Yes - Mach 3 is simple, because it uses the parrallel port. I think that was what attracted me to it.
I have read your post again - and in the last two lines you are saying "The only SPEED issue is that if you use a USB and the associated and required external hardware then the USB must be able to send data fast enough that the SERIAL data can be parallelized in the same time as a parallel port could have sent it."
Perhaps we are coming from different angles - but that is what I was saying.
The PC collects date for the LPT1 port in three addresses. It amalgamates that into 17 lines of data and puts it out on the parrallel port. It is hard to know the internal workings of the CPU, but somewhere between 4 and no more than 8 moves would be required. For serial work, the same data has to be amalgamated, then moved to a buffer for one bit at a time for transmission. The transmitters I am familiar with only go at CPU clock speed. It follows, therefore that in simple terms the CPU will take at least 25 moves more to put out the data.
I must admit - and I know nothing of USB systems - if the CPU can put the parrallel data into a buffer, and then a seperate system moves the data serially at an enhanced rate - the the time difference may be small.
With the dissapearance of LPT1 ports, it would be a great benefit if someone could produce a USB2 to Parrallel output that could guarantee the speed and reliability of the LPT1. Even better if it had reversible pins. The plug and play type of connection could be a great enhancement.