I was told to ask Mesa 5/6i25 questions here.

Hi Andy, Peter

Thank you so much for advice, i changed steplen and stepspace and 5i25 can generate 3MHz ( confirmed by logic analyzer).
But as you told my conversion board cannot capture stepgen at 3Mhz so i need to change method, can you help to point out any register on FPGA which is storing postion from linuxcnc ( motor.post - cmd )? I want to use this data to send to my board per 1ms cycle

There is no register that stores the commanded position

Basic operation of the step generator is that the velocity is set
by LinuxCNC and the actual step count is read back by LinuxCNC
to correct for minor differences in clocks, frequency setting
granularities, position sample and velocity update timing variations,
etc. etc.

Hi Peter

I used quadrature stepgen and added in filter on my board then it can work at high step rate now

Thank you for ideas