SNAX/APN Application Programming Manual

The CRT Interface
SNAX/APN Application Programming Manual—420111-001
2-2
Sending and Receiving 3270 Data Streams
Sending and Receiving 3270 Data Streams
The CRT protocol enables the transmission of 3270 (or SCS) data streams between an
application process and a device (either a terminal or a printer). Data streams consist of
data as well as processing and formatting information. The application process
maintains a full-screen buffer as the unit of transmission.
The data stream transmitted is preceded by several headers and followed by a trailer, as
shown in Figure 2-2
.
Figure 2-1. Typical Subsystem Use of the CRT Interface
Figure 2-2. Headers and Trailer of the 3270 Data Stream
201CDT .CDD
Device
User
Application
User
Application
SNAX/APN
CRT
Tandem NonStop System
SNA Subarea Network
202CDT .CDD
LH
TH
RH
LT
-
-
-
-
Link Header
Transmission Header
Request/Response Header
Link Trailer
3270 Data
Stream
LH TH RH LT
Application
Controlled
SNAX/APN-Controlled
Application
SNAX/APN-Controlled
Application