SSI encoder on MESA 7i96s

A friend gave me some AMS AS5311 absolute encoders , which I'd like to use to add Z-axis feedback to my little lathe. I have an encoder attached to my spindle already, so I'm out of quadrature encoder inputs on my 7i96, but the AS5311 also has an SSI interface. I'm wondering if I can configure the 7i96 to use some GPIO pins or something to read the encoder's SSI interface. I've seen folks talking about flashing different firmware onto MESA cards, but I can't for the life of me figure out where all these firmware files live.

Is it possible to read an SSI interface with the 7i96 or will I need to buy another board from MESA to get another quadrature input and wire the Z encoder to that?

I thought about bit-banging an SSI interface but I think that would be quite slow. I also read in another thread about an existing SSI component, but I don't see anything in the component documentation .

I feel kind of clueless, but am slowly learning the ins and outs of Linux CNC. I appreciate anyone's advice, thanks in advance.

I don't think the firmware SSI encoder interface will work with the AS5311
as it does not have a standard SSI interface. Normal SSI encoders do not
have a CS signal, the get all timing information from their clock input.

Ah, thank you for the pointer. Yeah, I think PCW is certainly right that the encoders won't work with the SSI firmware module. I think the documentation is incorrect in calling it an SSI interface. It seems like it's pretty clearly an SPI interface. I don't know if leaving CS permanently low would work, but based on how the documentation talks about selecting between magnetic strength mode and absolute position mode, I'm kind of doubtful. 

It is likely fairly simple to ad a CS output to the SSI interface firmware
though its a bit awkward for a normal cabled interface