step/dir vs analog / servo encoder vs output

Hi I am trying to decide between 7i77 or 7i76 for interface with 2 delta asd-a2 drives and dmm dyn3h drive.
The machine is a lathe, around 12x12" X Z travel, with 5mm pitch ballscrews. driven by delta 400w servos
I plan to have spindle C axis , which is a 750w servo driven 2 to 1.
Here is where I am unsure about which to choose.
Both brands of drive take position or analog speed/torque input.

The delta drives have 1,280,000 pulse encoders going to the drive, but the drive will output to linuxcnc between 20 and 320k pulses per rev,
If I use position input, it seems the drive would have better control, having access to finer resolution encoder signals.
The electronics gear ratio is adjustable from 2 to 500k pulse per rev (what a range)
If my math is right, a ratio of 508:100 or 1968.5 pulses per rev, would give me 1 pulse per 0.0001" travel on a 5mm pitch screw. (1:1 = 10k pulses per rev)
the 7i76 doesnt have encoder inputs, so I wouldn't be able to/ or need to run the encoders output back to linuxcnc.
That all seems fine, either mode should work, but analog would give me DRO's that reflect actual position.

However for the DMM drive, it has a 16,384 pulse encoder on the motor, but will only output to linuxcnc 500 - 2048 pulses per rev.
which would be 2000 - 8056 quadratures per revolution. I don't know if linuxcnc will see a max of 2048 pulses, or 8056 quadratures,
but even assuming 8056 positions, I would only have 0.044 degrees per step. On a 6" diameter part, that would be like 0.0015" per step. (2:1 at the chuck)
Or would it be the worst case of that scenario, 2048 positions, 0.087 chuck degrees per step, or 0.0045" per step on a 6" diameter part?
The first case seems fair, the second case seems too coarse.
Now if I chose position mode for the spindle servo drive, I would have a max of 16,384 steps per revolution,\
which would give me 0.0219 servp degrees per step, 0.0109 chuck degrees per step which would be 0.00057" per step on a 6" diam part.

So with position mode, I can double my resolution on the C axis.

Does this all sound right? or am I missing something?
I will admit, that I will probably never need that much resolution for any parts I will be making, and the machine construction is probably not stiff enough to realize that resolution either, but I understand that with C axis positioning, The higher the resolution, the better.

Here is what I can understand for pros/cons of analog and position

position
pros - ease of setup, positioning accuracy as fine as highest resolution of input.
cons - max speed limited by max pulse frequency

analog
pros - ability to tune on linuxcnc side, DRO has encoder feedback.
cons - positional resolution limited to simulated encoder feedback to linuxcnc.

Thanks for any replies, I am really excited to get this thing going, just dont want to make any great mistakes in doing so.

Theres yet another option:

Closed loop step/direction

With our hardware you can do this with a 7I85S (4 encoder inputs + 4 step/dir output pairs + 1 channel sserial expansion) and perhaps a 7I84 connected to the 7I85S's sserial expansion for misc I/O

Maximum step rate is not likely to be an issue (if used with a 5I25, the maximum step rate is ~8 MHz)

Thanks, would my positional resolution, still be limited by the simulated encoder output, or would that only be used to feed back for the DRO?
If only used for DRO, then the resolution would be limited by the encoder output, right? but in this case the DRO would only be for visual feedback, and not used in any calculations on the linuxcnc side?

In other words the encoder feedback would only be used for eye candy?

It can be used for eye candy or feedback, your choice

Note that the "finer resolution encoder signals"
are always used, their chief purpose is improved velocity feedback at
low speeds, not position feedback.

With common ~5mm pitch ballscrew drive systems your accuracy is so limited
by the mechanics that little is gained over say 50000 counts/turn
of position resolution.

Thanks again,
In reference to the DMM drive,
I guess I am confused on how linuxcnc would react to the encoder signals, being that the encoders on the motor are only sent to the drive, and a simulated encoder output of lower resolution is sent to linuxcnc. In other words, linuxcnc can command the drive to move as little as 1/16384 of a revolution, but the drive will only report back to linuxcnc a move as little as 1/8056 of a revolution.

As far as the 5mm pitch screws, you mean they are too coarse pitch? or being cheap screws the positional accuracy over the range of the screw wastes the resolution of the motor? I am not too worried about the X and Z setup, it seems there is enough configuration range on the delta drives to make it work with any setup.
I am more worried about the C axis positioning with the DMM drive with lower resolution.
Is my thinking right? open loop would have better resolution than closed loop, because of the low resolution of the simulated encoder output back to linuxcnc?

Sorry for the many questions, and thanks so much for your time!

Just saying for the linear axis that at a common 5mm pitch at say 20000 counts per turn (1/4 u per step) you have much better resolution than accuracy already (your ball screws/ thrust bearings will have errors much bigger than this)

Its unfortunate that the DMM driver doesn't forward the full encoder resolution, but
rotary axis typically have a much higher (that 2 --> 1) motor-axis gear ratio

PCW wrote:

Theres yet another option:

Closed loop step/direction

With our hardware you can do this with a 7I85S (4 encoder inputs + 4 step/dir output pairs + 1 channel sserial expansion) and perhaps a 7I84 connected to the 7I85S's sserial expansion for misc I/O

Maximum step rate is not likely to be an issue (if used with a 5I25, the maximum step rate is ~8 MHz)

PCW,

This is very interesting to me, but I have a question, could it also be accomplished with a 6i25, 7i76 and 7i77? The reason I ask is I have these boards and am interested in pulse controlled motion with feedback positioning.. or would it have to be the boards you listed?

Thanks in advance, I know this is an old thread, but it may be a potential solution to a dilemma I'm currently in.

Much appreciated,

Chris