ACT apricot - Page 182

PARALLEL INTERFACE
The constraints of the various hardware inputs and'
outputs to and from the PIO dictates the mode and direction
of operation of
the three ports. Port A, Port B and the 4 most
significant bits (MSB) of Port C are configured as outputs
and the 4 least significant bits (LSB) of Port C are configured
as inputs,
to provide the interface with a printer and the
system connections.
The operating mode required is the basic input/output
mode, defined as Mode o. The control word to be written to
the control register to set the three ports into the required
operating configuration
is thus as follows:
D7 DO
11101010101010111
t
III
L
Lset Port C(LSB) as inputs
Set Port B as an output port
, Set Mode 0 for Port B/Port C(LSB)
Set Port C(MSB) as outputs
Set Port A as an output port
Set Mode 0 for Port A and Port C (MSB)
Select Mode definition
After the mode and direction of operation of the three
ports is defined by the control word, the ports remain in the
set operating condition unless redefined by a different
control
word or reset by the system reset. The system reset,
when active (logic high), configures all lines of the three
ports as inputs.
Reading data from and writing data to the ports of the PIO
is performed by simple input/output operations, using the
address locations detailed above. Data written to a port is
latched into the PIO, whilst data read from the PIO is not
latched.
Printer Interface
Eight bits of parallel data are supplied to the printer via
Port A of the PIO and the transceiver. To set the transceiver
to transmit data to the printer, the MSB of Port B (PB7) is set
to logic low. The transceiver and the buffer for the printer
control signals are TTL devices.