Repair manual

2-70
THEORY OF OPERATION
F. Computer Interface
The Digital Palette is designed to operate in the IBM PC, Apple Macintosh (MAC-II),
and UNIX based engineering workstation operating environments. It is connected to a
host computer via a standard Centronics parallel interface, or optionally by a small
system computer interface (SCSI).
1. Centronics Interface
The Centronics Interface is an industry standard interface defined by the
American National Standards Institute (ANSI). This interface is a parallel
communication port that provides a communication link between the host
computer and the Digital Palette.
Note
Parallel communication means that eight bits
in each byte of data move along eight
separate parallel lines inside the host
interface cable.
This interface (Figure 2-9, sheet 6) consist of a host interface cable and a
Centronics interface network internal to the Digital Palette. The Centronics
interface network is on the Logic Controller P.C. Board.
A bi-directional octal latch is used to read/write the contents of the 8-bit data lines
onto the Centronics connector. The B data lines (B1 - B8) from the devices run
to the data lines of the Centronics connector and the A data lines (A1 - A7, A11)
are connected to the low bytes of the microprocessor’s address/data bus
(ADO - AD7). Four control lines control the operation of the bi-directional octal
latch: /DATAG, CPDIR, /CPSTB, and /CPDATWR. Table 2-13 lists and
describes the control lines of the bi-directional octal latch.
The control lines for the parallel port are controlled by an octal address/data latch.
The outputs of this octal address/data latch feed an inverting buffers. The output
of these buffers run to the Centronics connector. Since all of the control lines are
driven with open collector outputs, the port can serve as either an input or output
port.