Many years ago, the parallel/centronic port was the best port for all the electronic hackers and DIY experts. This port provided 8 TTL/Logic lines and was easy to use. Sadly, our modern computers don't have this port anymore. However, direct access logic lines are still useful, for mount guiding or camera control.
Sure, there is some USB/Printer adapter, but this adapters really need to be connected to a printer, they can't act as a simple logic interface. Some full compatible USB/Parallel also exists, but they are quite expensive. So, how could we get a simple, low cost adapter with TTL outputs ? We just have to use an USB/Printer adapter and simulate a printer.
In ordrer to get our electronic board to simulate a printer, we will have to bufferize all the data lines (D0 to D7) and handle some control lines. Our board will also be powered by remaining control lines.
Our fake printer is never busy, so, BUSY and PAPEROUT will grounded, and STROBE and /ACK will be linked. To bufferize the data lines, we will use a 74573 or 74574 latch (LS or HC series). The output should be always available, /OE is grounded. To store the datas in the latch, we need to invert the STROBE line.
The INIT line is allways high, it will be our power line. We don't need to manage SELECT, AUTOFEED, ERROR ans SELECTIN.
CAUTION : centronic's lines aren't supposed to drive power lines. Our outputs MUST be connected to high impedance pins.
The board :
My board looks like that :
Copper layout :
It works like a charm ;-)
If you want to know how to use this hack on GNU/Linux, just have a look at this page : Add TTL outputs to your USB Laptop (Part 2, Software on Linux)
If you want to know how to use this hack on MS Windows, just have a look at this page : Add TTL outputs to your USB Laptop (Part 3, Software on Windows)