USB to parallel adapters can be thought of as a data stream flow device. They should generally work for talking to printers on systems where the OS level provides a standardized device driver for printer access. The drivers supplied for these know how to take the stream of bytes and send it through the USB protocol and out to the target end electronics which will place data bytes out on the data out lines and pulse the data strobe line so as to transfer the bytes to the printer. The driver also will know how to read the printer status lines and send the states of these back to the driver as a logical state indication. Many of the adapters sold in the market have the 36-pin 'centronics' type connector on the end for direct attachment to the printer. A few adapters in the market have the female DB25 type connector.

USB parallel adapters are NOT a direct replacement for the good old parallel port DB25 connector that you would find on the back of a legacy computer. I say this in the sense that there are not a series of registers present that the PC programmer can use direct OUT DX,AL or IN AL,DX instructions to access. This means that any and all software that used direct register I/O to bit bang the parallel port does not work with USB parallel adapters.

It is always possible that there may be USB to parallel type devices available that the manufacturer has provided extra functionality to their USB driver to permit special commands to be sent that would set/clr output bits at the end of the cable or read back the state of individual input bits. I have not seen this available on any of the commercial USB-->PARALLEL adapters but it may exist. [Note that FTDI makes some USB adapter devices where they provide a USB driver interface that permits the programmer to send commands to operate the pins at the adapter chip like GPIOs]. In any case the ability to leverage this type of discrete I/O manipulation at the end of a USB adapter device takes extra software. Any pre-existing software that expected to be able to access some registers, (@ 0x378 or 0x3BC for example), will not be functional in this USB scenario. Also any software written for the more modern versions of Windows that has leveraged access to direct hardware parallel ports through the use of the INPOUT32 or GIVEIO DLLs will not work with the USB parallel adapter scenario.

In conclusion the use of parallel port attached devices such as:

- Do it yourself (DIY) type programmer devices

- Parallel port software protection dongles

- Parallel port JTAG adapters such as the Altera ByteBlaster FPGA/CPLD programmer cable

- Parallel port attached logic analyzer devices

- Parallel port attached access pods for I2C or SPI bus access

- Parallel port ...this, that and ...most other minimal hardware devices

All of these are history in the realm of USB parallel adapters....unless you are prepared to find a specially programmable adapter and make full source code level changes to the programs that utilize the connections.

(There is another whole subject concerning the use of PCI and PCIe addin parallel port boards for modern WinXP and WinVista computers that has a similar sad outcome that we can discuss another day).

Michael Karas