AM3270/TR3271 Reference Manual

TR3271-Specific Operations
A Closer Look at TR3271
086705 Tandem Computers Incorporated 5–7
Read-Modified TR3271 response to a read-modified command from the host depends on the
specification of TR3271 macro modifiers, which are set during SYSGEN:
READMODAFTERPOL
NOREADMODAFTERPOL
If NOREADMODAFTERPOL is specified, any read-modified command from the host
is rejected (NOREADMODAFTERPOL is the default).
If READMODAFTERPOL is specified, TR3271 will accept read-modified commands
from the host—but only if the command is issued just after TR3271 sends data in
response to a poll (or in response to another read-modified). When the read-modified
arrives, the data sent in response to the previous poll is resent to the host. If a read-
modified arrives just after an interaction that was not a poll (or a read-modified), the
command is rejected.
Multiple-Block
Transmissions
It is possible for an IBM host to send multiple blocks of text to TR3271 after one select
of an emulated subdevice. TR3271 supports multiple block transmissions through the
MULTIBLOCK system generation parameter.
Without MULTIBLOCK specified, TR3271 uses a “flow control” type of strategy to
handle multiple block transmissions. When the host attempts to send a multi-block
transmission, the first block of data is sent to the application or to AM3270 (for PTP
mode). TR3271 rejects a second block and reports a DB (Device Busy) status.
In order to avoid acknowledging data that cannot be delivered, TR3271 does not
accept a block if there is no READ outstanding. (In the PTP case, new data is not
accepted until previous data is delivered. When TR3271 can accept the second block, it
sends a DE (Device End) to the IBM host.
In order for this technique to work properly, the LINEBUFFERSIZE system generation
modifier must be defined as large enough (in words) to contain both the first and the
second block. If LINEBUFFERSIZE is defined large enough, TR3271 sends an EOT in
response to the second block; if it is not, TR3271 sends a NAK. (This NAK can result in
problems with error recovery.)
With the MULTIBLOCK parameter specified, TR3271 combines blocks into a message
which is handled as the unit of information that is sent to the application (in CRT
mode, or to AM3270 in PTP mode). The combined message contains only one 3270
command and WCC, followed by the concatenation of text from all the blocks.
Multiblock Transmission—How It Works
This is how a multiple block transmission is handled when MULTIBLOCK is specified:
The application issues a READ with a byte count large enough to accept the entire
message. (In the PTP case, the READ is considered to be issued when the previous
message sent to AM3270 is delivered.)
The count should not be larger than the record size that is set during system
generation. TR3271 allocates one buffer for the entire message.