AM3270/TR3271 Reference Manual

Details of the CRT Protocol
AM3270 Subdevice Protocols
086705 Tandem Computers Incorporated 4–13
General Information
When a text message is received or a line error occurs, the CONTROL request
completes by returning a file-system error 0 if the read was successful; otherwise,
it returns a specific error number. If the file-system error is 0, a READ must be
issued as soon as possible to pick up the input buffer.
The read count parameter of the READ procedure is checked to see that it is
greater than or equal to the actual count read from the device. If the read count
parameter is less than the actual read count, file system error 177 (text overrun
(insufficient buffer space for data transfer)) is returned to the application process
and no data is returned.
If another CONTROL 22 is issued by the same opener before the first one has
completed, file-system error 88 is returned (a CONTROL READ is pending; a new
READ is invalid).
If a CONTROL 22 has completed and the application program logic states to
discard the received data without issuing a READ, the application program may
issue a CONTROL 22 with parameter set to -1. AM3270 will interpret this form of
the CONTROL 22 as a “discard data” request and release the data buffer.
If the CONTROL portion of a two-step READ has completed and a request other
than a READ (or a CONTROL with a parameter of -1) is issued, file-system error
87 is returned (awaiting a new READ request but did not receive it).
READ Request A normal READ request (a CONTROL 22 is not used) causes the subdevice to be
added to the poll list. When the device responds with text, the blocks received by
AM3270 are collected and stripped of any communication control characters. When
the final block is received, the READ is completed by transferring pure text to the
application process. Status messages sent by a device are not passed to the
application. Instead, the message is evaluated and, if necessary, the READ is
completed with an error.
SETMODE 53 Operation SETMODE 53 enables or disables the receipt of status messages from a subdevice.
This SETMODE is applicable only to subdevices that have the CRT or PRT protocol
assigned to them and just one SETMODE 53 at a time can be in effect for a particular
subdevice.
Note SETMODE 53 will cause the line to poll, and when you disable SETMODE 53, polling is not stopped until
something other than an EOTis received.