IBM 3741 Data Station Reference Manual page 82

In the expanded communications feature receive portion
of the transmit/receive mode, the 3741 has the capability
of receiving non-transparent transmission blocks containing
up to 512 data bytes. Each record must be the same length
within each data set. Records must be separated by IRS
characters if more than one record per transmission block
is to be received. Records can be spanned between two
transmission blocks. Transmission block lengths must be
greater than the length of each record, except for the last
transmission block of the data set. One record per trans-
mission block in either transparent or non-transparent text
can also be received with the expanded communications
feature. When the transmission block length equals the
record length, no I RS character is required. The first
IRS, ETX or ETB (OLE ETX or OLE ETB in transparent
text) of the first transmission block determines the record
length for records in that data set. An I RS character pre-
ceding the ETX or ETB character is mandatory on all
multiple record transmission blocks except the last block.
Expanded Communications Transmit/Receive Mode (K)
The expanded communications feature transmit/receive
mode (K) is similar to the transmit/receive mode (B) except
that multiple records are transmitted in each transmission
block. Each transmission block can contain up to 512
bytes of data. When the expanded communications feature
transmit/receive mode is selected, a K is displayed in the
status line. For the transmit portion, the last character of
each record in the buffer is followed by an IRS (inter-
record separator) character which defines the end of that
record. Records are read from disk into the 512-byte
buffer until a complete record and I RS character cannot
be contained in the buffer. Thus, records are not spanned
between transmission blocks.
The I RS and I FS (interfield separator) characters cannot
be sent as data in the K mode of operation. If these or any
other data link control characters are read from disk a trans-
parency error will result. The I RS or I FS character can be
transmitted in T, B, P, or 0 mode.
The operator identification card data is transmitted as a
separate block without a following I RS character. The
format of this block is described in the IBM 3740 BTAM/
TeAM
Programmer's Guide, GC21-5071.
76
Transmit-Transparent/Receive Mode (D)
The transmit-transparent/receive mode of operation is
used to transmit data in the transparent mode and receive
data without operator intervention.
After loading the disk(s), an operator can place the 3741
in a transmit-transparent/receive mode by pressing FUNCT
SEL upper, COMM, and O. A 0 will then be displayed on
the display's status line to indicate the transmit-transparent/
receive mode.
This mode of operation is identical to the transmit/receive
mode with the exception that data is transmitted in the
transparent mode. OT, 01, OJ, and DC have meanings
corresponding to BT, BI, BJ and BC in the transmit/receive
mode.
Writing of Received Data Over Previously Transmitted Data
It is possible on the 3741 Model 2 or 4 in transmit/receive
mode to transmit data, and then receive data and write the
data over the same space on the disk. Before using the fol-
lowing technique, be sure that there is no requirement to
keep the data that was transmitted to another location.
Since the technique used for transmit/receive mode is to
start writing received data in the next available sector past
the EOE of the last data set transmitted, it is possible to
write received data over a portion of the disk from which
data was previously transmitted. This can be done by
transmitting as the last data set a null (empty) data set
with the BOE, EOO, and EOE set to the desired address
where you want the received data to be written on the disk.
On a 3741 Model 2 or 4 with two disk drives, the preceding
procedure can be used to transmit one full disk of data
and then receive 1897 records on disk 1 and 1898 records
on disk 2. Disk 1 can contain only 1897 records since re-
ceived records are written to disk starting at EOE+1. Thus
if EOE is 01001, received records are written to disk start-
ing at 01002.