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5-14 Lucent Technologies Inc.
USS-720
Instant USB
Preliminary Data Sheet, Rev. 5
USB-to-
IEEE
1284 Bridge September 1999
13
IEEE
1284 Port
The
IEEE
1284 port on the USS-720 is compliant with
the
IEEE
1284-1994 standard. The parallel port oper-
ates in two distinct modes. In fully Automatic Mode, the
IEEE
1284 protocol is implemented completely in hard-
ware. Compatibility Mode, Nibble Mode, and ECP
Mode (with or without RLE compression) are
supported, with all negotiation, termination, and other
features of the protocol handled transparently by the
hardware. The USS-720 also features a Register
Mode, which presents a standard register interface to
the host. These two modes provide the host with two
distinct operating paradigms. In Automatic Mode, the
host software interacts with the USS-720 as if with a
USB-capable printer; while in Register Mode, the USS-
720 emulates standard PC parallel port hardware. In
the Automatic Mode, the host application software
doesn’t know that the USB data stream is being
converted to
IEEE
1284 protocol. In the Register Mode,
it need not know that its interaction with parallel port
registers is actually talking place remotely over a USB
link.
Automatic mode is recommended for almost all appli-
cations. (Note: The drivers included in the Lucent
Technologies USS-720 Evaluation Kit use only Auto-
matic Mode.) Automatic Mode implements all negotia-
tion handshakes automatically for Compatibility, Nibble,
and ECP modes. In Register mode, the user must do
the negotiations manually in software. Register mode
can be useful when implementing a nonstandard
parallel port interface.
Register-Based Operation
In its Register Mode of operation, the USS-720 emu-
lates standard host-side parallel port hardware, with
the register accesses being performed remotely over a
USB connection. As in the standard register set, the
interface mode is controlled by the Mode field in the
Extended Control Register. The supported modes and
their meanings are given in the Extended Control Reg-
ister section on page 5-20, and operation and required
software interaction for each of the supported modes
are described in the sections that follow.
Standard Mode (000)
In this mode, the parallel port is under full software con-
trol, with no form of hardware assist. Software has
complete control of all parallel port signals. This mode
can be used for negotiations, terminations, proprietary
handshake sequences, etc. As in standard host-side
parallel port hardware, the parallel port data lines are
unidirectional outputs in this mode.
Bidirectional Mode (001)
This mode is identical to Standard Mode (000), except
that the direction of the parallel port data lines may be
controlled with the Direction bit in the Control Register.
Compatibility Mode (010)
This mode provides hardware-based Compatibility
Mode data transfers. Data sent to the USS-720 over
the Bulk Out pipe will be transferred automatically to
the peripheral using Compatibility Mode.
ECP Mode (011)
This mode provides hardware-based ECP Mode data
transfers. To use ECP, the host software should negoti-
ate for ECP Mode via the Control and Status Registers,
then set Mode to 011. At this time, the Compress
Enable bit in the USS-720 Control Register should also
be set appropriately.
EPP Mode (100)
This mode provides hardware-based EPP Mode data
transfers. To use EPP, the host software should negoti-
ate for EPP Mode via the Control and Status Registers,
and then set Mode to 100.
EPP mode in the USS-720 has the following data
transfer rate characteristics. Note that the rates are
approximate.
Because the reverse channel operation is implemented
in the USB Control Pipe, the transfer rates are limited
according to the particular implementation of the host
controller, either UHCI or OHCI.
The high performance of the forward direction in EPP
mode makes it attractive for implementations requiring
little reverse channel traffic.
Table 18. Transfer Rates
Direction UHCI OHCI
Forward 1 Mbyte/s 1 Mbyte/s
Reverse 250 bytes/s 1 Kbyte/s