Lucent Technologies IEEE 1284 Port, Register-Based Operation, USS-720 Instant USB, September

USS-720Instant USB	Preliminary Data Sheet, Rev. 5
USB-to-IEEE1284 Bridge	September 1999

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.

5-14

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.

Table 18. Transfer Rates

Direction	UHCI	OHCI
Forward	1 Mbyte/s	1 Mbyte/s
Reverse	250 bytes/s	1 Kbyte/s

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.

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