How to get Index pulse from Mesa

Hi!
I use 7i43 card.
There are 3 connections to encoder - A,B,Z from outside. This terminators connected to encoder phisicaly.
How can I determine the internal Index pulse input from HAL?
The Mesa hardware knows about it (it listen Z phace).
But what encoder's pin in hm2 know about input Index pulse?

The HostMot2 driver supports the low level index hardware
in the same way as the software encoder, with a bidirectional
latched index-enable pin.

When the index-enable pin to the encoder is set true, the encoder
counter waits for an index pulse, when an index pulse occurs with index enable set,
the scaled encoder count and the index enable signal from the encoder cleared (the raw count is not changed)

You can test this by setting index enable true manually and then turning the encoder shaft.
index enable will be cleared when the encoder reaches the index