Repair manual

2-71
THEORY OF OPERATION
Table 2-13. Control Lines for Bi-Directional Octal Latch
Control Line Function
CPDIR This control line indicates whether the port is configured as
an input or output port. When this signal is LOW, the port
is an input port (host to the Digital Palette). CPDIR is a
latch control bit from the octal address/data latch. It is
controlled by microprocessor address/data bit 14 (AD14).
/DATAG This control line is the data enable line. It will enable
either the A or B lines depending on the state of CPDIR
control line.
When CPDIR is LOW (input mode), /DATAG will enable
latched data onto the microprocessor address/data bus.
When CPDIR is HIGH, the assertion of /DATAG will
place data onto the Centronic data lines.
/CPSTB This control line is the STROBE line from the Centronics
Connector.
When it is asserted LOW, the contents of the Centronics
data lines are latched into internal latches of the
bi-directional octal latch.
/CPDATWR This control line is generated by the interface control PAL
when an I/O data write cycle occurs. The data on the
microprocessor’s address/data bus will be latched into the
bi-directional octal latch on the falling edge of this signal.
When configured as an input port, the CPDIR line is LOW. The host computer
will assert the data lines and then assert /CPSTB. This will latch the data from
the bi-directional octal latch bus (B1 - B8) into it’s internal latch. The assertion
of /CPSTB from the interface control PAL will assert CPBUSY. This action
asserts /CPBSY on the Centronics BUSY line. The interrupt line of the
microprocessor (CPRTINT) is also set. At a later time, the microprocessor reads
the control port and senses the /CPBSY line. The /CPBSY line, being asserted,
indicates that data from the host computer is available. First, it reads the data port
which asserts /DATAG and enables data onto the microprocessor’s address/data
bus (A data read instruction will clear CPRTINT). Next, the acknowledge line
(/CPACK) is pulsed low. This action causes the interface control PAL to