7i76E with 7i77 or better 7i92 with 7i76 and 7i77

Hi !

I do have a very similar setup than @ newbynobi so I hope it's fine to continue the discusion in this topic.

I'm looking for retrofiting a 3 axis mill with 3 servo amps with analog in. The motors tachymeters are connected directly to the servo amps, and the linear scales should go to mesa cards for position feed back.

I do have a few aditionnal questions :

- In my case, the 7i76E + 7i77 looks like a very flexible solution as I could choose to drive the servos with either the Analog outputs (from the 77) or the the Step+Dir (from 76E) (so I could change the motors and inverters some day in the future and the MESA setup would remain compatible for both).   
Question 1 : On a purely theorical standpoint ; between analog output and step+dir output ; which is the most recommended way to have a fast and precise control/loop (especially if the machine has scales) ?

-The 7i76 boasts 48 I/O + 6 axis outputs + 6 encoders inputs (so 72+ signals which obviously can't be one-to-one signals going though the 25 pins connector.
Question 2 : What is the multiplexing/communication protocol going through thoses connnectors between FPGA and daughter board? Isn't there a performance loss for critical signals (axis control)?

 - I could choose a setup 7i97 +7i84+7i74. (and add later a 7i78 if I want to change to step+dir instead of analog out)
   This setup is more expensive, but I have analog outputs and encoders input (which are obviously critical in the control loop) directly on the FPGA board and not through a daughter-board.
  Question 3 : Is there a performance benefit to have encoders and analog outputs on the FPGA board instead of daughter board ? Would that justify the added cost and loss of flexibility ?

- The 7i97 description mentions  "6 high speed encoder interfaces are provided axis feedback  and for spindle synchronized motion" which is not in the 7i77 description.
 Question 4 Is it a special kind of encoder input peripheral that doesn't exist on the 7i77 ?
 


Wishing you the best for this new year !

Those are very precise and specfic answers ! Thanks.

For question 2 , you're right. I meant 7i77.


Ok, so for further reference I note that, on MESA boards :
      - no performance difference between I/Os on FPGA board and daughter boards. (that's kinda hard to wrap my head around that, but multiplexing can sometimes be magical, I guess)
      - there are two kinds of encoder input on MESA boards : normal encoder = high speed encoder (~10 Mhz)   and slow encoder = MPG encoder for HMI purposes.

Muxing does seem rather magical, but in the case of a 7i77 card most of the magic is from Smart Serial connections.

So for example a 7i77 card only uses 17 io pin connections to the FPGA. 10 pins are for the high speed muxed encoder inputs (18 encoder pins), 6 pins for 3 smart serial interfaces, and one pin as the enable. One of the smart serial pairs handles all of the analog outputs, one does all of the 32 IO pins, and one is simply passed through for smart serial expansion. And there are 3 optional low speed encoders included with the 32 smart serial IO points.

Actually there are at least 3 types of encoder inputs on Mesa cards. High speed, muxed high speed, and the low speed for MPGs. I believe that all of the high speed inputs on the cards you are considering are all muxed encoder inputs. I think the straight through high speed inputs are mostly on the older daughter boards for the 50 pin FPGA cards. The advantage of the muxed encoders is that they use half plus one the number of pins on the FPGA card, the disadvantage is that they have about half the performance advantage (but can still read at multiple megahertz rates.) The low speed MPG inputs are fed through GPIO on a smart serial connection, so use very few actual FPGA pins (2 per smart serial device.)

(I'm sure if my assumptions are wrong someone will correct me and I'll feel very humbled.)