This project is an adapter for a DB-25 parallel port to USB; it converts your computer's USB female connector to a DB-25 female connector. This supports all parallel port modes named SPP, EPP and ECP, with 16Byte FIFO depth. The USB's signaling speed is 480Mbit/s; 8kByte/s at I/O instructions. The overall current consumption of this device in USB idle state should now be about 500µA, as required by USB specification. There are four headers in the circuit for configuration. For assignment of pin 25 of D-sub female connector J1; right connected: pin 25 to ground (default), not connected: pin 25 unconnected, left connected: pin 25 to 5V (or 3.3V if J4 is right-connected). For deactivation of boot EEPROM J2; connected: EEPROM active (connected to microcontroller), not connected: EEPROM disabled (disconnected from microcontroller). For write-protection of boot EEPROM J3; right connected: EEPROM writable, left connected: EEPROM write protected. For pull-up voltage J4; left connected: 5V pull-up (compatible to true parallel ports), right connected: 3.3V pull-up (recommended for non-5V-capable inputs, like Xilinx Spartan FPGAs).
The USB has been widely used as an ideal interface for plug-and-play applications eliminating the D-sub's bulky size and pins that are easily bent and misaligned. And also, the size and cost of the USB is far more efficient than D-subs. Despite the widespread of USB, still we cannot completely eliminate the use of D-subs because there are still lots of devices that are functioning with D-sub connectors. This is the purpose of the project, allowing your new computer to still communicate with your old devices.
