TL16C451, TL16C452

ASYNCHRONOUS COMMUNICATIONS ELEMENTS

SLLS053C – MAY 1989 – REVISED AUGUST 1999

21

POST OFFICE BOX 655303

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