AM3270/TR3271 Reference Manual

TR3271-Specific Operations
A Closer Look at TR3271
5–8 086705 Tandem Computers Incorporated
TR3271 issues a READ I/O to receive one block. The block is moved into the
message buffer.
TR3271 sends an ACK and issues another I/O to receive the next block of the
message. If a block is received, it is appended to the end of the previously-
received block in the message buffer.
TR3271 checks the 3270 command within the block to verify that it is “chainable”
to the previous block (see “Functional Limitations” below).
If the combination of the commands is valid, an ACK is sent. If the combination is
not valid, the flow control strategy described above is used to reject the second
block.
The process of receiving a block, verifying the command, and sending an ACK is
repeated until an EOT is sent by the host. When the EOT is received, TR3271 formats
the message by combining the 3270 commands and WCCs into one command and
WCC, strips the BSC framing characters from the blocks and compacts the blocks into
a message. The message is then sent to the application or to the AM3270 process
(framing characters are not stripped in PTP mode).
Functional Limitations of Multiblock Implementation
TR3271 does not provide a true emulation of a 327x control unit. Limitations are as
follows:
As mentioned previously, TR3271 combines the commands from each block into
one command for the entire message. This places a restriction on the the type of
commands that can be chained or received in the blocks. Only the following
command sequences are accepted:
ERASE/WRITE in the first block, followed by blocks with a WRITE command.
WRITE followed by WRITEs.
ERASE/WRITE alternate followed by WRITES.
Note that it is possible for an ERASE/WRITE to follow a WRITE. In this situation,
TR3271 will accept the WRITE but reject the ERASE/WRITE and the flow control
procedures described previously will be used.
TR3271 simply treats the WCCs of each block as though an OR operation
combined them into one WCC for the entire message. For example, if the
keyboard unlock bit is set in one of the WCCs, then after the OR operation the
keyboard unlock will be set in the WCC for the entire message. (The OR operation
actually takes place using the EBCDIC value of each WCC.)
TR3271 does not provide full support for the Reset Modified Data Tag (MDT) bit
of the WCC. That is, TR3271 will not handle a situation where a Reset MDT bit of
a WCC in a later block in the sequence is meant to reset MDT tags of fields in a
previous block.
Example of Reset MDT