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PROTOCOL CONVERTER MODEL A/S-4
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Appendix A: 3274 SNA
Communication Operations
This appendix is intended to give a VDU operator the basic description of the System Network
Architecture (SNA) for a 3278 CRT (or like device) in the area of remote communications.
First the 3278 CRT is connected to a 3274 controller that allows 32 3278 CRT’s to communicate to an
IBM mainframe computer. SNA calls the 3274 controller a PU (physical unit) and the 3278 CRT a LU
(logical unit).
Communications with the host computer and the CRT basically follow the following steps.
1. The line is activated - This means a phone line is established between the host computer and
the 3274 controller. This maybe done by dialing a phone for dialup applications or having a
host computer operator connect a leased phone line.
2. The PU is activated - The host sends the first message down the phone line to the 3274
controller (PU) to tell it to start communications. The 3274 controller then tells the host
when it is ready to start.
3. The LUs are activated - The host now tells all the 3278 CRTs (LUs) to start communications.
The CRTs then tell the host when they are ready to start.
4. SSCP (System Services Control Point) sends a message to the LU - The CRT at this point
receives an enter request logon message from SSCP if the host system is GENed to send a
SSCP message. A GEN is how the host system operators set up different CRT options. At this
point the CRT is considered to be in a SSCP to LU session. Session means that two devices are
talking with each other.
5. The CRT operator may now enter a request logon message for an application - This allows the
operator to request a logon to different applications like CICs, IMS or TSO on any available
system. (example; Test System A or B or a Production System). If the CRT will always be
assigned to one application, the host system may be GENed (optioned) to automatically
request an application logon, in which case this setup does not need to be performed by the
VDU operator.
6. The system now BINDs the CRT to the application - This establishes communication between
the CRT and the application. The 3274 controller will usually clear the CRT screen when this
happens. The CRT is now considered to be in a LU to LU session.
7. The application sends a welcome or signon application message to the CRT - This now allows
the operator to run his application or transactions. While communicating with the
application, it is usually on a demand-response basis. The operator enters data or answers a
question and the host sends back the information or more questions. This proceeds until the
operator has completed the assignment.