Repair manual

2-72
THEORY OF OPERATION
de-assert the /CPBSY line. Also, the assertion of /CPACK and the associated
de-assertion of /CPBSY indicate to the host computer that the data has been
accepted.
If, at any time, the control code senses an error, it can assert either the ERROR
line (/CPERR) or the paper end line (CPPE).
a. Host Interface Cable
The Host Interface Cable is a 36-pin Centronics connector that is supplied
with a mating cable terminating in a DB-25 pin connector for connection
to an IBM-PC or compatible host computer.
This interface cable provides the I/O signal lines that allows the host
computer and the Digital Palette to communicate with each other. The
signal protocol that takes place between the host computer and the
Digital Palette only uses fourteen of the I/O signal lines of the host
interface cable. All I/O signal lines that are not used are terminated in the
Digital Palette with a 3.3 Kohm resistor to +5 volts dc.
Table 2-14 lists and describes the function of each I/O signal line used to
interface the host computer with the Digital Palette.
Refer to the provided Centronics Parallel Interface Specifications Manual
for a detailed description of the communication signals between the
Digital Palette and the host computer.
b. Centronics Interface Network
The Centronics Interface Network in conjunction with the host interface
cable provides the following:
o Processes the transmitted data from the host computer to make it
compatible with the Logic Controller P.C. circuitry.
o Places the processed data onto the microprocessor address lines. The
microprocessor network decodes the processed data on the address
lines and flags the applicable program internal to the EPROM. The
instructions from the EPROM are processed by the microprocessor.
The processed data is applied on its output port to initiate the
specified Digital Palette operation.