servo amps and power supplies; terms of endearment

I've got a servo amp with 4 boards, each rated to run a (permanent
magnet DC servo) motor 70-100 VDC at 15 cont / 25 peak amps. The unit
has an integrated power supply for its own requirements--but zilch
for the motors.

What I'm wondering is how the amp communicates with the power supply
so as to vary the power supplied to the motors? Does the amp just
tell the individual motors to run faster or slower and so each just
pulls more or less from the well? Maybe the amp sends voltage and
amperage codes to the power supply and it adjusts its output (but
this would require a power chanell for each motor, right?) Maybe the
full output of the power supply runs through the amp and four demons,
one for each motor, regulate the flow to each motor.

As I've stated before, I'm not an EE. I figure I could probably
figure out how to make a simple DC power supply, but depending on the
complexity of the controll scheme I might just try and find one
surplus.

As always, thanks in advance.

--Jack