte transfers occur
with execution of these commands;
accordingly, Interface Stop is
never signaled.
The Interface Stop signal causes
this command to end immediately
with Channel End and Device End
status (CSW bits 36 and 37).
A Halt I/O (HIO) instruction can be issued to the 2703 during the execution of a command. When HIO is
issued, various actions take place, depending on the
command being executed:
Command Set Mode HIO Action
If HIO is received before the Set Mode byte has been accepted
from main storage, the com­
mand ends immediately with
Channel End, Device End, and Unit Exception status (CSW bits
36, 37, and 39). The SF is
reset. If HIO is issued at other
Enable
Dial
Write
Read
Prepare
times, it has no effect on the Set Mode I s execution. On a switched network, if the HIO is received when the data
set is not operational (i. e. ,
before the call has been answer­
ed), the command is ended with
Channel End, Device End, and Unit Exception status (CSW bits
36, 37, and 39). Otherwise, HIO has no effect on the normal
execution of the Enable command
and the SF is not reset.
An HIO received before the
Interface Stop signal has been
transmitted to the SF (e. g., the
entire number has not been
dialed) results in aborting the
dialing operation, the resetting
of the enable and dial circuitry,
and the immediate ending of the
command with Channel End,
Device End, and Unit Exception
status (CSW bits 36, 37, and
39). If HIO is received after the
Interface Stop signal has been
received, the dialing operation
proceeds normally. HIO causes the SF to transmit
any currently buffered characters
and then terminate the com mand. HIO causes the command to
stop transferring data to the
channel. The Unit Check status (CSW bit 38) and Lost Data
(sense-bit 6) bits are set to one. Part of the data will always be
lost when the HIO is issued
during a Read command execu­
tion. The command ends im­
mediately.
If HIO is issued before character
phase is established, the com­
mand ends immediately with
Channel End, Device End, and Unit Exception status (CSW bits
36, 37, and 39). If HIO is
issued after character phase has
been established, there is no
effect.
63