command ends normally when the ENQ is detected.
All conditions pertaining to the Read command apply
to the Search command under this circumstance. PROGRAMMING NOTE: No fast-selection operation is permitted
with the Search command.
Address Prepare
This command (Adprep) is used by the tributary
station equipped with the Station-Selection feature.
The Adprep command is a pseudo-Read command in
Address Prepare CON 1 Et fIt f2t f3 I IE I Data Address
o 78 31
Count
32 3940 4748 63
that the SF goes through all the actions of a Read
command, yet does not transfer data to main storage.
When the SF accepts the Adprep, the SF monitors
the receive line, following all data on the line. If
the Adprep command is accepted and character
phase is not established (the SYN pattern has not
been detected), the SF starts a three-second timeout,
during which it must recognize SYN followed by one
of the following three conditions:
a. 2703 Address--If a selection address is
detected, the command will end with Channel End and Device End status (CSW bits 36 and
37), thus allowing chaining to a Read command;
or if the polling address is detected, the
command ends with Channel End, Device End,
and Status Modifier status (CSW bits 36, 37,
and 33 respectively). This allows command
chaining to a Read command other than the
Read command used when the selection address
is detected.
b. Text-Mode Entry--Upon entering into text
mode (as a result of recogni zing STX or SOH), the SF will cease looking for its address until
a new SYN SYN EOT sequence is detected.
The SF monitors the data stream while in text
mode, using the three-second timeout for
recognition of SYN characters ... similar
to the Read command operation, but with the
exception that the Address Prepare command
does not end.
c. End-Character Recognition--The recognition
of a new SYN SYN and an End character will
cause the three-second timeout to be reini­
tiated, and the SF will continue to monitor for
a new SYN SYN and its address or an End
character.
If a timeout occurs or if sync is established when the
Adprep command is issued, the SF looks for a SYN
SYN EOT sequence before any of the preceding
condi tions are sought.
The SF decodes up to three addresses. The
addresses are known as the group address, the poll
address, and the selection address.
The group address is used when a station wishes
to receive a transmission as part of a group of
stations, with one of the group of stations pre­
designated as the responding station.
The poll address is the address of the tributary
station used in a polling operation (transmitting),
while the selection address is used as the address of
the tributary station in a selection operation. Poll This command is a pseudo Write command in that
the SF goes through the actions of a normal Write
command until the ENQ control character is detected
Data Address
o 78 31
Count
32 3940 4748 63
in the data received from main storage. The index
character should always follow ENQ, and this index
will be the first character sent to main storage when
the positive response is received. After the ENQ
has been transmitted to the line, the SF enters
receive mode (however, Channel End and Device
End are not sent to main storage). When in receive
mode, the SF looks for the receipt of character
phase. The following conditions may occur:
a. Ti meout - - Following the trans mi tting of the
ENQ, a three-second timeout is initiated.
At the end of three seconds, if the SF is not in
character phase, the Poll command ends with Channel End, Device End, and Status Modifier
status (CSW bits 36, 37, and 33).
b. Recognition of Character Phase-- Upon receipt
of character phase, the operation ends with Channel End, Device End, and Status Modifier
status if the first non-SYN character following
recognition of character phase is not an EOT character. If the first non -SYN character is
an EOT, the SF retui'ns to transmit mode and
transmits the next character immediately
61

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.