Hi crchisholm,
I have been reading the thread you started and thought I may be able to pass on at a layman level, some of what I have learned. I too am a confused beginner at this CNC stuff, but have slowly gathered enough information to probably be somewhat dangerous.
I have servo motors on my CNC, so am using the Gecko 320X, but I am using the ESS, so I may be able to let you know about what I have found out.
One potential issue with using parallel port connection is the limited amount of i/o (in and out) available. If you intend on using your mouse and PC monitor to control the machine, then perhaps you will only need inputs for things like E-STOP and Limit Switches.
If you would like to control the machine via external buttons for things like Cycle Start, Feed Hold, Jogging, MPG etc., then it is likely unless you implement more parallel ports, that you will quickly run out of inputs and perhaps outputs.
If you go for the ESS, then it can be advantageous to use a good BOB (Breakout Board). There are several good ones on the market for the ESS, personally I have the MB2 from CNCRoom in Thailand.
It is a very good board and if you buy the ESS from him as well, it comes installed onto the MB2 and utilises all three ports on the ESS. The MB2 has quite a lot of I/O, as well as some other nice features, like industry standard 24V inputs and outputs. There is also an on-board Analog speed control, which can be used to control the speed of you Spindle via a VFD (Variable Frequency Drive) or a drive that uses +/-10V.
This is the link.
http://www.cncroom.com/interface-board-mach3-mach4/smooth-stepper-ess-mb2-bobYou asked if the ESS would connect to the g540 via a parallel port cable. That is correct, although if you use a break out board with the ESS, then you would connect each wire at the ESS end of the PP cable into its respective input or output on the Break-Out Board.
There are other really good CNC motion controllers on the market. The ESS is a very good board, and has the big advantage of many users around the world, which helps for support. Another very good board for the price is the PoKeys57CNC. A lot of well thought out features for a great price. And from experience, very timely and helpful support. I own the PoKeys 57E, which I am using to increase the amount of inputs on my front panel.
Hope there is something in there that you may find interesting. And I hope you find it all as stimulating as I have.
Craig