SNAX/APN Application Programming Manual

The CRT Interface

SNAX/APN Application Programming Manual—420111-001

2-3

Outbound Data Stream

An outbound data stream is sent from an application process on a Tandem system to a

device on an IBM system. For outbound data, the application process explicitly controls

the screen format by including standard 3270 screen orders within the outgoing block of

text. An application process uses a WRITE procedure call to send an outbound data

stream to a device.

Inbound Data Stream

An inbound data stream is sent from a device on an IBM system to an application

process on a Tandem system. For inbound data, SNAX/APN passes the appropriate

3270 attention identifier (AID) character, followed by the incoming text, to the

application process.

An application process uses a READ operation to acquire 3270 data. SNAX/APN

removes the link header, transmission header and the request/response header and then

passes only the AID character and screen information (screen data and orders sent by the

device) to the application process, as shown in Figure 2-3

. The AID character identifies

which key (from the group of attention keys) the operator pressed to cause transmission

of the particular screen orders or block of data.

Two-Step Reads

The CRT protocol provides an additional way to read inbound data. This read

capability, called “two-step read,” enables an application process to make the most

efficient use of available buffer space by determining the size of the inbound data stream

before actually reading the data.

A two-step read involves two separate read operations:

1. The first read operation is initiated by a WRITEREAD procedure call with a write

count of 0 and a read count of 2, that is:

CALL WRITEREAD (filenum, countbuffer, 0, 2);

filenum specifies the remote LU representing the device, and countbuffer

specifies an array within the application process. A write count of 0 indicates that

nothing is being written to the device, and a read count of 2 indicates two bytes is to

be read from the device.

The call returns a two-byte integer to countbuffer identifying how many bytes

of data are in the incoming data block.

Figure 2-3. Format of the Inbound Data Stream

203CDT .CDD

Attention

Identifier (AID)

Character

Screen Information