Lucent Technologies manual Description continued, Typical Circuit Showing the USS-720, February

Typical Circuit Showing the USS-720	Application Note
Bridging USB to Parallel Port	February 1999

Description (continued)

Whenever device descriptor data is requested, the USS-720 drives both control pins CS (pin 35, Serial ROM Chip Select) and SK (pin 34, Serial ROM Clock). The USS-720 then looks for a response on DIO

(pin 36, Serial ROM Data Signal). If there is no external device connected, and no data is present on the DIO pin, then the descriptor data is taken from the internal ROM. The USS-720 accesses this data through a MicroWire* ROM Interface. One method for this is to use an EEPROM such as the 93CS56L/66L. The USS- 720 Evaluation Kit is specifically designed for use with this part. Substitution components must be pin and functional compatible with the 93CS56L/66L. 93C56L/ 66L, 93CS46L, and 93C46L EEPROM parts will not work. The connection scheme for this device is shown on the attached schematic. The circuit diagram assumes that a preprogrammed EEPROM is used.

Note: Developers using the USS-720 must use an external serial EEPROM (or the equivalent) in their design and create their own hex data file for use in programming the EEPROM at their site. The configuration data stored in this serial EEPROM is used by the Microsoft host software during the enumeration to load the appropriate drivers. Using unique identification data in the EEPROM provides a means for the developer to ensure that only their software is loaded for use with their device. Otherwise, the enumeration of a camera using the USS-720 could cause the software for a USS-720-based printer to be loaded, resulting in a system that does not func- tion correctly.

See the Device Descriptor, Configurations, and Interfaces section in USS-720USB-to-IEEE 1284 Bridge Data Sheet for more information on device descriptors.

*MicroWire is a registered trademark of Advanced Interconnection Technology, Inc.

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