Program Control Functions - Varian Data 620/i System Reference Manual

4.3 PROGRAM CONTROL
FUNCTIONS
4.2.2 Input/Output Bus Structure
The basic DATA 620/i computer (620/i-OO, 622/i-OO) is
equipped with a positive-voltage-level party:"line I/O bus.
The party line is
Q
bi~djrectional
common communication
channel containing the data and control lines required for
system communication. Each transmission on the party line
has two phases: The first phase is the route set-up (i .e. device
selection); the second is the data transmission.
The party line permits plug-in expansion of all peripheral
devices. The party line contains line drivers and line
receivers to service up to ten standard peripheral devices.
Modifications to the computer are not required to add
periphera Is. Each standard periphera I devi ce contains a
party-line data buffer. Thus, no device can tie-up the party
line. The party line technique solves the troublesome
problems usually encountered with on-site system expansion.
4.2.3
I
nput/Output Operations
During information transfers over the I/O bus, the E-bus lines
may carry control codes, addresses, or data, dependi ng upon
the type of operation being performed. Table 4-1 defines
the I/O cable control signals used to synchronize all I/O
operations. Table 4-2 summarizes the signals on the interrupt
cable.
NOTE
An I/O command is not transmitted intact over the
E bus. Bits 11-15 are decoded in the central pro-
cessor. The processor then generates an E-bus bit
(EB 11-EB15). Only one of these bits is true for
each type of command. Bits
0-8
of the command
are transm itted unchanged on the I/O cab Ie.
Interfacing functions fall into two major categories: pro-
grammed operations and automatic operations. The
programmed operations are: external control (single-bit out),
sense operations (testing a single bit), data transfer in (full-
word input) and data transfer out (full-word output). The
following paragraphs describe the programmed operations and
examples of their use. The I/O instruction group is sum-
marized in table G -11, appendix G. Th is group of
instructions is standard for the DATA 620/i.
4-3