Information Technology Reference
In-Depth Information
Enhanced Parallel Port
17
17.1 Introduction
The Centronics parallel port only allows data to be sent from the host to a peripheral. To
overcome this the IEEE published the 1284 standard, entitled 'Standard Signaling Method
for a Bidirectional Parallel Peripheral Interface for Personal Computers'. It allows for bi-
directional communication and high communication speeds, while being backwardly com-
patible with existing parallel ports.
The IEEE-1284 standard defines the following modes:
•
Compatibility mode (forward direction only) - This mode defines the transfer of data
between the PC and the printer (Centronics mode, as covered in the previous chapter).
•
Nibble mode (reverse direction) - This mode defines how four bits are transferred, at a
time, using status lines for the input data (sometimes known as Hewlett Packard Bi-
tronics). The Nibble mode can thus be used for bidirectional communication, with the
data lines being used as outputs. To input a byte, requires two nibble cycles.
•
Byte mode (reverse direction) - This mode defines how eight bits are transferred at a
time.
•
Enhanced parallel port (EPP) - This mode defines a standard bidirectional communica-
tions and is used by many peripherals, such as CD-ROMs, tape drives, external hard
disks, and so on.
In the IEEE 1284 standard the control and status signal for nibble, byte and EPP modes have
been renamed. It also classifies the modes as forward (data goes from the PC), reverse (data
is input into the PC) and bidirectional. Both the compatibility and nibble modes can be im-
plemented with all parallel ports (as the nibble mode uses the status lines and the compatibil-
ity mode only outputs data). Some parallel ports support input and output on the data lines
and thus support the byte mode. This is usually implemented by the addition of a direction
bit on the control register.
17.2 Compatibility mode
The compatibility mode was discu
ssed in C
hapter 16. In this mode, the program sends data
to the data lines and then sets the
STROBE
LOW and then HIGH. These then latch the data
to the printer. The operations that the program does are:
1. Data is written to the data register.
