Exchange/SNA Manual

IBM 3770 SNA MLU Terminals
Introduction
1–2 104700 Tandem Computers Incorporated
Card Punches
Card punches are an optional output device used by the IBM 3770 SNA MLU
terminals. Output from the host system is sent to a card punch attached to an IBM
3770 SNA MLU terminal for punching on cards at the remote site. Card punches are
used when the remote site requires the output in machine-readable form.
Diskette Storage Units
Diskette storage units are used for both input and output, as well as offline tasks such
as reading a deck of cards and writing the information to diskette. Aside from their
use as input and output devices, the diskette storage units provide a convenient
medium for exchanging data between remote sites.
Console Keyboard and Display
The console, which consists of a keyboard, a control panel, and a display, is used by
the operator to control the terminal and to communicate with the host remote job entry
(RJE) subsystem, which is the subsystem in the host that handles communications with
IBM 3770 SNA MLU terminals. The operator can send commands to and receive
output from the host RJE subsystem using the console keyboard and display.
Terminal Control Unit Each IBM 3770 SNA MLU terminal contains a control unit that controls the input and
output devices and handles communications with the host RJE subsystem. The control
unit implements the SNA protocols required for communication with the host system.
Communications
Capabilities
The IBM 3770 SNA MLU terminals communicate with the host system using SNA
protocols, including the synchronous data link control (SDLC) line control protocol.
These protocols define the format of the communications between the terminal and the
host system.
The major communication feature of the IBM 3770 SNA MLU terminals is that they
support the SNA multiple logical unit protocol. Each terminal supports up to six
separate logical units (LUs), thus allowing up to six separate LU-LU sessions with the
host system. Each LU-LU session supports separate, independent communications
with the host, in effect providing a separate communications line to the host (although
all communications take place over a single physical data communications line).
By using the MLU protocol, each IBM 3770 SNA MLU terminal can support up to six
simultaneous data streams to and from the host system. This allows several input and
output devices to be in use at once, some sending data to the host and some receiving
data from the host.