I am looking to develop a Mach 4 Plugin to support an Arduino Mega 2560 with a RAMPs 1.4 driver board for small low budget desktop cnc machines.

How do I get access to the SDK for Mach 4?  Do you require an NDA?

Any other advice would be greatly appreciated. 

Note this would not be a Smooth Stepper replacement.  This would be a low-budget hobbyist CNC primer board set ($25-$50) similar to what is available on the CNC 3018 as a single board only with Mach 4 Control Software vs the Open Source CNC Control Software. 

Why you want to write a plugin for Mach4(3) to "degrade" it to an ordinary G-code sender?

To operate on a common interface as a stepping stone to better hardware and software.

Why would I want to operate multiple software interfaces when I have one already?

Hi,
in order to write a plugin you will need in-depth information about Mach4 and particularly its data IO.
In order to get that info you need to contact NFS and sign a Non-Disclosure-Agreement. NFS are very keen
to have enthusiastic individuals and companies write Mach4 plugins for their hardware......it all benefits
NFS sales.

I must say I echo Zasto's comment that the effort you're going to go to to write a plugin is such that any
commercial return would price it well outside of your projected sale price range.

Given that Mach4Hobby is $200USD, even if you could build a $50 external motion controller, is $250USD
a 'low budget hobbyist CNC primer' solution?

My personal experience of Mach4, and I've been using it for six years is that it is far from a 'low-budget CNC primer' solution,
but competes very favourably with industrial CNC solutions costing many tens of thousands of dollars. It's not so much
that Mach4 (and compatible hardware) is too expensive for low-budget CNC primer but rather that it is a very  cheap
alternative to high end industrial solutions.

Craig

Given I already have 2 smooth steppers (1-USS and 1-ESS)two mills, 6 arduinos, and a 7th on it's way. One less SW baseline to maintain, and the knowledge I would gain is worth it to me.  A guy has to have a hobby 

I already put in a request to machsupport.  I was given access to the SDK about 5 years ago (V1), but a life event tied me up for about five years.  Now I have some time to get reacquainted with Mach 4 and it's updates.

An Arduino with motor drivers is about $50 and the SW is Opensource or with CNC 3018 $212.  Add some modbus features + Mach Hobby $200 to get you going and your old machine still can provide some small project milling support.  Purchase a smoothstepper and then a PMDX board $400 or purchase an ESS and BOB from Warp9TD $480.

$212....then add $200 Mach....then add $400-$500 for ESS/BOB...Make your machine larger....$1000.  No this is not a cheap Hobby but it does have some stepping stones.  I agree Mach 4 is not a low budget solution.  I am saying that the arduino-RAMPS or CNC3018 is a low HW budget solution. Just to get you started.