Datasheet

TL16C451, TL16C452
ASYNCHRONOUS COMMUNICATIONS ELEMENTS
SLLS053C – MAY 1989 – REVISED AUGUST 1999
21
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
parallel port registers
The parallel port registers interface either device to a Centronix-style printer interface. When chip select 2 (CS2)
is low, the parallel port is selected. Tables 7 and 8 show the registers associated with this parallel port. The read
or write function of the register is controlled by the state of the read (IOR
) and write (IOW) terminal as shown.
The read data register allows the microprocessor to read the information on the parallel bus.
The read status register allows the microprocessor to read the status of the printer in the five most significant
bits. The status bits are printer busy (BUSY
), acknowledge (ACK) which is a handshake function, paper empty
(PE), printer selected (SLCT), and error (ERROR). The read control register allows the state of the control lines
to be read. The write control register sets the state of the control lines, which are interrupt enable (IRQ ENB),
select in (SLIN), initialize the printer (INIT
), autofeed the paper (AFD), and strobe (STB), which informs the
printer of the presence of a valid byte on the parallel bus. These signals are cleared when a reset occurs. The
write data register allows the microprocessor to write a byte to the parallel bus. The parallel port is completely
compatible with the parallel port implementation used in the IBM serial/parallel adaptor.
Table 7. Parallel Port Registers
REGISTER
REGISTER BITS
REGISTER
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Read data PD7 PD6 PD5 PD4 PD3 PD2 PD1 PD0
Read status BUSY ACK PE SLCT ERROR 1 1 1
Read control 1 1 1 IRQ ENB SLIN INIT AFD STB
Write data PD7 PD6 PD5 PD4 PD3 PD2 PD1 PD0
Write control 1 1 1 IRQ ENB SLIN INIT AFD STB
Table 8. Parallel Port Register Select
CONTROL TERMINALS
REGISTER SELECTED
IOR IOW CS2 A1 A0
REGISTER
SELECTED
L H L L L Read data
L H L L H Read status
L H L H L Read control
L H L H H Invalid
H L L L L Write data
H L L L H Invalid
H L L H L Write control
H L L H H Invalid