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PROTOCOL CONVERTER MODEL A/S-4
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Appendix B: General 3770 SNA
Communication Operations
This appendix is intended to give a basic description of the System Network Architecture (SNA)
operation for an IBM 3776/77 Model 3 and 4 multiple logical unit (MLU) work station in the area of
remote communications.
The 3776/77 Model 3 and 4 is a stand alone work station consisting of; 1) console with keyboard and
display. 2) console-mounted line printer with optional card reader and line printer. SNA calls the
controller port a PU (Physical Unit) and the attached printers, consoles, card readers and disk LUs
(Logical Unit).
Communications with the host computer and the work station basically follows the following steps.
1. The line is activated - This means a phone line is established between the host computer and
the work station. 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 3776/77
work station’s controller (PU) to tell it to start communications. The work station then tells
the host when it is ready to start.
3. The LUs are activated - The host now tells all the attached devices (LUs) to start
communications. The devices respond to host when they are ready to start.
4. SSCP (System Services Control Point) may send a message to the console requesting a logon
message for the active LUs if the host system is GENed to send a SSCP message. A GEN is how
the host system operators set up different work station options. At this point the console is
considered to be in a SSCP to LU session. Session means that two devices are talking with
each other.
5. The work station operator may now enter a request logon message for an application - This
allows the operator to request a logon to different applications like JES2, POWER or CICs on
any available system. For example; Test System A or B or a Production System. If the work
station 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 work station operator.
6. After a good logon message is received by the host it will BIND the requested LUs to the
application - This establishes communication between the work station and the application.
The devices (console, printer, readers, disk) are now considered to be in a LU-to-LU session.
7. The application usually sends an active session message to the console - This allows the
operator to run his application or transactions. At this time the operator can down-line load a
job to the host, receive a job from the host, receive print, ask for status of jobs running on the
system and many other operations.