Is there any reason why mach 3 could not drive older controls formerly driven by dos software?
It seems to me if you know the pin assignments of the old controller you should be able to
drive it with Mach. Or is this wishfull thinking?
 I have an old Servo Impact 2 machine, nice little machine but old controls from the late 1990's

If it accepts Step/Dir signals or even quadrature signals then it should be possible, the latter will be restricted to certain pin assignment if using the parallel port.
Does this system run from the parallel port or is it serial or a dedicated PCI or ISA card?
Hood

It outputs from the pc through a serial port. The motors are brushless dc servos.
The servo impact tech claims it's not step and direction. I was surprised to hear that,
since it's sending pulses, (even from the pendant without the PC on) I assumed it is step and direction.
If not step and direction, what else could the old dos control software be?

If its via serial then its not just a case of hooking to the parallel port unfortunately. Even worse if your drives are not Step/Dir then that is no use. The drives are likely analogue input (-10v to 10v) and there will be a board in the controller that converts what ever info the serial is sending out.
If you knew the make/model of drives you could maybe get info but likely your best bet would either stick with what you have or get some new drives for the motors.
Hood

Thanks for your input. I guess for now I'll stick with it as it is.