AM3270/TR3271 Reference Manual

Details of the QUO Protocol
AM3270 Subdevice Protocols
086705 Tandem Computers Incorporated 4–21
File-System Procedures
Although the QUO protocol is transparent to the AM3270 I/O process, including the
QUO protocol module, request handling for QUO subdevices is substantially different
from request handling for CRT subdevices.
The CRT protocol module of the AM3270 I/O process handles READ and WRITE
requests using a single queue. This single queue limitation means that if a WRITE
request comes in from the application while a READ request is posted for an AM3270
logical CRT subdevice, and if the physical device has started to transmit data back to
AM3270, the application will send a CANCEL request, and data received from the the
physical device by the AM3270 I/O process will be discarded.
Because data to and from multiple Quotron devices is multiplexed, these devices must
be able to have an outstanding READ request while a WRITE request is processed. To
accommodate the QUO protocol, the AM3270 I/O process has been modified to have
one queue for READ requests and another one for WRITE requests. This two-queue
scheme applies only when the QUO protocol is specified for a subdevice through SCF.
A Note About Configuring QUO Subdevices
When you specify the QUO protocol, you must also specify the following modifiers:
RETRIES
This modifier defines the number of retries permitted. Either specify this modifier
in SYSGEN, or set the SCF attribute RETRIES.
RVIIFNOREAD/EOTIFNOREAD
The default for this modifier is RVIIFNOREAD. Either specify EOTIFNOREAD in
SYSGEN, or set EOTIFNOREAD to ON using SCF.
EOTIFNOREAD results in the CLIP sending an EOT indication to the controller
concentrator when the number of times the controller concentrator can retransmit
data (that has received a NAK) has been exceeded.
The purpose of the EOT is to alert the controller concentrator that none of the data
responded to with a NAK has been accepted. The controller concentrator must
retransmit the entire data message. If you use the default modifier
RVIIFNOREAD, there is no way to be certain which data has been accepted by the
CLIP. This can result in data loss.