Double Pulse For Each Incoming Pulse -> Double Stepping Stepper Motor Control

As Nick T mentioned, TCP/IP is unfiltered and non-deterministic. You said you want to also interface with SPI and UART devices, and previous versions used USB. These are difficult to do in real time, and probably require significant code space.

On the other hand, you want real-time control of your stepper motors, which requires deterministic execution and rapid interrupts but not a lot of program space.

While you said you wanted to avoid using a separate microcontroller because it would complicate things, this is a perfect example of when you need a co-processor, and I believe that it would make things simpler in the end.

You can develop simple stepper motor controller firmware that run on small, cheap microcontrollers. Design (or discover) a protocol that can use a communication bus available in hardware on whatever controller you choose. I'd recommend SPI for this (on a different channel than the device you mentioned in your question; your master should have two SPI peripherals), you don't want to mess with address conflicts given the small number of controllers you'll have. Don't do anything with this controller but run the stepper according to the data it receives over the protocol. You can probably buy something that does this already. The goal is to be able to treat this as a hardware peripheral.

Then, you can develop USB or Ethernet controllers that will plug into these devices. You will have flexibility in the host protocols (Need to use RS485 this time? No problem!), you can use different numbers of motors, and you have greater modularity for designing, testing, debugging, repairing, and replacing. The only reasons I'd try to implement something like this in a single controller would be extreme cost/size restrictions, and/or a lack of hardware modules (like PWM? Did I miss something?) that could do this without interrupting the host processor.

A stepper motor can certainly be used as a rotary encoder, with the caveat that both reliability and precision will be questionable at low rotation speeds and small numbers of steps.

• Question indicates need for absolute angular movement detection, i.e. a "zero detection"
  • No convenient way to detect zero from a stepper
• Question does not specify angular resolution of stepper motor, nor angular precision required.
  • A stepper with more steps (not microsteps) per rotation will provide greater precision
• Speed of rotation of arm not specified
  • Slow rotation will result in weaker step signal, hence greater likelihood of missed steps

Now, working towards a solution:

• Energize the stepper motor's coils with current-regulated DC at a small fraction of the rated operating current for the stepper. For a 3 Ampere rated stepper motor, 10 mA per coil is ample, step detection should work at even less.
• Read the voltage pulses (back-EMF) generated across the stepper coils, as the stepper is rotated.
• The sequence of the pulses indicates direction of rotation, the pulse count is the number of angular steps traversed, and the amplitude of the pulses relates to the angular rotation speed.
• Some calibration will be required if anything other than number of steps traversed is to be detected
• Do not expect lab-quality rotary encoder reliability from this solution: Occasionally steps may be missed, and some spurious steps may get detected due to EMI picked up by the coils.

This YouTube video demonstrates the use of a Hard Drive motor as a rotary encoder.

This article discusses the use of stepper motors as rotary encoders in some detail.