I IOpera-ICommand I I I ITP Opl I Ition I Code IAddress IFlagslCode ICountl 1-------------------------------------1 IWrite 01 Area CC, I 10 Ivari-I Itext SILII lable I I 1 IRead 02 IResponsel SILl I 11 2 I IRe- I I I I I I sponse I I I I I , I In situations where the line is found to be in text mode, CP can
issue a write reset sequence to put the binary synchronous line in
control mode. The following format illustrates the write reset CCI. I , IOpera- ICommand I I ITP Op I I Ition I Code IAddress IFlagslCode ICountl I I I Write 01 Table SILl I 09 1 I I EOT I I In situations where the expected response from a remote station was not received or was invalid, the channel program may request the remote
station to retransmit the response. The following write ENQ for.at
shows this sequence. The remote station, upon receipt of the ENQ message, responds by transmitting the expected or valid response to the
response area indicated by the second CCI. • Opera-ICommandl I ITP opt I tion I Code IAddress IFlagslCode ICountl ----------------------------------------1 Write 01 ENQ I Table I CC, I 03 SILl I 1 I I --------------------------------------1
Read 02 Re- I sponsel IResponse I SIL! 1 11 I I I I I I 2 I I I ,
Read operations occur following a general poll or a specific poll for
text messages. In a general poll sequence, CP transmits the general
poll characters to the attached control unit on the bisync line. The
control unit recognizes the polling request, then the list (referred to
in the poll CCW) of enabled devices is scanned for any messages that are
queued and ready for trans.ission. 1 positive acknowledgement (yes, I
have a message to trans.it) from any of the attached devices causes the
next ccw to be skipped. The last CCI provides the read buffer and the
count necessary for the incoming data block from the first remote CP Introduction 1-101

station on the list that had a message queued for transmission. If,
however, all remote stations respond with negative acknowledgement (no messages queued) or any station queried for a response fails to respond,
then the channel program ends with the third CCW. The following read init{al format shows the initial read CCW sequence.
r IOpera- ICommand I I ITP Opl ItioD I Code IAddress IFlagslCode ICount I IWrite 01 Table CC, I 02 1 I EaT SILl I I IPol1 09 List CC, I 03 ILIST I SILII I I !T/O 0 ! 07 1 INo- I lopera-I Ition I I IRead 02 Area SILl I 10 162 IText I I After CP receives a message from a remote station, it .ay reissue the
initial read sequence to poll the remaining stations on the list (assuming the list of enabled devices was not exhausted on the first
pass of the initial read sequence). In the event that the list was
exhausted on either the first or a subsequent initial read sequence, CP starts the poll delay, then allows the poll delay interval to expire
before starting another read scan to the line (assuming CP has no higher
line priority tasks to process). If, in the process of receiving
aessages froa remote stations, CP receives a message block that is
invalid or its beginning or ending bisync control characters are not
recognized, CP can elect to send a negative response back to the reaote station. This negative response, the NAK control character, causes the remote station to retransmit the previous message to CPt this incoming aessage is processed by the second CCW of the read repeat sequence as
shown in the format below. I I IOpera-ICommand I I ITP Opl I Ition Code IAddress IFlagslCode ICount I I I IWrite 01 Table CC, I 06 1 I I NAK SILII I I I IRead 02 Area SILl I 10 162 I IText I I , I Once CP message processing receives an error-free aessage fro a a reaote station, CP sends an RVI control character to the remote station
before processing the message. The remote station, upon recognition of
the RVI character, halts the sending of additional queued data and
responds with EaT (instead of the noraal lCKO/1CK1 response). The 1-102 IBK VK/370 System Logic and Problem Deter.ination--Voluae 1