ST78C34 Exar Corporation, ST78C34 Datasheet - Page 6

• Manufacturer Part Number: ST78C34
• Description: General Purpose Parallel Printer Port With 83 Byte Fifo
• Manufacturer: Exar Corporation

PRINTER PORT PROGRAMMING TABLE:
PRINTER FUNCTIONAL DESCRIPTION
The ST78C34 parallel port is designed to operate as
a normal CENTRONICS printer interface. The port
contains 83 byte FIFO that may be enabled via bit-7
of the Alternate Function Register (AFR). After reset,
the FIFO is disabled and the part will function identical
to the ST16C552. Once the FIFO is enabled via AFR
bit-7, the port will enter FIFO mode after the first byte
of data is strobed to the printer and the printer re-
sponds with either an -ACK or BUSY signal.
The ST78C34 will remain in FIFO mode until the part
is reset or INIT is brought low. While in FIFO mode,
data transfer to the printer will be controlled by the
printer without any user intervention. The printer port
also contains a FIFO byte counter that maintains a
count of the number of bytes remaining in the FIFO.
The FIFO and the FIFO byte counter are cleared by a
reset or by a change of state of the INIT pin. All FIFO
related timing is derived from the clock input to pin 17
of the part.
A special parallel port write / read mode is activated
when INIT is held low, either by writing a “0” to Control
Register bit-2 or by forcing the INIT pin low. In this
mode the FIFO read pointer is advanced by reading
the parallel port instead of the -ACK or BUSY signals.
The -STROBE output is forced high. This allows the
user to perform parallel port write and read from
operations without strobing data to the printer.
ST78C34
* Reading the status register will reset the INT output.
Rev. 3.00
A1
0
0
1
1
A0
0
1
0
1
WRITE MODE
PORT REGISTER
CONTROL REGISTER
ALTERNATE FUNCTION REGISTER
6-8
Following an INIT, the parallel port will not be in the
FIFO mode. Control Register bit-0 is used as the -
STROBE, Status Register bit-7 is the inverse of the
BUSY signal, and INT is derived from -ACK. The
transition into FIFO mode will occur after the first -
STROBE is generated and the printer responds with
either an -ACK or BUSY. In FIFO mode, -STROBE is
generated automatically and writing to Control Regis-
ter bit-0 has no effect on -STROBE. Alternate Func-
tion Register bit 0-2 are used to control the delay and
width of -STROBE. Handshaking between the printer
and the ST78C34 may be controlled by bit-3 of the
Alternate Function Register. Setting this bit to a “1” will
result in the use of BUSY instead of -ACK for FIFO
reading and interrupt control. INT will transition low
when a “1” is written to Control Register bit-0 and will
transition high when a write to parallel port is per-
formed. In FIFO mode, data transfer to the printer will
be controlled by the printer and will occur at the
printer’s maximum data rate.
The FIFO byte counter is incremented one count for
each parallel port write and decremented one count
for each FIFO read (data taken by printer). A FIFO
read will be generated at the falling edge of either -
ACK or BUSY. The byte counter will require two to
three clock cycles to update. Hence, a read of FIFO
Byte Count Register (FBCR) should only be per-
formed a minimum of three clock after the falling edge
of either -ACK or BUSY. The counter is reset when-
ever the FIFO is reset. If write to parallel port operation
is attempted when the FIFO is full, the data will not be
READ MODE
STATUS REGISTER *
COMMAND REGISTER
PORT REGISTER
FIFO BYTE COUNT REGISTER