IBM 3741 Data Station Reference Manual page 75

The binary synchronous communications feature checks
to determine which data set labels are set up for data set
transmission (transmit mode), which data set labels are
to be used to identify data sets received (receive mode),
or which data set labels are set up for data set transmission
and which data set labels are to be used to identify data
sets received (transmit/receive mode). Figure 34 sum-
marizes the effect of data set label status on data sets in
the various BSCA modes.
The following further describes the individual BSCA modes.
Transmit Mode (T)
Transmit mode is used to transmit data from one disk or
two disks (if the second disk feature is installed) to a remote
location. Transmission begins with the first data set label or
record selected and continues to the end of the data set
labels unless the labels are deleted, bypassed, or a label error
is detected.
After loading the disks to be transmitted, an operator places
the 3741 in transmit mode by pressing FUNCT SE L upper,
COMM, and T. A T will then be displayed on the status line
of the display to indicate the transmit mode.
When the 3741 is in transmit mode, it can be connected to
the telephone line through a modem, and subsequently
transmit data from the disk to the remote location.
The procedure in the previous paragraph was for transmit-
ting one or two disks in their entirety. If the operator
desires to start transmission from any position on the first
disk, the operator can position the disk to any record of
any data set with normal key entry operations, and sub-
sequently start the transmission of data from that point.
If transmission occurs with no errors, a TT is displayed on
the display's status line to indicate a completed transmis-
sion. If a disk error, invalid data set label, or a transparency
error occurs, TI will be displayed in the display's status line
to indicate an incompleted transmission. If the mode in-
dicator is TT or TI, the 3741 will not automatically answer
a telephone call. If the mode indicator remains T (as it
would if a line check, remote abort, or message aborted
error occurred), the 3741 will answer a telephone call and
will retransmit the data beginning at the same place as the
original call. However, on early shipmentsof the 3741
Model 2, if the communications mode was selected with a
data set record displayed, the operator must reposition the
disk to the original starting position. Your 3741 can be
field upgraded so that the operator does not have to re-
position the disk to the original starting position. Contact
your I BM representative for more information. If your
3741 has not been field upgraded and you are performing
unattended operations, it is recommended that the com-
munications mode be selected while the label is displayed.
If you intend to use any transmit mode, do not write a %
character as data in position 1 of disk address 01001 since
this unique condition is reserved for initiating a BSCA diag-
nostic procedure.
Expanded Communications Transmit Mode (J)
The expanded communications transmit mode
(J)
is similar
to the transmit mode (T) of operation except that multiple
records are transmitted in each transmission block. The
last character of each record in the buffer is followed by an
I RS (inter-record separator) character which defines the
end of that record. The I RS character occupies a byte
position in the 512-byte buffer and is transmitted to the
receiving system. Records are read from disk into the 512-
byte buffer until a complete record and I RS character can-
not be contained in the buffer. Thus, records are not
spanned between transmission blocks. Refer to the host
system's reference manual for more information.
The IRS (X'1 E') and IFS (X'1C') characters cannot be sent
as data in the
J
mode of operation. If these or any other
data link control characters are read from disk, a transpar-
ency error results. The I RS or I FS character can be trans-
mitted in transmit (T), transmit/receive (B), transmit-
transparent (P), or transmit-transparent/receive (0) mode.
The operator identification card data is transmitted as a
separate block without a following I RS character. The for-
mat of this block is described in the IBM 3740 BTAM/
TeAM
Programmer's Guide, GC21-5071.
Communications 69