• Text messages to and from remote terminals and printers can only be
achieved when the bisync line is in text mode. • Text messages from a remote device can be the result of a general
poll or specific poll operation to the related device or devices on
the bisync This polling communication interface is
accomplished by each line-connected control unit having unique
specific poll and general poll recognition circuitry and by the CP terainal list of valid bisync lines and 3270 remote control unit
addresses. This list, the terminal list, is generated by VM/370 system generation procedures employing TERMINAL and CLUSTER macros. For more details about terminal list generation, see the g.!l£ • Reliability and dependability of line operation is achieved by the
use of: a double addressing scheme, control characters with a rigid
message protocol, and complex redundancy-check characters appended to
transmission messages. Examples of these techniques are shown in the
formats that follow. • Every message (text or that is by CP may .ay not be responded to by the remote station or control unit. The type of
response (or absence of response) that CP receives depends on the
receptiveness of that device or control unit to the previously sent
message (is the device ready and enabled and accurately addressed)
and the content and correctness of the aessage (no line errors). • To establish the relationship of the line of terminal response to a
particular line or device write or read operation, CP employs an
operation "tracking" facility (TP op code) imbedded 1n the issued CCWs. The function performed by the CP op code is described in the
following CCW formats. I IOperationl Address IFlags ITP Op 1 Count 1 Code 1 Field 1 1 Code I 1 I 1 byte 1 3 bytes 11 by tell bytel2 bytes 1 I o 7 8 31 32 39 40 47 48 63
Operation Code
contains the hexadeciaal value of the type of operation
perforaed by the command. Valid operation codes are: X'01' WRITE X'02' READ X'03' NO-OP X'09' POLL 1'23' SET MODE X'27' ENABLE X'2F' DISABLE
Address Field
Depending on CCW usage, this field may address an:
1-98 IB" '"1370 System Logic and Problem Deteraination--Volu.e 1

Area
Table
The address of the data area (read buffer) located in the BSCBLOK at BSCREAD.
The appropriate location in the table of data-link control
characters provided in the module D8KGRF (Exaaple: ENQ).
Response
List (BSCRESP). The address location of the response aessage in
the BSCBLOK. The appropriate entry in terminal list (NICBLOKS) associated
with the READ or WRITE operation. The entry for WRITE operation is at location BSCSEL. The entry for the READ
operation is at location BSCPOLL. Note: To see how the key words AREA, and LIST are refer to the CCW sequences described in "I/O Program Routines for Bisync
Lines and 3270 Remote Devices" in this section.
Flags
TP op Code Count
The flag bits turned on in the CCW: CC (channel CD
(chained data), SILl (suppress incorrect length skip (suppress data transfer to main storage) and PCI (progra&-controlled interrupt).
An imbedded teleprocessing operation code in the CCWs used in
bisync line communications. This code is inspected by the
secondary interruption when channel end and
device end are received. The code is also used by the error
processing module, DMKBSC. The code indicates the function
being performed by the associated command. For use of the TP op codes, refer to the formatted CCWs that follow.
Refers to the byte length of the CCW READ or WRITE operation. 1/0 PROGRAMS FOR BISYNCHRONOUS LINES AND REMOTE 3270S Before data communication to re.ote 3270 equipment can take the
re.ote teleprocessing line, the control unit and the device(s) .ust be
enabled for co •• unication. This occurs when control unit hardware
recognizes a unique string of characters transmitted on the line from CP. Disabling a line occurs in a siailar .anner. The following is the
format of the CCWs used in the enabling/disabling operation: CP Introduction 1-99