When you write an HP 3000 program designed to communicate
with a host application that will not be modified, you need to know
what the host screen formats are and how the host program works.
You must know when the host program requires input, when it sends
data, and how to determine when the host has finished sending a
particular internal screen image. You should also know if a single
screen of data will be transmitted in more than one data transmission.
You can use the automatic print feature in Pass Thru to show
when and how the host program writes to your screen. Start Pass
Thru specifying format = 3 in the info string of the RUN command, and manually access your host program. Your
internal screen image will be printed after every call to either
the TRAN3270 or the RECV3270 intrinsic. The printed screen images show you
the screens sent by the host, with attribute and null characters
differentiated from blank characters. If the host sends a single
screen in more than one transmission, you will be able to determine
that from the printed images. See Using SNA IMF Pass
Thru for more information on Pass Thru and its automatic
print feature.
Because your HP program emulates a 3278 display station or
3287 printer, you must design your program to respond to the host
program whenever it modifies your internal screen image. Your program
must know what the host system will do in a particular situation.
If the host may do one of several things at a particular time, your
program must be written to handle variable situations.
When you can modify existing host applications, or when you
can write local and host applications in tandem, you have the opportunity
to write simpler programs. Screens do not need operator instructions
or even much structure. In fact, you can use an unformatted screen
with no fields at all. Unformatted screens are often more efficient
for new applications. You can write the programs to suit your local
needs, possibly with the HP 3000 application controlling the host
program.
