AM3270/TR3271 Reference Manual

Details of the CRT Protocol
TR3271 Subdevice Protocols
6–2 086705 Tandem Computers Incorporated
As illustrated in Figure 6-1, TR3271 adds an STX at the beginning of all blocks. It also
adds the control unit and device address at the beginning of the first block and an ETB
at the end of all blocks except the last, after which an ETX is added, followed by a
BCC. For Figure 6-1, assume that all data fits into one block.
Message Format—Inbound
Data
For inbound data, as shown in Figure 6-2, the buffer passed back to the application
process contains the complete data message received from the host. The message
begins with the command (CMD) character; all communication characters, including
the escape (ESC) character, are removed.
Figure 6-2. CRT Protocol Message Format—Inbound Data
020
W
C
C
Screen Information
Read Count
Returned to the
Application Process
S
T
X
E
S
C
C
M
D
W
C
C
E
T
X
B
C
C
Screen Information
Sent by the Host
C
M
D
Applicable File-System
Procedures
For TR3271/CRT protocol, the following file-system procedures apply:
AWAITIO CONTROL OPEN SETMODENOWAIT
CANCEL DEVICEINFO READ WRITE
CLOSE FILEINFO SETMODE
Note that all file-system procedures are documented in the System Procedure Calls
Reference Manual. File-system procedures CONTROL 22, READ, SETMODE (30, 50, 51,
and 59), and WRITE have special significance for this protocol and are also discussed
in this section.
CONTROL 22 (Two-Step READ Operation)
The CONTROL procedure is used to perform device-dependent I/O operations. A
CONTROL 22 initiates a two-step READ operation, which is designed to conserve
buffer resources.
A file-system READ request requires the file-system to allocate buffer space in the
amount specified by the COUNT parameter of the READ call. The space must be
allocated for the lifetime of the READ request. Assuming that for a given
configuration many subdevices exist simultaneously, each with one or more out-